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Warming has broad and often nonlinear impacts on organismal physiology and traits, allowing it to impact species interactions like predation through a variety of pathways that may be difficult to predict. Predictions are commonly based on short‐term experiments and models, and these studies often yield conflicting results depending on the environmental context, spatiotemporal scale, and the predator and prey species considered. Thus, the accuracy of predicted changes in interaction strength, and their importance to the broader ecosystems they take place in, remain unclear. Here, we attempted to link one such set of predictions generated using theory, modeling, and controlled experiments to patterns in the natural abundance of prey across a broad thermal gradient. To do so, we first predicted how warming would impact a stage‐structured predator–prey interaction in riverine rock pools between Pantala spp. dragonfly nymph predators and Aedes atropalpus mosquito larval prey. We then described temperature variation across a set of hundreds of riverine rock pools (n = 775) and leveraged this natural gradient to look for evidence for or against our model's predictions. Our model's predictions suggested that warming should weaken predator control of mosquito larval prey by accelerating their development and shrinking the window of time during which aquatic dragonfly nymphs could consume them. This was consistent with data collected in rock pool ecosystems, where the negative effects of dragonfly nymph predators on mosquito larval abundance were weaker in warmer pools. Our findings provide additional evidence to substantiate our model‐derived predictions while emphasizing the importance of assessing similar predictions using natural gradients of temperature whenever possible. [ABSTRACT FROM AUTHOR]

We define coral reef BCMs as a functional grouping that broadly encompasses taxonomically and functionally rich, conspicuous benthic consortia primarily comprised of prokaryotic members of the phyla Cyanobacteria (dominating the construction and maintenance of mat structural complexity), Proteobacteria, and Bacteroidetes (Cissell & McCoy, [7]; Stuij et al., [20]). Keywords: bacteriophage; coral reef; Cyanobacteria; microbial mat; molecular ecology; species interactions; top-down; trophic control EN bacteriophage coral reef Cyanobacteria microbial mat molecular ecology species interactions top-down trophic control 1 6 6 09/05/23 20230901 NES 230901 Benthic cyanobacterial mats (BCMs) are becoming increasingly dominant members of coral reef communities worldwide due to the influence of both local and global stressors (Cissell & McCoy, [8]; de Bakker et al., [11]; Ford et al., [13]). The proliferation of BCMs on coral reefs is both a product of and itself exacerbates reef degradation, with feedback among BCM cover and major drivers of reef decline such as shifting reef carbon chemistry and coral disease transmission (Brocke, Polerecky, et al., [3]; Cissell et al., [6]). Sampling during the collapse of an extensive BCM bloom event on a coral reef in Moreton Bay, Australia, revealed viruses as a potential driver of rapid bloom mortality (Hewson et al., [15]). [Extracted from the article]

Keywords: clam shrimps; Crustacea; fairy shrimps; predation; temporary waters EN clam shrimps Crustacea fairy shrimps predation temporary waters 1 5 5 12/03/21 20211201 NES 211201 It is conventionally understood that large branchiopod crustaceans are limited to temporary waters because they are sensitive to fish predation in permanent habitats (Kerfoot and Lynch 1987, Brendonck et al. 2008). We simultaneously subjected field-collected populations of the commonly occurring fairy shrimp Streptocephalus bidentatus and the clam shrimp Cyzicus sp. to predation by the co-occurring killifish Nothobranchius furzeri. Clam shrimps tend to occur later than fairy shrimps in the sequence of ecological succession (Jocque et al. 2007, Brendonck et al. 2017), and our results show that they are less susceptible to killifish predation, at least when more vulnerable fairy shrimp prey are present. To each of the tanks, 5 small killifish, 20 fairy shrimp, and 11 clam shrimps were added during the first trial and 4 large killifish, 15 fairy shrimp, and 10 clam shrimps in the second trial. [Extracted from the article]

In Southeast Asia, biodiversity‐rich forests are being extensively logged and converted to oil palm monocultures. Although the impacts of these changes on biodiversity are largely well documented, we know addition to samples we collected in 201 little about how these large‐scale impacts affect freshwater trophic ecology. We used stable isotope analyses (SIA) to determine the impacts of land‐use changes on the relative contribution of allochthonous and autochthonous basal resources in 19 stream food webs. We also applied compound‐specific SIA and bulk‐SIA to determine the trophic position of fish apex predators and meso‐predators (invertivores and omnivores). There was no difference in the contribution of autochthonous resources in either consumer group (70–82%) among streams with different land‐use type. There was no change in trophic position for meso‐predators, but trophic position decreased significantly for apex predators in oil palm plantation streams compared to forest streams. This change in maximum food chain length was due to turnover in identity of the apex predator among land‐use types. Disruption of aquatic trophic ecology, through reduction in food chain length and shift in basal resources, may cause significant changes in biodiversity as well as ecosystem functions and services. Understanding this change can help develop more focused priorities for mediating the negative impacts of human activities on freshwater ecosystems. [ABSTRACT FROM AUTHOR]

Abstract: Consumer‐driven nutrient recycling can have substantial effects on primary production and patterns of nutrient limitation in aquatic ecosystems by altering the rates as well as the relative supplies of the key nutrients nitrogen (N) and phosphorus (P). While variation in nutrient recycling stoichiometry has been well‐studied among species, the mechanisms that explain intraspecific variation in recycling N:P are not well‐understood. We examined the relative importance of potential drivers of variation in nutrient recycling by the fish Gambusia marshi among aquatic habitats in the Cuatro Ciénegas basin of Coahuila, Mexico. There, G. marshi inhabits warm thermal springs with high predation pressure as well as cooler, surface runoff‐fed systems with low predation pressure. We hypothesized that variation in food consumption among these habitats would drive intraspecific differences in excretion rates and N:P ratios. Stoichiometric models predicted that temperature alone should not cause substantial variation in excretion N:P, but that further reducing consumption rates should substantially increase excretion N:P. We performed temperature and diet ration manipulation experiments in the laboratory and found strong support for model predictions. We then tested these predictions in the field by measuring nutrient recycling rates and ratios as well as body stoichiometry of fish from nine sites that vary in temperature and predation pressure. Fish from warm, high‐predation sites excreted nutrients at a lower N:P ratio than fish from cool, low‐predation sites, consistent with the hypothesis that reduced consumption under reduced predation pressure had stronger consequences for P retention and excretion among populations than did variation in body stoichiometry. These results highlight the utility of stoichiometric models for predicting variation in consumer‐driven nutrient recycling within a phenotypically variable species. [ABSTRACT FROM AUTHOR]

Allochthonous resource movement across ecosystem boundaries creates episodic linkages between ecosystems. The sensitivity of the community to external resources of varying duration can alter the baseline upon which future pulses of allochthony can act. We explored the terrestrial ecosystem response to pulsed inputs of lake-derived resources with a manipulative experiment in a subarctic heathland where we assessed plant community and nutrient availability responses to additions of midge carcasses (Diptera: Chironomidae). Insect carcasses were added as either a one-time pulse or a 4-yr press to simulate differing durations of allochthony, which is common in the area. We found that midge pulses significantly elevated soil inorganic [N] in the first year (7× over background levels) but were significantly diminished (1.5×) by the second year after the initial pulse. The press treatment continued to elevate total soil inorganic [N] to 13× over background levels by the fourth year of midge additions, but then declined to 3.6× background in year 5 when experimental midge additions had ceased. In contrast to the soil inorganic N response, plant biomass was similar in pulse-addition and control plots over the course of the experiment. However, by the second year of the study plant biomass in press-addition plots were significantly higher than controls (>50%), and continued to increase over the 4 yr of the press treatment. Midge addition stimulated dominance of graminoids and thatch litter in plots that had previously been primarily heathland vegetation, a response that persisted 4 yr post-midge addition. Our findings suggest that soil and plant community responses to persistent insect carcass deposition (e.g., press) into heathland vegetation has the potential to carry forward in a way that modifies the baseline ecosystem conditions upon which additional allochthony may act. [ABSTRACT FROM AUTHOR]

