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Metacommunity theory suggests that relationships between diversity and ecosystem stability can be determined by the rate of species dispersal among local communities. The predicted relationships, however, may depend upon the relative strength of local environmental processes and disturbance. Here we evaluate the role of dispersal frequency and local predation perturbations in affecting patterns of diversity and stability in pond plankton metacommunities. Pond metacommunities were composed of three mesocosm communities: one of the three communities maintained constant 'press' predation from a selective predator, bluegill sunfish (Lepomis macrochirus); the second community maintained 'press' conditions without predation; and the third community experienced recurrent 'pulsed' predation from bluegill sunfish. The triads of pond communities were connected at either no, low (0.7%/d), or high (20%/d) planktonic dispersal. Richness and composition of zooplankton and stability of plankton biomass and ecosystem productivity were measured at local and regional spatial scales. Dispersal significantly affected diversity such that local and regional biotas at the low dispersal rate maintained the greatest number of species. The unimodal local dispersal diversity relationship was predator-dependent, however, as selective press predation excluded species regardless of dispersal. Further, there was no effect of dispersal on beta diversity because predation generated local conditions that selected for distinct community assemblages. Spatial and temporal ecosystem stability responded to dispersal frequency but not predation. Low dispersal destabilized the spatial stability of producer biomass but stabilized temporal ecosystem productivity. The results indicate that selective predation can prevent species augmentation from mass effects but has no apparent influence on stability. Dispersal rates, in contrast, can have significant effects on both species diversity and ecosystem stability at multiple spatial scales in metacommunities. Key words: dispersal rate; diversity--stability; food web; Kellogg Biological Station, Hickory Corners, Michigan, USA; Lepomis macrochirus; mass effects; metacommunity; plankton; predation; spatial stability; species sorting; temporal stability.

The temporal stability of aggregate community and ecosystem properties is influenced by the variability of component populations, the interactions among populations, and the influence of environmental fluctuations on populations. Environmental fluctuations that enhance population variability are generally expected to destabilize community and ecosystem properties, but this will depend on the degree to which populations are synchronized in their dynamics. Here we use seminatural experimental ponds to show that reduced synchrony among zooplankton taxa increases the temporal stability of zooplankton density, abundance, and ecosystem productivity in fluctuating environments. However, asynchrony only occurs at long timescales (~80-day periods) and under recurring environmental perturbations. At shorter timescales (~10-day periods) and in constant environments, synchronous dynamics dominate. Our findings support recent theory indicating that compensatory dynamics can stabilize communities and ecosystems. They further indicate that environmental fluctuations can enhance the likelihood of long-period asynchrony and thus stabilize community and ecosystem properties despite their short term destabilizing effects. Key words: compensatory dynamics; ecosystem processes; food web dynamics; mesocosm; phytoplankton; stability; synchrony; wavelet analysis; zooplankton.

We examine co-occurrence patterns of microorganisms to evaluate community assembly 'rules.' We use methods previously applied to macroorganisms, both to evaluate their applicability to microorganisms and to allow comparison of co-occurrence patterns observed in microorganisms to those found in macroorganisms. We use a null model analysis of 124 incidence matrices from microbial communities, including bacteria, archaea, fungi, and algae, and we compare these results to previously published findings from a meta-analysis of almost 100 macroorganism data sets. We show that assemblages of microorganisms demonstrate nonrandom patterns of co-occurrence that are broadly similar to those found in assemblages of macroorganisms. These results suggest that some taxon co-occurrence patterns may be general characteristics of communities of organisms from all domains of life. We also find that co-occurrence in microbial communities does not vary among taxonomic groups or habitat types. However, we find that the degree of co-occurrence does vary among studies that use different methods to survey microbial communities. Finally, we discuss the potential effects of the undersampling of microbial communities on our results, as well as processes that may contribute to nonrandom patterns of co-occurrence in both macrobial and microbial communities such as competition, habitat filtering, historical effects, and neutral processes. Key words: assembly rules; community composition; co-occurrence; microbial diversity; microbial ecology.

