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Unprecedented rates of species extinctions have prompted extensive research into the consequences of biodiversity losses on ecosystem functioning. While aquatic species are most threatened, research with freshwater and marine model systems has lagged behind progress made in terrestrial environments. This editorial to a special feature summarizes the main outcomes of a conference aimed at setting the stage for exploring the potential of aquatic systems to assess the role of biodiversity in ecosystem functioning. This series of papers proposes fresh approaches to the study of biodiversity effects on ecosystem functioning, outlines a new way of analyzing experimental data, presents a model that considers scale as an important factor determining outcomes, explores the effects of multiple stressors on species richness and ecosystem processes, and develops a food-web perspective that relates ecosystem properties to biodiversity. An insightful synthesis of lessons learned from aquatic systems is premature at present, but the papers clearly demonstrate the role that marine and freshwater systems can play in resolving open questions. The implications go well beyond the biodiversity in, and functioning of, ecosystems shaped by free-flowing or standing water. [ABSTRACT FROM AUTHOR]

Research on ecological communities, and plant–pollinator mutualistic networks in particular, has increasingly benefited from the theory and tools of complexity science. Nevertheless, up to now there have been few attempts to investigate the interplay between the structure of real pollination networks and their dynamics. This study is one of the first contributions to explore this issue. Biological invasions, of major concern for conservation, are also poorly understood from the perspective of complex ecological networks. In this paper we assess the role that established alien species play within a host community by analyzing the temporal changes in structural network properties driven by the removal of non-native plants. Three topological measures have been used to represent the most relevant structural properties for the stability of ecological networks: degree distribution, nestedness, and modularity. Therefore, we investigate for a detailed pollination network, 1) how its dynamics, represented as changes in species abundances, affect the evolution of its structure, 2) how topology relates to dynamics focusing on long-term species persistence; and 3) how both structure and dynamics are affected by the removal of alien plant species. Network dynamics were simulated by means of a stochastic metacommunity model. Our results showed that established alien plants are important for the persistence of the pollination network and for the maintenance of its structure. Removal of alien plants decreased the likelihood of species persistence. On the other hand, both the full network and the subset native network tended to lose their structure through time. Nevertheless, the structure of the full network was better preserved than the structure of the network without alien plants. Temporal topological shifts were evident in terms of degree distribution, nestedness, and modularity. However the effects of removing alien plants were more pronounced for degree distribution and modularity of the network. Therefore, elimination of alien plants affected the evolution of the architecture of the interaction web, which was closely related to the higher species loss found in the network where alien plants were removed. [ABSTRACT FROM AUTHOR]

In this paper, we tested whether the spatial distribution of a given species in more or less fragmented and disturbed landscapes depends on the species habitat specialization. We studied 891 spatial replicates from the French Breeding Bird Survey (FBBS) monitored at least two years during 2001–2005, and two independent landscape databases measuring respectively landscape fragmentation and recent landscape disturbance on each FBBS replicate. We used a continuous habitat specialization index for the 105 most common bird species monitored by the FBBS. We further modelled the spatial variation in abundance of each species according to fragmentation and disturbance across FBBS replicates, accounting for habitat differences and spatial trends. We then tested whether more or less specialized species responded to landscape fragmentation and disturbance. We found that the more specialist a species, the more negative its spatial response to landscape fragmentation and disturbance. Although there was a very high variation around these tendencies indicating that there are many other drivers of species distribution, our results suggest that measuring specialization may be helpful in predicting which species are likely to thrive in human degraded landscapes. We also emphasize the need to consider both positive and negative species responses when assessing consequences of habitat change in communities. [ABSTRACT FROM AUTHOR]

Species distributions often show an aggregated pattern, which can be due to a number of endo- and exogenous factors. While autologistic models have been used for modelling such data with statistical rigour, little emphasis has been put on disentangling potential causes of aggregation. In this paper we ask whether it is possible to infer sources of aggregation in species distributions from a single set of occurrence data by comparing the performance of various autologistic models. We create simulated data sets, which show similar occupancy patterns, but differ in the process that causes the aggregation. We model the distribution of these data with various autologistic models, and show how the relative performance of the models is sensitive to the factor causing aggregation in the data. This information can be used when modelling real species data, where causes of aggregation are typically unknown. To illustrate, we use our approach to assess the potential causes of aggregation in data of seven bird species with contrasting statistical patterns. Our findings have important implications for conservation, as understanding the mechanisms that drive population fluctuations in space and time is critical for the development of effective management actions for long-term conservation. [ABSTRACT FROM AUTHOR]