Changes in the distribution of rainfall and the occurrence of extreme rain events will alter the size and persistence of aquatic ecosystems. Such alterations may affect the structure of local aquatic communities in terms of species composition, and by altering species interactions. In many aquatic ecosystems, leaf litter sustains detrital food webs and could regulate the responses of communities to changes in rainfall. Few empirical studies have focused on how rainfall changes will affect aquatic communities and none have evaluated if basal resource diversity can increase resistance to such rainfall effects. In this study, we used water-holding terrestrial bromeliads, a tropical aquatic ecosystem, to test how predicted rainfall changes and litter diversity may affect community composition and trophic interactions. We used structural equation modeling to investigate the combined effects of rainfall changes and litter diversity on trophic interactions. We demonstrated that changes in rainfall disrupted trophic relationships, even though there were only minor direct effects on species abundance, richness, and community composition. Litter diversity was not able to reduce the impact of changes in rainfall on trophic interactions. We suggest that changes in rainfall can alter the way in which species interact with each other, decreasing the linkages among trophic groups. Such reductions in biotic interactions under climate change will have critical consequences for the functioning of tropical aquatic ecosystems. [ABSTRACT FROM AUTHOR]

Species interactions can be important mediators of community and ecosystem responses to environmental stressors. However, we still lack a mechanistic understanding of the indirect ecological effects of stress that arise via altered species interactions. To understand how species interactions will be altered by environmental stressors, we need to know if the species that are vulnerable to such stressors also have large impacts on the ecosystem. As predators often exhibit certain traits that are linked to a high vulnerability to stress (e.g., large body size, long generation time), as well as having large effects on communities (e.g., top-down trophic effects), predators may be particularly likely to mediate ecological effects of environmental stress. Other functional groups, like facilitators, are known to have large impacts on communities, but their vulnerability to perturbations remains undocumented. Here, we use aquatic insect communities in bromeliads to examine the indirect effects of an important stressor (drought) on community and ecosystem responses. In a microcosm experiment, we manipulated predatory and facilitative taxa under a range of experimental droughts, and quantified effects on community structure and ecosystem function. Drought, by adversely affecting the top predator, had indirect cascading effects on the entire food web, altering community composition and decomposition. We identified the likely pathway of how drought cascaded through the food web from the top-down as drought→predator→shredder→decomposition. This stress-induced cascade depended on predators exhibiting both a strong vulnerability to drought and large impacts on prey (especially shredders), as well as shredders exhibiting high functional importance as decomposers. [ABSTRACT FROM AUTHOR]

In fluctuating environments, matching breeding timing to periods of high resource availability is crucial for the fitness of many vertebrate species, and may have major consequences on population health. Yet, our understanding of the proximate environmental cues driving seasonal breeding is limited. This is particularly the case in marine ecosystems, where key environmental factors and prey abundance and availability are seldom quantified. The Northern Humboldt Current System ( NHCS) is a highly productive, low-latitude ecosystem of moderate seasonality. In this ecosystem, three tropical seabird species (the Guanay Cormorant Phalacrocorax bougainvillii, the Peruvian Booby Sula variegata, and the Peruvian Pelican Pelecanus thagus) live in sympatry and prey almost exclusively on anchovy, Engraulis ringens. From January 2003 to December 2012, we monitored 31 breeding sites along the Peruvian coast to investigate the breeding cycle of these species. We tested for relationships between breeding timing, oceanographic conditions, and prey availability using occupancy models. We found that all three seabird species exhibited seasonal breeding patterns, with marked interspecific differences. Whereas breeding mainly started during the austral winter/early spring and ended in summer/early fall, this pattern was stronger in boobies and pelicans than in cormorants. Breeding onset mainly occurred when upwelling was intense but ecosystem productivity was below its annual maxima, and when anchovy were less available and in poor physiological condition. Conversely, the abundance and availability of anchovy improved during chick rearing and peaked around the time of fledging. These results suggest that breeding timing is adjusted so that fledging may occur under optimal environmental conditions, rather than being constrained by nutritional requirements during egg laying. Adjusting breeding time so that fledglings meet optimal conditions at independence is unique compared with other upwelling ecosystems and could be explained by the relatively high abundances of anchovy occurring throughout the year in the NHCS. [ABSTRACT FROM AUTHOR]

The species richness hypothesis, which predicts that species-rich communities should be better at resisting invasions than species-poor communities, has been empirically tested many times and is often poorly supported. In this study, we contrast the species richness hypothesis with four alternative hypotheses with the aim of finding better descriptors of invasion resistance. These alternative hypotheses state that resistance to invasions is determined by abiotic conditions, community saturation (i.e., the number of resident species relative to the maximum number of species that can be supported), presence/absence of key species, or weighted species richness. Weighted species richness is a weighted sum of the number of species, where each species' weight describes its contribution to resistance. We tested these hypotheses using data on the success of 571 introductions of four freshwater fish species into lakes throughout Sweden, i.e., Arctic char ( Salvelinus alpinus), tench ( Tinca tinca), zander ( Sander lucioperca), and whitefish ( Coregonus lavaretus). We found that weighted species richness best predicted invasion success. The weights describing the contribution of each resident species to community resistance varied considerably in both strength and sign. Positive resistance weights, which indicate that species repel invaders, were as common as negative resistance weights, which indicate facilitative interactions. This result can be contrasted with the implicit assumption of the original species richness hypothesis, that all resident species have negative effects on invader success. We argue that this assumption is unlikely to be true in natural communities, and thus that we expect that weighted species richness is a better predictor of invader success than the actual number of resident species. [ABSTRACT FROM AUTHOR]

Terrestrial ecosystems export large amounts of organic carbon (t-OC) but the net effect of this OC on the productivity of recipient aquatic ecosystems is largely unknown. In this study of boreal lakes, we show that the relative contribution of t-OC to individual top consumer (fish) biomass production, and to most of their potential prey organisms, increased with the concentration of dissolved organic carbon (DOC; dominated by t-OC sources) in water. However, the biomass and production of top consumers decreased with increasing concentration of DOC, despite their substantial use (up to 60%) of t-OC. Thus, the results suggest that although t-OC supports individual consumer growth in lakes to a large extent, t-OC input suppresses rather than subsidizes population biomass production. [ABSTRACT FROM AUTHOR]

Parks and protected areas have been instrumental in reducing anthropogenic sources of damage in terrestrial and aquatic environments. Pathogen invasion often succeeds physical wounding and injury, yet links between the reduction of damage and the moderation of disease have not been assessed. Here, we examine the utility of no-take marine reserves as tools for mitigating diseases that affect reef-building corals. We found that sites located within reserves had fourfold reductions in coral disease prevalence compared to non-reserve sites (80466 corals surveyed). Of 31 explanatory variables assessed, coral damage and the abundance of derelict fishing line best explained differences in disease assemblages between reserves and non-reserves. Unexpectedly, we recorded significantly higher levels of disease, coral damage, and derelict fishing line in non-reserves with fishing gear restrictions than in those without gear restrictions. Fishers targeting stocks perceived to be less depleted, coupled with enhanced site access from immediately adjacent boat moorings, may explain these unexpected patterns. Significant correlations between the distance from mooring sites and prevalence values for a ciliate disease known to infest wounded tissue (r =-0.65), coral damage (r =-0.64), and the abundance of derelict fishing line (r =-0.85) corroborate this interpretation. This is the first study to link disease with recreational use intensity in a park, emphasizing the need to evaluate the placement of closures and their direct relationship to ecosystem health. Since corals are modular, ecological processes that govern reproductive and competitive fitness are frequently related to colony surface area therefore, even low levels of cumulative tissue loss from progressing diseases pose significant threats to reef coral persistence. Disease mitigation through reductions in physical injury in areas where human activities are concentrated is another mechanism by which protected areas may improve ecosystem resilience in a changing climate. [ABSTRACT FROM AUTHOR]