The stoichiometric light:nutrient hypothesis (LNH) links the relative supplies of key resources with the nutrient content of tissues of producers. This resource-driven variation in producer stoichiometry, in turn, can mediate the efficiency of grazing. Typically, discussions of the LNH attribute this resource--stoichiometry link to bottom-up effects of light and phosphorus, which are mediated through producer physiology. Emphasis on bottom-up effects implies that grazers must consume food of quality solely determined by resource supply to ecosystems (i.e., they eat what they are served). Here, we expand upon this largely bottom-up interpretation with evidence from pond surveys, a mesocosm experiment, and a model. Data from shallow ponds showed the 'LNH pattern' (positive correlation of an index of light:phosphorus supply with algal carbon:phosphorus content). However, algal carbon:phosphorus content also declined as zooplankton biomass increased in the ponds. The experiment and model confirmed that this latter correlation was partially caused by the various bottom-up and top-down roles of grazers: the LNH pattern emerged only in treatments with crustacean grazers, not those without them. Furthermore, model and experiment clarified that another bottom-up factor, natural covariation of nitrogen:phosphorus ratios with light:phosphorus supply (as seen in ponds), does not likely contribute to the LNH pattern. Finally, the experiment produced correlations between shifts in species composition of algae, partially driven by grazing effects of crustaceans, and algal stoichiometry. These shifts in species composition might shape stoichiometric response of producer assemblages to resource supply and grazing, but their consequences remain largely unexplored. Thus, this study accentuated the importance of grazing for the LNH; deemphasized a potentially confounding, bottom-up factor (covarying nitrogen:phosphorus supply); and highlighted an avenue for future research for the LNH (grazer-mediated shifts in producer composition). Key words: algal species composition; ecological stoichiometry; grazing; light : nutrient hypothesis; nitrogen : phosphorus supply; ponds.

The neutral theory for community structure and biodiversity is dependent on the assumption that species are equivalent to each other in all important ecological respects. We explore what this concept of equivalence means in ecological communities, how such species may arise evolutionarily, and how the possibility of ecological equivalents relates to previous ideas about niche differentiation. We also show that the co-occurrence of ecologically similar or equivalent species is not incompatible with niche theory as has been supposed, because niche relations can sometimes favor coexistence of similar species. We argue that both evolutionary and ecological processes operate to promote the introduction and to sustain the persistence of ecologically similar and in many cases nearly equivalent species embedded in highly structured food webs. Future work should focus on synthesizing niche and neutral perspectives rather than dichotomously debating whether neutral or niche models provide better explanations for community structure and biodiversity. Key words: coexistence; ecological similarity; equivalence; neutral theory; niche differentiation; speciation.

A current principle of ecological stoichiometry states that the nitrogen to phosphorus ratio (N:P) of primary producers should closely match that from environmental nutrient supplies. This hypothesis was tested using data from ponds in Michigan, USA, a freshwater mesocosm experiment, a synthesis of studies from diverse systems (cultures, lakes, streams, and marine and terrestrial environments), and simple dynamic models of producer growth and nutrient content. Unlike prior laboratory studies, the N:P stoichiometry of phytoplankton in Michigan ponds clustered around and below the Redfield ratio (7.2:1 by mass), despite wide variation in N:P supply ratios (2:1-63:1 by mass) and the presence of grazers. In a mesocosm experiment, the N:P stoichiometry of phytoplankton cells again deviated from a nearly 1:1 relationship with N:P supply. Phytoplankton seston exhibited lower N:P content than expected at high N:P supply ratios, and often higher N:P content than anticipated at low N:P supply ratios, regardless of herbivore presence. Similar deviations consistently occur in the N:P stoichiometry of algae and plants in the other diverse systems. The models predicted that both high loss rates (sinking, grazing) and physiological limits to nutrient storage capacity could attenuate producer stoichiometry. In the future, research should evaluate how limits to elemental plasticity of producers can influence the role of stoichiometry in structuring communities and ecosystem processes. Key words: aquatic ecosystems; food webs; grazers; nitrogen:phosphorus ratio; N:P; primary producers; quota saturation; Redfield ratio; stoichiometry.

Species-area relationships have been observed for virtually all major groups of macroorganisms that have been studied to date but have not been explored for microscopic phytoplankton algae, which are the dominant producers in many freshwater and marine ecosystems. Our analyses of data from 142 different natural ponds, lakes, and oceans and 239 experimental ecosystems reveal a strong species-area relationship with an exponent that is invariant across ecosystems that span > 15 orders of magnitude in spatial extent. A striking result is that the species-area relationship derived from small-scale experimental studies correctly scales up to natural aquatic ecosystems. These results significantly broaden our knowledge of the effects of island size on biodiversity and also confirm the relevance of experimentally derived data to the analysis and understanding of larger-scale ecological patterns. In addition, they confirm that patterns in microbial diversity are strongly consistent with those that have been repeatedly reported in the literature for macroorganisms. biodiversity | island biogeography | species-area | scale-invariance