Species and trophic richness often increase with habitat size. Although many ecological processes have been evoked to explain both patterns, the environmental stress associated with small habitats has rarely been considered. We propose that larger habitats may be species rich simply because their environmental conditions are within the fundamental niche of more species; larger habitats may also have more trophic levels if traits of predators render them vulnerable to environmental stress. We test this hypothesis using the aquatic insect larvae in water-filled bromeliads. In bromeliads, the probability of desiccation is greatest in small plants. For the 10 most common bromeliad insect taxa, we ask whether differences in drought tolerance and regional abundances between taxa predict community and trophic composition over a gradient of bromeliad size. First, we used bromeliad survey data to calculate the mean habitat size of occurrence of each taxon. Comparing the observed mean habitat size of occurrence to that expected from random species assembly based on differences in their regional abundances allowed us to obtain habitat size sensitivity indices (as Z scores) for the various insect taxa. Second, we obtained drought sensitivity indices by subjecting individual insects to drought and measuring the effects on relative growth rates in a mesocosm experiment. We found that drought sensitivity strongly predicts habitat size sensitivity in bromeliad insects. However, an increase in trophic richness with habitat size could not be explained by an increased sensitivity of predators to drought, but rather by sampling effects, as predators were rare compared to lower trophic levels. This finding suggests that physiological tolerance to environmental stress can be relevant in explaining the universal increase in species with habitat size. [ABSTRACT FROM AUTHOR]

Global change is altering the movement of materials across landscapes in ways that likely have major consequences for the functioning and stability of ecosystems. For example, the export of dissolved organic carbon (DOC) from terrestrial to aquatic ecosystems is increasing globally. This browning phenomenon is expected to alter the stability of recipient aquatic ecosystems, but theory provides contrasting predictions about the form and direction of this response. We created a gradient in terrestrial DOC supply by adding humic substances on weekly basis to 10 experimental ponds (106 L each) over a growing season. The manipulation of terrestrial DOC supply had strong effects on the chemical, physical, and biological properties of the pond ecosystems. Light attenuation linearly increased with terrestrial DOC supply, which created a shading effect that negatively influenced whole-pond gross primary production and respiration. Despite this, bacterial contributions to basal energy mobilization and respiration increased with terrestrial DOC supply indicating that aquatic food webs were subsidized by terrestrial inputs. After establishing the DOC gradient, we used dynamic linear models to test the subsidy-stability hypothesis by measuring the resilience and sensitivity of each pond to a pulse nutrient perturbation. We found that recovery from the perturbation decreased nonlinearly along a gradient in terrestrial DOC supply. Reciprocal transplant experiments indicated that owing primarily to its light attenuating properties and recalcitrant nature, terrestrial DOC diminished aquatic ecosystem stability by reducing nutrient turnover rates (NTR). Together, our results demonstrate that global-changemediated alterations in the movement of material and energy between habitats can have unpredictable and dramatic impacts on the reliability of ecosystem services. [ABSTRACT FROM AUTHOR]

Adjacent ecosystems are influenced by organisms that move across boundaries, such as insects with aquatic larval stages and terrestrial adult stages, which transport energy and nutrients from water to land. However, the ecosystem-level effect of aquatic insects on land has generally been ignored, perhaps because the organisms themselves are individually small. At the naturally productive Lake Mývatn, Iceland, we used two readily measured quantities: total insect emergence from water and relative insect density on land, to demonstrate an approach for estimating aquatic insect deposition (e.g., kg N.m-2.yr-1) to shore. Estimates from emergence traps between 2008 and 2011 indicated a range of 0.15-3.7 g.m-2.yr-1, or a whole-lake emergence of 3.1-76 Mg/yr; all masses are given as dry mass. Using aerial infall trap measurements of midge relative abundance over land, we developed a local-maximum decay function model to predict proportional midge deposition with distance from the lake. The dispersal model predicted midge abundance with R² = 0.89, a pattern consistent among years, with peak midge deposition occurring 20-25 m inland and 70% of midges deposited within 100 m of shore. During a high-midge year (2008), we estimate midge deposition within the first 50 m of shoreline to be 100 kg.ha-1.yr-1, corresponding to inputs of 10 kg N.ha-1.yr-1 and 1 kg P.ha-1.yr-1, or about three to five times above background terrestrial N deposition rates. Consistent with elevated N input where midges are most dense, we observed that soil available nitrate in resin bags decreases with increasing distance from the lake. Our approach, generalizable to other systems, shows that aquatic insects can be a major source of nutrients to terrestrial ecosystems and have the capacity to significantly affect ecosystem processes. [ABSTRACT FROM AUTHOR]

Climate gradient-focused ecological research can provide a foundation for better understanding critical ecological transition points and nonlinear climate-ecological relationships, which is information that can be used to better understand, predict, and manage ecological responses to climate change. In this study, we examined the influence of freshwater availability upon the coverage of foundation plant species in coastal wetlands along a northwestern Gulf of Mexico rainfall gradient. Our research addresses the following three questions: (1) What are the regional-scale relationships between measures of freshwater availability (e.g., rainfall, aridity, freshwater inflow, salinity) and the relative abundance of foundation plant species in tidal wetlands; (2) how vulnerable are foundation plant species in tidal wetlands to future changes in freshwater availability; and (3) what is the potential future relative abundance of tidal wetland foundation plant species under alternative climate change scenarios? We developed simple freshwater availability-based models to predict the relative abundance (i.e., coverage) of tidal wetland foundation plant species using climate data (1970-2000), estuarine freshwater inflow-focused data, and coastal wetland habitat data. Our results identify regional ecological thresholds and nonlinear relationships between measures of freshwater availability and the relative abundance of foundation plant species in tidal wetlands. In drier coastal zones, relatively small changes in rainfall could produce comparatively large landscape-scale changes in foundation plant species abundance that would affect some ecosystem good and services. Whereas a drier future would result in a decrease in the coverage of foundation plant species, a wetter future would result in an increase in foundation plant species coverage. In many ways, the freshwater-dependent coastal wetland ecological transitions we observed are analogous to those present in dryland terrestrial ecosystems. [ABSTRACT FROM AUTHOR]

Aquatic ecosystems play an important role in the global carbon (C) cycle and represent substantial source of greenhouse gases to the atmosphere. However, little attention has been paid to quantifying how aquatic community respiration (CR), a major source of CO2, will respond to warming temperatures expected under climate change, and whether landscape features affect temperature modulation of CR. We quantified how temperature sensitivity of CR varied among streams in southwestern Alaska, a region with one of the fastest warming trends globally. We incubated sediments from streams spanning a geomorphic gradient and estimated the degree to which CR responded to increased water temperature as described by Arrhenius kinetics. As expected, CR increased with temperature across all streams, and the average temperature sensitivity was similar to theoretical predictions for heterotrophic metabolism. However, temperature sensitivity was significantly higher in streams with higher quantity but lower quality organic C substrates, conditions that were strongly associated with watershed geomorphic characteristics. These results suggest that basic geomorphic features of landscapes will control the rates at which C is lost or sequestered from watersheds under new climate regimes. [ABSTRACT FROM AUTHOR]

Riparian vegetation research has traditionally focused on channel-related processes because riparian areas are situated on the edge of aquatic ecosystems and are therefore greatly affected by the flow regime of streams and rivers. However, due to their low topographic position in the landscape, riparian areas receive significant inputs of water and nutrients from uplands. These inputs may be important for riparian vegetation, but their role for riparian plant diversity is poorly known. We studied the relationship between the influx of groundwater (GW) from upland areas and riparian plant diversity and composition along a stream size gradient, ranging from small basins lacking permanent streams to a seventh-order river in northern Sweden. We selected riparian sites with and without GW discharge using a hydrological model describing GW flow accumulation to test the hypothesis that riparian sites with GW discharge harbor plant communities with higher species richness. We further investigated several environmental factors to detect habitat differences between sites differing in GW discharge conditions. Vascular plant species richness was between 15% and 20% higher, depending on the spatial scale sampled, at riparian sites with GW discharge in comparison to non-discharge sites, a pattern that was consistent across all stream sizes. The elevated species richness was best explained by higher soil pH and higher nitrogen availability (manifested as lower soil C/N ratio), conditions which were positively correlated with GW discharge. Base cations and possibly nitrogen transported by groundwater may therefore act as a terrestrial subsidy of riparian vegetation. The stable isotopes 15N and 13C were depleted in soils from GW discharge compared to non-discharge sites, suggesting that GW inputs might also affect nitrogen and carbon dynamics in riparian soils. Despite the fact that many flows of water and nutrients reaching streams are filtered through riparian zones, the importance of these flows for riparian vegetation has not been appreciated. Our results demonstrated strong relationships between GW discharge, plant species richness and environmental conditions across the entire stream size gradient, suggesting that both river hydrology and upland inputs should be considered to fully understand riparian vegetation dynamics. [ABSTRACT FROM AUTHOR]

Predation is known to have both direct and indirect effects on nutrient cycling in terrestrial and aquatic ecosystems, and the general stress paradigm (GSP) has been promoted as a theory for describing predator-mediated indirect effects on nutrient cycling. The GSP predicts that prey exposed to predators will produce glucocorticosteroids, which have a host of physiological effects including gluconeogenesis, increased respiration, excretion of N and P, and increases in body C:N. We tested the nutrient predictions of the GSP using anuran larvae, which exhibit morphological defenses in addition to behavioral defenses for which the GSP was conceived. Genetically similar Hyla versicolor tadpoles were placed in mesocosms either in the presence or absence of a fed predator (Dytiscus verticalis), and after two weeks, tadpoles exposed to predators exhibited strong induced defenses with large, tubular bodies, larger tails, and reduced activity. Tadpole body %C and N:P increased with no change in C:N, which is contrary to expectations from the GSP. Statistical models suggested that changes in body morphology (e.g., tail muscle width) rather than behavioral defenses (i.e., reduced activity) were most likely responsible for predator-mediated differences in body stoichiometry. This study suggests that strong morphological defenses may overwhelm or counteract the nutrient predictions of the GSP. [ABSTRACT FROM AUTHOR]

Many different drivers, including productivity, ecosystem size, and disturbance, have been considered to explain natural variation in the length of food chains. Much remains unknown about the role of these various drivers in determining food chain length, and particularly about the mechanisms by which they may operate in terrestrial ecosystems, which have quite different ecological constraints than aquatic environments, where most food chain length studies have been thus far conducted. In this study, we tested the relative importance of ecosystem size and productivity in influencing food chain length in a terrestrial setting. We determined that (1) there is no effect of ecosystem size or productive space on food chain length; (2) rather, food chain length increases strongly and linearly with productivity; and (3) the observed changes in food chain length are likely achieved through a combination of changes in predator size, predator behavior, and consumer diversity along gradients in productivity. These results lend new insight into the mechanisms by which productivity can drive changes in food chain length, point to potential for systematic differences in the drivers of food web structure between terrestrial and aquatic systems, and challenge us to consider how ecological context may control the drivers that shape food chain length. [ABSTRACT FROM AUTHOR]

With many ecosystems now supporting multiple nonnative species from different trophic levels, it can be challenging to disentangle the net effects of invaders within a community context. Here, we combined wetland surveys with a mesocosm experiment to examine the individual and combined effects of nonnative fish predators and nonnative bullfrogs on aquatic communities. Among 139 wetlands, nonnative fish (bass, sunfish, and mosquitofish) negatively influenced the probability of occupancy of Pacific treefrogs (Pseudacris regilla), but neither invader correlated strongly with occupancy by California newts (Taricha torosa), western toads (Anaxyrus boreas), or California red-legged frogs (Rana draytonii). In mesocosms, mosquitofish dramatically reduced the abundance of zooplankton and palatable amphibian larvae (P. regilla and T. torosa), leading to increases in nutrient concentrations and phytoplankton (through loss of zooplankton), and rapid growth of unpalatable toad larvae (through competitive release). Bullfrog larvae reduced the growth of native anurans but had no effect on survival. Despite strong effects on natives, invaders did not negatively influence one another, and their combined effects were additive. Our results highlight how the net effects of multiple nonnative species depend on the trophic level of each invader, the form and magnitude of invader interactions, and the traits of native community members. [ABSTRACT FROM AUTHOR]

Aquatic communities have been altered by invasive species, with impacts on native biodiversity and ecosystem function. At the same time, native biodiversity may mitigate the effects of an invader. Common carp (Cyprinus carpio) is a ubiquitous, invasive fish species that strongly influences community and ecosystem processes. We used common carp to test whether the potential effects of an invasive species are altered across a range of species diversity in native communities. In mesocosms, treatments of zero, one, three, and six native fish species were used to represent the nested subset patterns observed in fish communities of lakes in Illinois, USA. The effect of the invader was tested across fish richness treatments by adding common carp to the native community and substituting native biomass with common carp. Native species and intraspecific effects reduced invader growth. The invader reduced native fish growth; however, the negative effect was minimized with increasing native richness. The zooplankton grazer community was modified by a top-down effect from the invader that increased the amount of phytoplankton. Neither the invader nor richness treatments influenced total phosphorus or community metabolism. Overall, the invader reduced resources for native species, and the effect scaled with how the invader was incorporated into the community. Higher native diversity mitigated the impact of the invader, confirming the need to consider biodiversity when predicting the impacts of invasive species. [ABSTRACT FROM AUTHOR]

Until the last decade it was assumed that most marine species have pronounced gene flow over vast areas, largely because of their potential for dispersal during early life stages. However, recent genetic, modeling, and field studies have shown that marine populations may be structured at scales that are inconsistent with extensive dispersal of eggs and larvae. Such findings have stimulated the birth of new studies explaining the mechanisms that promote population structure and isolation in the oceans, in the face of high potential for dispersal. Here we study the vertical and horizontal distribution of cod (Gadus rnorhua) eggs in relation to small-scale circulation and water column hydrography in a coastal location of southern Norway. Previous studies conducted in this region have shown that cod populations inhabiting fjord locations, which are on average 30 km apart, are genetically differentiated, a remarkable outcome considering that Atlantic cod have pelagic egg stages and long pelagic larval duration. We document that cod eggs are found in greater abundance in shallow water layers, which on average are flowing up the fjord (away from the open ocean), and in the inner portion of the fjord, which is subject to lower current speeds compared to the outer or mouth of the fjord. Eggs were found to be neutrally buoyant at shallow depths, a trait that also favors local retention, given the local circulation. The same patterns held during two environmentally contrasting years. These results strongly suggest that population structure of Atlantic cod is favored and maintained by a balance between Water circulation and egg characteristics. [ABSTRACT FROM AUTHOR]

The global ocean and atmosphere are warming. There is increasing evidence suggesting that, in addition to other environmental factors, climate change is affecting species distributions and local population dynamics. Additionally, as a consequence of the growing levels of atmospheric carbon dioxide (CO2), the oceans are taking up increasing amounts of this CO2, causing ocean pH to decrease (ocean acidification). The relative impacts of ocean acidification on population dynamics have yet to be investigated, despite many studies indicating that there will be at least a sublethal impact on many marine organisms, particularly key calcifying organisms. Using empirical data, we forced a barnacle (Semibalanus balanoides) population model to investigate the relative influence of sea surface temperature (SST) and ocean acidification on a population nearing the southern limit of its geographic distribution. Hindcast models were compared to observational data from Cellar Beach (southwestern United Kingdom). Results indicate that a declining pH trend (-0.0017 unit/yr), indicative of ocean acidification over the past 50 years, does not cause an observable impact on the population abundance relative to changes caused by fluctuations in temperature. Below the critical temperature (here Tcrit = 13.1°C), pH has a more significant affect on population dynamics at this southern range edge. However, above this value, SST has the overriding influence. At lower SST, a decrease in pH (according to the National Bureau of Standards, pHNBS) from 82 to 7.8 can significantly decrease the population abundance. The lethal impacts of ocean acidification observed in experiments on early life stages reduce cumulative survival by ~25%, which again will significantly alter the population level at this southern limit. Furthermore, forecast predictions from this model suggest that combined acidification and warming cause this local population to die out 10 years earlier than would occur if there was only global warming and no concomitant decrease in pH. [ABSTRACT FROM AUTHOR]