Mechanisms that explain shifts in species composition over environmental gradients continue to intrigue ecologists. Ecological stoichiometry has recently provided a new potential mechanism linking resource (light and nutrient) supply gradients to grazer performance via elemental food-quality mechanisms. More specifically, it predicts that light and nutrient gradients should determine the relative dominance of P-rich taxa, such as Daphnia, in grazer assemblages. We tested this hypothesis in pond mesocosms (cattle tanks) by creating gradients of resource supply and predation risk, to which we added diverse assemblages of algal producer and zooplankton grazer species. We then characterized the end-point composition of grazer assemblages and quantity and elemental food quality of edible algae. We found that somatically P-rich Daphnia only dominated grazer assemblages in high-nutrient, no-predator treatments. In these ecosystems, P sequestered in producers exceeded a critical concentration. However, other grazers having even higher body P content did not respond similarly. These grazers were often abundant in low-nutrient environments with poorer food quality. At face value, this result is problematic for ecological stoichiometry because body composition did not correctly predict response of these other species. However, two potential explanations could maintain consistency with stoichiometric principles: species could differentially use a high-P resource (bacteria), or body composition might not always directly correlate with nutrient demands of grazers. Although our data cannot differentiate between these explanations, both suggest potential avenues for future empirical and theoretical study. Keywords: Daphnia; grazers; light; nutrients; phosphorus; predation; predation-risk gradients; resource-supply gradients; species composition, explaining shifts in; stoichiometty; zooplankton.

One of the more intriguing and challenging developments in ecology and in limnology and oceanography is the expansion of the temporal and spatial scales that are being addressed by current work. Researchers are realizing that individual communities and ecosystems are not isolated from each other but rather that they are connected by exchanges of individuals (through dispersal) and materials (through spatial fluxes). From a conceptual perspective, there is a need to develop theories and hypotheses about the roles of these exchanges on different community and ecosystem attributes. Here we focus on the concept of metacommunities, to address how plankton communities are structured and how this may generate patterns of variation in planktonic ecosystems. Our premise is that planktonic systems can be understood in the framework of metacommunities and that the regulation of these metacommunities alter how they work as 'complex adaptive systems.' We hypothesize that connectivity through dispersal of local communities that are embedded in aquatic metacommunities show a range of dynamic behaviors related to their capacity to respond adaptively to environmental change.

Empirical studies show that the relationship between species diversity and productivity can be strongly scale dependent. At the local scale of population dynamics the relationship is commonly unimodal or humped, but it is frequently monotonically increasing at larger spatial scales. Such contrasting patterns can occur if beta diversity (compositional dissimilarity among sites) also increases with productivity, but mechanistic reasons for this remain unresolved. Here we use theoretical models to explore the influence of ecological assembly on the diversity of multitrophic communities along gradients of productivity. We show that assembly dynamics can generate scale-dependent patterns consistent with those observed in nature; while unimodal productivity-diversity patterns are produced at the scale of local communities, positive monotonic patterns emerge when diversity is measured at larger spatial scales (among communities). This occurs due to increases in beta diversity with productivity. Our results suggest that increases in beta diversity, and resultant scale dependency, can depend on three vital and interactive elements: the presence of more than one consumer trophic level, a greater propensity for cyclic compositional change at high productivities, and the stochastic nature of species dispersal and invasion history. Key words: assembly; beta diversity; heteroclinic cycles; invasibility; metacommunity; permanence; productivity-diversity relationships, scale-dependent; scale; species diversity; species turnover.

A three-part model is presented to identify dominant pattern characteristics within a given species population, consisting of coherence, species turnover, and boundary clumping. Coherence depicts the degree of collapsibility of a pattern into a single dimension; species turnover depicts species replacements within this dimension; and boundary clumping depicts the distribution of species boundary edges along this dimension.

Author(s): Amy L. Downing (corresponding author) [1]; Mathew A. Leibold Resolving current concerns about the role of biodiversity on ecosystems calls for understanding the separate roles of changes in species [...]

Author(s): Jonathan M. Chase (corresponding author) [1]; Mathew A. Leibold [2] The diversity of life is heterogeneously distributed across the Earth. A primary cause for this pattern is the heterogeneity [...]