Cascading trophic interactions mediated by consumers are complex phenomena, which encompass many direct and indirect effects. Nonetheless, most experiments and theory on the topic focus uniquely on the indirect, positive effects of predators on producers via regulation of herbivores. Empirical research in aquatic ecosystems, however, demonstrate that the indirect, positive effects of consumer-mediated recycling on primary producer stocks may be larger than the effects of herbivore regulation, particularly when predators have access to alternative prey. We derive an ecosystem model with both recipient- and donor-controlled trophic relationships to test the conditions of four hypotheses generated from recent empirical work on the role of consumer-mediated recycling in cascading trophic interactions. Our model predicts that predator regulation of herbivores will have larger, positive effects on producers than consumer-mediated recycling in most cases but that consumer-mediated recycling does generally have a positive effect on producer stocks. We demonstrate that herbivore recycling will have larger effects on producer biomass than predator recycling when turnover rates and recycling efficiencies are high and predators prefer local prey. In addition, predictions suggest that consumer-mediated recycling has the largest effects on primary producers when predators prefer allochthonous prey and predator attack rates are high. Finally, our model predicts that consumer-mediated recycling effects may not be largest when external nutrient loading is low. Our model predictions highlight predator and prey feeding relationships, turnover rates, and external nutrient loading rates as key determinants of the strength of cascading trophic interactions. We show that existing hypotheses from specific empirical systems do not occur under all conditions, which further exacerbates the need to consider a broad suite of mechanisms when investigating trophic cascades. [ABSTRACT FROM AUTHOR]

The light nutrient hypothesis posits that herbivore growth is increasingly constrained by low food quality as the ratio of light to nutrients increases in aquatic ecosystems. We tested predictions of this hypothesis by examining the effects of large seasonal cycles in light and nutrients on the mineral content of periphyton and the growth rate of a dominant herbivore (the snail Elimia clavaeformis) in two oligotrophic streams. Streambed irradiances in White Oak Creek and Walker Branch (eastern Tennessee, USA) varied dramatically on a seasonal basis due to leaf phenology in the surrounding deciduous forests and seasonal changes in sun angle. Concentrations of dissolved nutrients varied inversely with light, causing light: nitrate and light: phosphate to range almost 100-fold over the course of any individual year. Periphyton nitrogen and phosphorus concentrations were much lower than the concentrations of these elements in snails, and they bottomed out in early spring when streambed irradiances were highest. Snail growth, however, peaked in early spring when light: nutrient ratios were highest and periphyton nutrient concentrations were lowest. Growth was linearly related to primary production (accounting for up to 85% of growth variance in individual years), which in turn was driven by seasonal variation in light. Conceptual models of herbivore growth indicate that growth should initially increase as increasing light levels stimulate primary production, but then level off, and then decrease as the negative effects of decreasing algal nutrient content override the positive effects of increased food production. Our results showed no, evidence of an inflection point where increasing ratios of light to nutrients negatively affected growth. Snail growth in these intensively grazed streams is probably unaffected by periphyton nutrient content because exploitative competition for food reduces growth rates to levels where the demand for nitrogen and phosphorus is small enough to be satisfied by even low levels of these nutrients in periphyton. Competition for limited food resources in habitats where herbivore densities are uncontrolled by predation or other mortality factors should strongly influence the potential for herbivores to be limited by mineral deficits in their food. [ABSTRACT FROM AUTHOR]

The compositional heterogeneity of biotic assemblages among sites, or diversity, regulates the relationship between local and regional species diversity across scales. Recent work has suggested. that increased harshness of environmental conditions tends to reduce β-diversity by decreasing the importance of stochastic processes in structuring assemblages. We investigated the effect of nutrient enrichment on the compositional heterogeneity of lake benthic invertebrate assemblages in Ireland at both local (within-lake) and regional (among-lake) scales. At local scales, we found that the compositional heterogeneity of benthic assemblages was related inversely to the extent of nutrient enrichment (as indicated by measurements of water column total phosphorus, total' nitrogen, and chlorophyll a), after effects of lake morphology (i.e., surface area, connectivity, and depth of sampling) and alkalinity were accounted for. At regional scales, we found that nutrient-rich lakes had significantly more homogenous benthic assemblages than nutrient-poor lakes, over and above the effect of alkalinity and across a similar range of lake morphologies. These findings have profound implications for global aquatic biodiversity, as the homogenization of benthic assemblages at both local and regional scales may have important and unpredictable effects on whole aquatic ecosystems, with potentially considerable ecological and evolutionary consequences. [ABSTRACT FROM AUTHOR]

In aquatic ecosystems, where organisms typically feed and grow by eating smaller individuals, a characteristic size spectrum emerges, such that large organisms are much more rare than small ones. Here, a stochastic individual-based model for the dynamics of size spectra is described, based on birth, growth, and death of individuals, using simple assumptions about feeding behavior, It is shown that the deterministic limit derived from the stochastic process is a partial differential equation previously used to describe the dynamics of size spectra. The equation has two classes of dynamics in the long term. The first is a steady state. A derivation under simple mass-balance assumptions shows that, at steady state, the linear size spectrum relating log abundance to log mass has a slope of approximately -1, similar to that often observed in natural size spectra. The second class of dynamics, not previously described, is a traveling-wave solution in which waves move along the size spectrum from small to large body size. Traveling waves become more likely when predators prefer prey much smaller than themselves and when they are specialized in the range of prey body sizes consumed. Wavelength depends on the size of prey relative to the size of predator, and wave speed depends on how fast mass moves through the spectrum. [ABSTRACT FROM AUTHOR]

Lichens are reputedly slow growing and become unhealthy or die in response to supplements of the usual limiting resources, such as water and nitrogen. We found, however, that the tripartite cyanobacterial lichen Lobaria pulmonaria doubled in annual biomass growth after a single 20-minute immersion in a phosphorus solution (K2HPO4), as compared to controls receiving no supplemental phosphorus. This stimulation of cyanolichens by phosphorus has direct relevance to community and population ecology of lichens, including improving models of lichen performance, in relation to air quality, improving forest management practices affecting old-growth associated cyanolichens, and understanding the distribution and abundance of cyanolichens on the landscape. Phosphorus may be as important a stimulant to cyanobacterial-rich lichen communities as it is to cyanobacteria in aquatic ecosystems. [ABSTRACT FROM AUTHOR]

A goal of phylogeography is to relate patterns of genetic differentiation to potential historical geographic isolating events. Quaternary glaciations, particularly the one culminating in the Last Glacial Maximum -21 ka (thousands of years ago), greatly affected the distributions and population sizes of temperate marine species as their ranges retreated southward to escape ice sheets. Traditional genetic models of glacial refugia and routes of recolonization include these predictions: low genetic diversity in formerly glaciated areas, with a small number of alleles/haplotypes dominating disproportionately large areas, and high diversity including "private" alleles in glacial refugia. In the Northern Hemisphere, low diversity in the north and high diversity in the south are expected. This simple model does not account for the possibility of populations surviving in relatively small northern periglacial refugia. If these periglacial populations experienced extreme bottlenecks, they could have the low genetic diversity expected in recolonized areas with no refugia, but should have more endemic diversity (private alleles) than recently recolonized areas. This review examines evidence of putative glacial refugia for eight benthic marine taxa in the temperate North Atlantic. All data sets were reanalyzed to allow direct comparisons between geographic patterns of genetic diversity and distribution of particular clades and haplotypes including private alleles. We contend that for marine organisms the genetic signatures of northern periglacial and southern refugia can be distinguished from one another. There is evidence for several periglacial refugia in northern latitudes, giving credence to recent climatic reconstructions with less extensive glaciation. [ABSTRACT FROM AUTHOR]

An important goal in ecology is developing general theory on how the species composition of ecosystems is related to ecosystem properties and functions. Progress on this front is limited partly because of the need to identify mechanisms controlling functions that are common to a wide range of ecosystem types. We propose that one general mechanism, rooted in the evolutionary ecology of all species, is adaptive foraging behavior in response to predation risk. To support our claim, we present two kinds of empirical evidence from plant-based and detritus-based food chains of terrestrial and aquatic ecosystems. The first kind comes from experiments that explicitly trace how adaptive foraging influences ecosystem properties and functions. The second kind comes from a synthesis of studies that individually examine complementary components of particular ecosystems that together provide an integrated perspective on the link between adaptive foraging and ecosystem function. We show that the indirect effects of predators on plant diversity, plant productivity, nutrient cycling, trophic transfer efficiencies, and energy flux caused by consumer foraging shifts in response to risk are qualitatively different from effects caused by reductions in prey density due to direct predation. We argue that a perspective of ecosystem function that considers effects of consumer behavior in response to predation risk will broaden our capacity to explain the range of outcomes and contingencies in trophic control of ecosystems. This perspective also provides an operational way to integrate evolutionary and ecosystem ecology, which is an important challenge in ecology. [ABSTRACT FROM AUTHOR]

While there is great interest in the degree to which local interactions "scale-up" to predict regional patterns of abundance, few studies in marine systems have simultaneously examined patterns of abundance at both the large scale (tens of kilometers) typical of, larval movement and the small scale (meters) typical of post-settlement interactions. We addressed this gap by monitoring larval supply, adult survivorship, and giant kelp (Macrocystis pyrifera, a primary habitat-forming species) abundance for 13 populations of kelp bass (Paralabrax clathratus) spread over ∼200 km in the Santa Barbara Channel, California, USA. At the small, within-site scale, both recruitment and adult survivorship of kelp bass were density-dependent and positively related to kelp abundance. At the larger, among-site scale, the spatial pattern of adult kelp bass abundance was predicted well by the pattern of kelp bass larval supply, but there was a consistent negative spatial relationship between kelp abundance and kelp bass larval supply despite the positive effects of kelp on kelp bass at the smaller spatial scale. This large-scale negative relationship was likely a product of a channel-wide spatial mismatch between oceanographic conditions that favor kelp survival and those that concentrate and distribute fish larvae. These results generally support the recruit-adult hypothesis: kelp bass populations are limited by recruitment at low recruit densities but by density-dependent competition for food resources and/or predator refuges at high recruit densities. At the same time, spatial variation in kelp abundance produced substantial spatiotemporal heterogeneity in kelp bass demographics, which argues for a multispecies, metacommunity approach to predicting kelp bass dynamics. [ABSTRACT FROM AUTHOR]

The rate, timing, and quality of resource supply exert strong controls on a wide range of ecological processes. In particular, resource-mediated changes in microbial activity have the potential to alter ecosystem processes, including the production and respiration of organic matter. In this study, we used field experiments and simulation modeling to explore how aquatic heterotrophic bacteria respond to variation in resource quality (low vs. high) and resource schedule (pulse vs. press). Field experiments revealed that one-time pulse additions of resources in the form of dissolved organic carbon (DOC) caused short-lived (≤48 h) peaks in bacterial productivity (BP), which translated into large differences across treatments: cumulative BP was twice as high in the pulse vs. press treatment under low resource quality, and five times as high under high resource quality. To gain a more mechanistic understanding of microbial productivity in variable resource environments, we constructed a mathematical model to explore the attributes of bacterial physiology and DOC supply that might explain the patterns observed in our field experiments. Model results suggest that the mobilization rate of refractory to labile carbon, an index of resource quality, was critical in determining cumulative differences in BP between pulse and press resource environments (BPPu:Pr ratios). Moreover, BPPu:Pr ratios were substantially larger when our model allowed for realistic changes in bacterial growth efficiency as a function of bacterial carbon consumption. Together, our field and modeling results imply that resource schedule is important in determining the flow of material and energy from microbes to higher trophic levels in aquatic food webs, and that the effects of resource quality are conditional upon resource schedule. An improved understanding of the effects of resource variability on microorganisms is therefore critical for predicting potential changes in ecosystem functioning in response to environmental change, such as altered DOC fluxes from terrestrial to aquatic ecosystems. [ABSTRACT FROM AUTHOR]

Dormant propagule banks are important reservoirs of biological and genetic diversity of local communities and populations and provide buffering mechanisms against extinction. Although dormant stages of various plant and animal species are known to remain viable for decades and even centuries, little is known about the effective influence of recolonization from such old sources on the genetic continuity of intermittent populations under natural conditions. Using recent and old dormant eggs recovered from a dated lake sediment core in Kenya, we traced the genetic composition of a local population of the planktonic crustacean Daphnia barbata through a sequence of extinction and recolonization events. This was combined with a phylogeographic and population-genetic survey of regional populations. Four successive populations, fully separated in time, inhabited Lake Naivasha from ca. 1330 to 1570 AD, from Ca. 1610 to 1720 AD, from ca. 1840 to 1940 AD, and from 1995 to the present (2001 AD). Our results strongly indicate genetic continuity between the 1840–1940 and 1995–2001 populations, which are separated in time by at least 50 years, and close genetic relatedness of them both to the 1330–1580 population. A software tool (Colonize) was developed to find the most likely source population of the refounded 1995–2001 population and to test the number of colonists involved in the recolonization event. The results confirmed that the 1995–2001 population most probably developed out of a limited number of surviving local dormant eggs from the previous population, rather than out of individuals from regional (central and southern Kenya) or ipore distant (Ethiopia, Zimbabwe) populations that may have immigrated to Lake Naivasha through passive dispersal. These results emphasize the importance of prolonged dormancy for the natural long-term dynamics of crustacean zooplankton in fluctuating environments and suggest an important role of old local dormant egg banks in aquatic habitat restoration. [ABSTRACT FROM AUTHOR]

Community biomass is often less variable than the biomasses of populations within the community, yet attempts to implicate compensatory dynamics between populations as a cause of this relationship often fail. In part, this may be due to the lack of appropriate metrics for variability, but there is also great potential for large-scale processes such as seasonality or longer-term environmental change to obscure important dynamics at other temporal scales. In this study, we apply a scale-resolving method to long-term plankton data, to identify the specific temporal scales at which community-level variability is influenced by synchrony or compensatory dynamics at the population level. We show that variability at both the population and community level is influenced strongly by a few distinct temporal scales: in phytoplankton, ciliate, rotifer, and crustacean communities, synchronous dynamics are predominant at most temporal scales. However, in phytoplankton and crustacean communities, compensatory dynamics occur at a sub-annual scale (and at the annual scale in crustaceans) leading to substantial reductions in community-level variability. Aggregate measures of population and community variability do not detect compensatory dynamics in these communities; thus, resolving their scale dependence unmasks dynamics that are important for community stability in this system. The methods and results presented herein will ultimately lead to a better understanding of how stability is achieved in communities. [ABSTRACT FROM AUTHOR]

While the number of studies investigating the effects of species diversity on ecosystem properties continues to expand, few have explicitly examined how ecosystem functioning depends quantitatively on the degree of niche complementarity among species. We report the results of a microcosm experiment where similarity in habitat use among aquatic snail species was evaluated as a predictor of changes in community and ecosystem properties due to increasing species richness. Replicate microcosms with all possible one- and two-species combinations of a guild of six snail species were stocked with identical initial snail biomass. Microcosms with two species of snails had greater final snail biomass, lower attached algae biomass, and less total organic matter than monocultures. Snail species differed in their use of five distinct habitat types in the microcosms. Similarity in habitat use between a species pair was negatively related to the magnitude of change (e.g., ΔEF [change in ecosystem function]) in dissolved oxygen, periphyton biomass, and accrual of organic matter with a change in diversity. However, using the most stringent criterion for complementarity effects (e.g., Dmax [proportional deviation of the total polyculture yield from the highest yielding monoculture]), a relationship between species' niche similarity and changes in function with increasing species richness was only observed for dissolved oxygen. The identity of snail species present in the microcosms had strong effects on total organic matter, snail biomass, dissolved oxygen, periphyton biomass, and sedimentation rate. In this study, herbivore identity, sampling effects, and niche complementarity all appear to contribute to species richness effects on pond ecosystem properties and community structure. The analytical approach employed here may profitably be used in other systems to quantify the role of niche complementarity in species richness- ecosystem function relationships. [ABSTRACT FROM AUTHOR]