Assemblages of coexisting species are formed by immigration from a regional pool of colonists and local interactions among species and with the physical environment. Theory suggests that the shape of the relationship between regional and local species richness may indicate the relative roles of dispersal and local interactions in limiting local diversity. Here we examine patterns of regional and local species richness in freshwater crustacean zooplankton to test whether linear (suggesting dispersal limitation) or curvilinear (suggesting saturation, via strong local control) functions best fit the data. Local richness appeared saturated when regions of different spatial extents were included on the same graph. However, this pattern was influenced by differences in scale among surveys. We corrected for the effects of regional scale by plotting mean local richness against the residuals of the species-landscape area relations. Controlling for the extent of the regional scale produced much more linear patterns, suggesting strong dispersal limitation. We present a simple graphical model to explain how variation among surveys in the geographic size of regions can produce apparent saturation of local diversity even if the underlying pattern of local and regional richness is linear. We also compare the predictive power of residual regional richness on local richness with that of several local features in a multiple regression model. Local richness exhibits strong relationships with both residual regional richness and pH. We argue that the relative strengths of local and regional processes depend on the definition of the regional scale. A variety of evidence suggests that local processes play a major role in generating differences in zooplankton diversity among lakes within a biogeographic region. Evidence for the importance of dispersal limitation comes largely from comparisons of lakes across very large scales. Our analysis suggests that linear patterns of local and regional diversity are not incompatible with strong local interactions. Keywords: cladocerans; copepods; dispersal limitation; diversity; saturation; scale-dependence; species richness.

In order to evaluate the patterns and processes of food web structure in grasslands, we compiled data from published studies on the relationship between precipitation (which is a predictor of primary productivity in grasslands), plant and herbivore standing crop biomass, and the results of large-herbivore exclosure experiments on plant abundance and composition. Three informative patterns emerged: (1) both producer and herbivore biomass increase across a natural precipitation gradient; (2) the relative effect of large herbivores on plant biomass, inferred from exclosure studies, decreases with increasing precipitation; and (3) the effect of herbivores on changes in plant species composition increases with precipitation. Simple resource-controlled and consumer-controlled food chain models can explain different subsets of these patterns. However, models of heterogeneous food webs that incorporate differences among species within trophic levels, and compositional turnover within such trophic levels, are consistent with all of the reviewed patterns. We suggest that this compositional turnover of plant species, combined with the interactive controlling effects of consumers and resources, may help to explain why studies performed in different areas, and with different methodologies, often draw different conclusions about the patterns and structuring processes in grassland ecosystems. Key words: compositional turnover; consumer control; food web; grasslands; herbivore exclosure; herbivory; heterogeneous food web models; plant defense; primary productivity; resource control.

Short-term experiments yield fewer changes in the composition of species than those observed in natural communities. This is due primarily to the different temporal scales governing experiments and naturally occurring phenomena. Experiments also lack the impacts of resource productivity and predators as found in natural environments, creating further research disparity.

The niche concept is a central organizing aspect of modern ecology. Although its history has often been reviewed, the structure of the concept and its connection to advances in ecological theory has received less recent attention. I review the niche concept using 'mechanistic' models of community theory to identify two distinct components. One describes the environmental requirements of organisms and the other describes the per capita impact of organisms on the environment. I argue that these correspond to significant differences between Grinnell's and Elton's concepts distinct from the previously discussed 'habitat' vs. 'functional' dichotomy. I illustrate the distinction between the requirement and impact components of the niche using models of resource competition and of keystone predators, and I discuss 'Gause's axiom' and conventional 'niche theory' in the context of these two distinct niche components. I suggest that the niche concept be elucidated by explicit reference to these two distinct components; the 'impact' niche (corresponding to Elton's concept) describing instantaneous per-capita effects of species on the environment, and the 'requirement' niche describing the response of species to the environment (corresponding to Hutchinson's definition). This approach connects conventional niche theory with 'mechanistic' individual-based ecological models and can help provide a more modern context for the niche concept. Key words: competition; niche; predation; species impact; species requirement; species role; zero net growth isoclines., Introduction Although most textbooks emphasize the use of the niche concept in community ecology (e.g., Ricklefs 1979, Begon et al. 1990, Pianka 1994), the concept (or a near synonym) is [...]

Ecology is a leading discipline in the synthesis of diverse knowledge. Ecologists have had considerable experience in bringing together diverse, multinational data sets, disciplines, and cultural perspectives to address a […]

Populations of organisms within an ecosystem react to changes in the food chain or web. In experimental mesocosms constructed to resemble natural ponds, populations of Rana utricularia tadpoles and Daphnia laevis responded to increasing amounts of plant biomass being available for food following the introduction of nutrients. Thus biomass, the food-web and the number of herbivores were all shown to act on each other as the productivity of their environment varied.

ALAN J. TESSIER [1,3] MATHEW A. LEIBOLD [2] JEAN TSAO [2] Abstract. The genus Daphnia (Crustacea: Branchipoda) includes a diversity of species that are generalist grazers on plankton in freshwater [...]

Recent efforts at modeling population dynamics have rapidly outstripped empirical tests of these theories. Costantino and Desharnais claim that we need more such empirical work and present the flour beetle [...]