Species may compete indirectly by altering the traits of a shared resource. For example, herbivore-induced responses in plants may make plants more resistant or susceptible to additional herbivorous insect species. Herbivore-induced plant responses can significantly affect interspecific competition and herbivore population dynamics. These herbivore—herbivore indirect interactions have been overlooked in aquatic ecosystems where previous studies used the same herbivore species to induce changes and to assess the effects of these changes. We asked whether seaweed grazing by one of two herbivorous, congeneric snail species (Littorina obtusata or Littorina littorea) with different feeding strategies and preferences would affect subsequent feeding preferences of three herbivore species (both snails and the isopod Idotea baltica) and population densities of three herbivore species (both snails and a third periwinkle snail, Lacuna vincta). In addition, we measured phlorotannin concentrations to test the hypothesis that these metabolites function as induced defenses in the Phaeophyceae. Snail herbivory induced cue-specific responses in apical tissues of the seaweed Fucus vesiculosus that affected the three herbivore species similarly. When compared to ungrazed controls, direct grazing by Littorina obtusata reduced seaweed palatability by at least 52% for both snail species and the isopod species. In contrast, direct grazing by L. littorea did not decrease seaweed palatability for any herbivore, indicating herbivore-specific responses. Previous grazing by L. obtusata reduced populations of L. littorea on outplanted seaweeds by 46% but had no effect on L. obtusata populations. Phlorotannins, a potential class of inducible chemicals in brown algae, were not more concentrated in grazed seaweed tissues, suggesting that some other trait was responsible for the induced resistance. Our results indicate that marine herbivores may compete via inducible responses in shared seaweeds. These plant-mediated interactions were asymmetric with a specialist (L. obtusata) competitively superior to a generalist (L. littorea). [ABSTRACT FROM AUTHOR]

After House Finches were introduced from the western to the eastern United States and rapidly increased in numbers, House Sparrows declined, leading to suggestions that the decline was caused by interspecific competition. However, other potential causes were not excluded. The rapid decline in House Finches following the emergence of a new disease (mycoplasmal conjunctivitis) caused by a novel strain of Mycoplasma gallisepticum (MG) in 1994 has provided a natural experiment and an opportunity to revisit the hypothesis that interspecific competition from House Finches drives population changes in House Sparrows. If true, the recent decline in House Finches should lead to an increase in House Sparrows. In this paper we test the hypothesis that House Sparrow and House Finch numbers in the northeastern United States vary inversely by examining data from three independent volunteer programs that monitor bird species' abundance and distribution (Christmas Bird Count, Project FeederWatch, and Breeding Bird Survey). In the first analysis we found that House Sparrow and House Finch numbers varied inversely during a time interval when House Finches were increasing and a time interval when House Finches were decreasing. In the second analysis, we found that the rates of geometric change in House Sparrow abundance (In[HOSPt+1/HOSPt]) were negatively correlated with initial House Finch (HOFIt) and sparrow (HOSPt) abundances at individual sites, irrespective of the time period. Given that finch range expansion and subsequent declines in abundance are the result of two very different phenomena, it would be very unlikely for apparent competition or spurious correlations to cause the observed concomitant changes in House Sparrow abundance. We conclude that interspecific competition exists between these two species. [ABSTRACT FROM AUTHOR]

Although an ecosystem's response to biodiversity loss depends on the order in which species are lost, the extinction sequences generally used to explore such responses in food webs have been ecologically unrealistic. We investigate how several extinction orders affect the minimum number of secondary extinctions expected within pelagic food webs from 34 temperate freshwater lakes. An ecologically plausible extinction order is derived from the geographically nested pattern of species composition among the lakes and is corroborated by species' pH tolerances. Simulations suggest that lake communities are remarkably robust to this realistic extinction order and highly sensitive to the reverse sequence of species loss. This sensitivity is not well explained by the known sensitivity of networks to the loss of highly connected species but appears to be better explained by our observation that trophic specialists preferentially consume widely distributed species at low risk of extinction. Our results highlight an important aspect of community organization that may help to maintain biodiversity amidst changing environments. [ABSTRACT FROM AUTHOR]

We conducted a study to determine the contribution of lethal and nonlethal effects to a predator's net effect on a prey's population growth rate in a natural setting. We focused on the effects of an invasive invertebrate predator, Bythotrephes longimanus, on zooplankton prey populations in Lakes Michigan and Erie. Field data taken at multiple dates and locations in both systems indicated that the prey species Daphnia mendotae, Daphnia retrocurva, and Bosmina longirostris inhabited deeper portions of the water column as Bythotrephes biomass increased, possibly as an avoidance response to predation. This induced migration reduces predation risk but also can reduce birth rate due to exposure to cooler temperatures. We estimated the nonlethal (i.e., resulting from reduced birth rate) and lethal (i.e., consumptive) effects of Bythotrephes on D. mendotae and Bosmina longirostris. These estimates used diel field survey data of the vertical gradient of zooplankton prey density, Bythotrephes density, light intensity, and temperature with growth and predation rate models derived from laboratory studies. Results indicate that nonlethal effects played a substantial role in the net effect of Bythotrephes on several prey population growth rates in the field, with nonlethal effects on the same order of magnitude as or greater (up to 10-fold) than lethal effects. Our results further indicate that invasive species can have strong nonlethal, behaviorally based effects, despite short evolutionary coexistence with prey species. [ABSTRACT FROM AUTHOR]

The response of ecological communities to anthropogenic disturbance is of both scientific and practical interest. Communities where all species respond to disturbance in a similar fashion (synchrony) will exhibit large fluctuations in total biomass and dramatic changes in ecosystem function. Communities where some species increase in abundance while others decrease after disturbance (compensation) can maintain total biomass and ecosystem function in the face of anthropogenic change. We examined dynamics of the Little Rock Lake (Wisconsin, USA) zooplankton community in the context of an experimental pH manipulation conducted in one basin of the lake. A novel application of wavelets was used to partition patterns of synchrony and compensation by time scale. We find interestingly that some time series show both patterns of synchrony and compensation depending on the scale of analysis. Within the unmanipulated basin, we found subtle patterns of synchrony and compensation within the community, largely at a one-year time scale corresponding to seasonal variation. Within the acidified lake basin, dynamics shifted to longer time scales corresponding to the pattern of pH manipulation. Comparisons between pairs of species in different functional groups showed both strong compensatory and synchronous responses to disturbance. The strongest compensatory signal was observed for two species of Daphnia whose life history traits lead to synchrony at annual time scales, but whose differential sensitivity to acidification led to compensation at multiannual time scales. The separation of time scales inherent in the wavelet method greatly facilitated interpretation as patterns resulting from seasonal drivers could be separated from patterns driven by pH manipulation. [ABSTRACT FROM AUTHOR]

The significance of spatial subsidies depends on consumer resource interactions in the recipient habitat. Lakes are subsidized by terrestrial carbon sources, but the pathways of allochthonous carbon through lake food webs are complex and not well understood. Zooplankton vertically partition resources within stratified lakes in response to life history trade-offs that are governed by predators, the quantity and quality of food, and abiotic conditions (e.g., UV, temperature, and viscosity). We measured habitat specialization of zooplankton in an oligotrophic lake where allochthonous and autochthonous resources varied with depth. During stratification, the quantity and quality of zooplankton food was highest in the hypolimnion. We used a yearlong time series of the δ13C of zooplankton and particulate organic matter (POM) to determine which zooplankton species exploited, hypolimnetic rather than epilimnetic resources. Because the δ13C of POM decreased with depth, we used the δ13C of zooplankton to detect inter- and intraspecific variation in habitat selection. We incubated Daphnia pulex at discrete depths in the water column to confirm that the δ13C of zooplankton can indicate habitat specialization. Zooplankton that specialized in the epilimnion relied more on allochthonous carbon sources than those that specialized in the hypolimnion. Therefore, the fate of allochthonous carbon subsidies to lakes depends on spatially explicit consumer-resource interactions. [ABSTRACT FROM AUTHOR]

Some marine species have been shown to target foraging at particular hotspots of high prey abundance. However, we show here that in the year after a nesting season, female leatherback turtles (Dermochelys coriacea) in the Atlantic generally spend relatively little time in fixed hotspots, especially those with a surface signature revealed in satellite imagery, but rather tend to have a pattern of near continuous traveling. Associated with this traveling, distinct changes in dive behavior indicate that turtles constantly fine tune their foraging behavior and diel activity patterns in association with local conditions. Switches between nocturnal vs. diurnal activity are rare in the animal kingdom but may be essential for survival on a diet of gelatinous zooplankton where patches of high prey availability are rare. These results indicate that in their first year after nesting, leatherback turtles do not fit the general model of migration where responses to resources are suppressed during transit. However, their behavior may be different in their sabbatical years away from nesting beaches. Our results highlight the importance of whole-ocean fishing gear regulations to minimize turtle bycatch. [ABSTRACT FROM AUTHOR]

Fungi and bacteria are key agents in plant litter decomposition in freshwater ecosystems. However, the specific roles of these two groups and their interactions during the decomposition process are unclear. We compared the growth and patterns of degradative enzymes expressed by communities of bacteria and fungi grown separately and in coexistence on Phragmites leaves. The two groups displayed both synergistic and antagonistic interactions. Bacteria grew better together with fungi than alone. In addition, there was a negative effect of bacteria on fungi, which appeared to be caused by suppression of fungal growth and biomass - accrual rather than specifically affecting enzyme activity. Fungi growing alone had a high capacity for the decomposition of plant polymers such as lignin, cellulose, and hemicellulose. In contrast, enzyme activities were in general low when bacteria grew alone, and the activity of key enzymes in the degradation of lignin and cellulose (phenol oxidase and cellobiohydrolase) was undetectable in the bacteria-only treatment. Still, biomass-specific activities of most enzymes were higher in bacteria than in fungi. The low total activity and growth of bacteria in the absence of fungi in spite of apparent high enzymatic efficiency during the degradation of many substrates suggest that fungi provide the bacteria with resources that the bacteria were not able to acquire on their own, most probably intermediate decomposition products released by fungi that could be used by bacteria. [ABSTRACT FROM AUTHOR]

Species richness patterns are remarkably similar across many marine taxa, yet explanations of how such patterns are generated and maintained are conflicting. I use published occurrence data to identify previously masked latitudinal and longitudinal diversity gradients for all genera of benthic marine macroalgae and for species in the Order Bryopsidales. I also quantify the size, location, and overlap of macroalgal geographic ranges to determine how the observed richness patterns are generated. Algal genera exhibit an inverse latitudinal gradient, with biodiversity hotspots in temperate regions, while bryopsidalean species reach peak diversity in the tropics. The geographic distribution of range locations results in distinct clusters of range mid-points. In particular, widespread taxa are centered within tight latitudinal and longitudinal bands in the middle of the Indo-Pacific and Atlantic Oceans while small-ranged taxa are clustered in peripheral locations, suggesting that variation in speciation and extinction are important drivers of algal diversity patterns. Hypotheses about factors that regulate diversity contain underlying assumptions about the size and location of geographic ranges, in addition to predictions as to why species numbers will differ among regions. Yet these assumptions are rarely considered in assessing the validity of the prevailing hypotheses. I assess a suite of hypotheses, suggested to explain patterns of marine diversity, by comparing algal-richness patterns in combination with the size and location of algal geographic ranges, to the richness and range locations predicted by these hypotheses. In particular, the results implicate habitat areas and ocean currents as the most plausible drivers of observed diversity patterns. [ABSTRACT FROM AUTHOR]

The consequences of declining biodiversity remain controversial, in part because many studies focus on a single metric of ecosystem functioning and fail to consider diversity's integrated effects on multiple ecosystem functions. We used tide pool microcosms as a model system to show that different conclusions about the potential effects of producer diversity on ecosystem functioning may result when ecosystem functions are measured separately vs. together. Specifically, we found that in diverse seaweed assemblages, uptake of either nitrate or ammonium alone was equal to the average of the component monocultures: However, when nitrate and ammonium were available simultaneously, uptake by diverse assemblages was 22% greater than the monoculture average because different species were complementary in their use of different nitrogen forms. Our results suggest that when individual species have dominant effects on particular ecosystem processes (i.e., the sampling effect), multivariate complementarity can arise if different species dominate different processes. Further, these results suggest that similar mechanisms (complementary nutrient uptake) may underlie diversity-functioning relationships in both algal and vascular-plant- based systems. [ABSTRACT FROM AUTHOR]

Foundation species in space-limited systems can increase diversity by creating habitat, but they may also reduce diversity by excluding primary space competitors. These contrasting forces of increasing associate diversity and suppressing competitor diversity have rarely been examined experimentally with respect to disturbance. In a benthic marine community in central California, where native oysters are a foundation species, we tested how disturbance influenced overall species richness, evenness, and diversity. Surprisingly, overall diversity did not peak across a disturbance gradient because, as disturbance decreased, decreases in overall species evenness opposed increases in overall species richness. Decreasing disturbance intensity (high oyster abundance) led to increasing species richness of sessile and mobile species combined. This increase was due to the facilitation of secondary sessile and mobile species in the presence of oysters. In contrast, decreasing disturbance intensity and high oyster abundance decreased the evenness of sessile and mobile species. Three factors likely contributed to this decreased evenness: oysters reduced abundances of primary sessile species due to space competition; oysters supported more rare mobile species; and oysters disproportionately increased the relative abundance of a few common mobile species. Our results highlight the need for further studies on how disturbance can differentially affect the evenness and richness of different functional groups, and ultimately how these differences affect the relationship between overall diversity and ecosystem function. [ABSTRACT FROM AUTHOR]

Some Bosmina water flea species develop morphological antipredatory defenses, such as long antennules and a high carapace, but in Bosmina (Eubosmina) coregoni gibbera these traits are larger and more variable in females than in males. Here we propose that this sexual dimorphism derives from differential costs of hydrodynamic drag and selection for mobility in males. We tested this hypothesis by estimating drag of several Bosmina morphologies by using scale models sinking in glycerin of different concentrations and viscosities. Body forms included males, sexual and asexual females of B. c. gibbera, and males and asexual females of Bosmina (Eubosmina) Ion gispina, a taxon with less variable body shape. For a given body length or body volume, male models had lower drag than models of sexual and asexual females, suggesting that males can swim 14-28% faster with the same energy consumption. Consistent with this conclusion, video recordings showed that males of B. c. gibbera advanced 55-73% farther than females in each swimming stroke. We conclude that hydrodynamic drag may have significant implications for swimming and evolution of sexual dimorphism in water fleas, and we suggest that males lack the defensive structures of females of B. c. gibbera (e.g., high carapaces) because com- petition over mates favors low drag. [ABSTRACT FROM AUTHOR]

Organisms in aquatic ecosystems must often tolerate variable environmental conditions, including an uncertain risk of predation. Individuals that can maintain plastic defenses against predation will increase their survival when predators are present, but will not incur the costs of these defenses when the risk of predation is low and the defense is not induced. Larvae of the pond-breeding anuran Hyla chrysoscelis develop a conspicuous phenotype in the presence of predators consisting of a brightly colored tail and a deeper tail fin. In this study, I attempted to identify the source of the chemical signal that induces this defensive morphology in this species. I tested whether metabolites alone, originating from the prey but passing through the predator, were able to induce the same morphological response as the combination of alarm signals released directly by attacked conspecifics, and metabolites. I used morphometric and tail conspicuousness data to assess tadpole response to the perceived risk of predation by larval odonate predators (Anax junius). I also tested whether this inducing cue could be recognized across species by measuring the morphological response of H. chrysoscelis tadpoles exposed to cues emitted when tadpoles of a closely related genus (Pseudacris crucifer) were consumed. Tadpoles exhibited a clean graded response of both overall shape and tail morphology in response to all cues, cor- responding to their relative reliability as indicators of a risk of predation. H. chrysoscelis tadpoles were also able to respond to cues emitted when tadpoles of a closely related genus were consumed by predators. These results illustrate that tadpoles of this species are able to respond to metabolites alone without alarm signals, and that interspecific chemical communication is a primary mechanism for predator avoidance in this inducible defense system. [ABSTRACT FROM AUTHOR]

The traditional emphasis in ecology on local species interactions has recently broadened to include landscape-level processes. I used data from a seven-year survey of ponds in northern Switzerland to judge the relative contributions of local and landscape influences to the occurrence and density of larvae of eight amphibian species. Each pond was sampled twice each year using pipe samples and dip nets. Analyses evaluated support for a series of alternative models, some suggested by current ideas in aquatic ecology and others designed to reveal the impact of key variables. Results strongly supported models that include both local and landscape covariates. Species differed in preference for permanent or temporary ponds, high- or low-predator ponds, and landscapes within forested or open areas. The occurrence of all species was positively correlated with the densities of other species, suggesting that competition was less important than variation in quality among sites. The results emphasize the importance of local processes in governing the status of populations, because even landscape-level effects were probably caused by availability of habitat during the terrestrial stage, rather than by metapopulation processes. [ABSTRACT FROM AUTHOR]