The species–time relationship (STR) describes how the species richness of a community increases with the time span over which the community is observed. This pattern has numerous implications for both theory and conservation in much the same way as the species–area relationship (SAR). However, the STR has received much less attention and to date only a handful of papers have been published on the pattern. Here we gather together 984 community time-series, representing 15 study areas and nine taxonomic groups, and evaluate their STRs in order to assess the generality of the STR, its consistency across ecosystems and taxonomic groups, its functional form, and its relationship to local species richness. In general, STRs were surprisingly similar across major taxonomic groups and ecosystem types. STRs tended to be well fit by both power and logarithmic functions, and power function exponents typically ranged between 0.2 and 0.4. Communities with high richness tended to have lower STR exponents, suggesting that factors increasing richness may simultaneously decrease turnover in ecological systems. Our results suggest that the STR is as fundamental an ecological pattern as the SAR, and raise questions about the general processes underlying this pattern. They also highlight the dynamic nature of most species assemblages, and the need to incorporate time scale in both basic and applied research on species richness patterns. [ABSTRACT FROM AUTHOR]

In this paper we investigate ecological flow networks via graph theory in search of the real sequential chains through which energy passes from producers to consumers in complex food webs. We obtain such fundamental pathways by identifying strongly connected components (SCCs), subsystems that groups species that take part in cycling, and performing topological sorting on the acyclic graphs that are obtained. Topological sorting identifies preferential directions for energy to flow from sources to sinks, while recycling remains confined within each SCC. Resolving food web networks for SCC highlights the possibility that compartments can be found in ecosystems, but this does not seem a general rule. The four aquatic food webs described in detail show a rather clear subdivision between benthic and pelagic subcommunities, a result that is discussed in the light of other studies. Should further research confirm these results, new insight into the way ecosystems use energy will be provided, with implications on cycling, reciprocal dependency of variables and indirect effects. [ABSTRACT FROM AUTHOR]

Human activities have differentially altered biogeochemical cycling at local, regional and global scales. We propose that a stoichiometric approach, examining the fluxes of multiple elements and the ratio between them, may be a useful tool for better understanding human effects on ecosystem processes and services. The different scale of impacts of the elements carbon, nitrogen and phosphorus and the different nature of their biogeochemical cycles, imply a large variation of their stoichiometric ratios in space and time and thus divergent impacts on biota.In this paper, we examine the effects of anthropogenic perturbations on nutrient ratios in ecosystems in two examples and one case study. Altered stoichiometry in agricultural systems (example 1) can affect not only crop yield and quality but also the interactions between plants and their pollinators, pests and pathogens. Human activities have also altered stoichiometry in coastal ecosystems (example 2). Increased N loading has especially lead to increased N:P and reduced Si:N ratios, with detrimental effects on ecosystem services derived from coastal pelagic food webs, such as fish yield and water quality. The terrestrial–aquatic linkage in stoichiometric alterations is illustrated with a case study, the Mississippi River watershed, where anthropogenic activities have caused stoichiometric changes that have propagated through the watershed into the northern Gulf of Mexico.Coupled with altered stoichiometric nutrient inputs are the inherent differences in variation and sensitivity of different ecosystems to anthropogenic disturbance. Furthermore, the connections among the components of a watershed may result in downstream cascades of disrupted functioning. Applying a multiple element perspective to understanding and addressing societal needs is a new direction for both ecological stoichiometry and sustainability. [ABSTRACT FROM AUTHOR]

Studies of microbial communities from aquatic ecosystems provide important insights into relations between various aspects of ecosystem functioning and changes in biodiversity. Aquatic microbial systems provide a valuable counterpoint to studies of terrestrial systems, because patterns reflect consequences of interactions occurring over many generations of community development, and are unlikely to represent artifacts of the initial conditions established in experimental communities. In this paper we re-analyse our previously published data to separate the contributions of temporal and spatial variation to overall variation in ecosystem functioning. A new analysis based on re-sampling confirms a negative relationship between richness and the variability of one ecosystem process, carbon dioxide flux. The negative relationship reflects high variation among communities of low species richness, rather than high temporal variation within communities of low richness. We also review the various transformations and summary statistics proposed as alternate measures of variability in ecosystem functioning, to point out that different measures are often appropriate for different kinds of data. Finally, we conclude that arguments about the cosmopolitan distribution of microbes do not preclude the existence of important relations between microbial species richness and ecosystem functioning. [ABSTRACT FROM AUTHOR]

The relationship between biodiversity and ecosystem functioning has emerged as a major scientific issue today. As experiments progress, there is a growing need for adequate theories and models to provide robust interpretations and generalisations of experimental results, and to formulate new hypotheses. This paper provides an overview of recent theoretical advances that have been made on the two major questions in this area: (1) How does biodiversity affect the magnitude of ecosystem processes (short-term effects of biodiversity)? (2) How does biodiversity contribute to the stability and maintenance of ecosystem processes in the face of perturbations (long-term effects of biodiversity)? Positive short-term effects of species diversity on ecosystem processes, such as primary productivity and nutrient retention, have been explained by two major types of mechanisms: (1) functional niche complementarity (the complementarity effect), and (2) selection of extreme trait values (the selection effect). In both cases, biodiversity provides a range of phenotypic trait variation. In the complementarity effect, trait variation then forms the basis for a permanent association of species that enhances collective performance. In the selection effect, trait variation comes into play only as an initial condition, and a selective process then promotes dominance by species with extreme trait values. Major differences between within-site effects of biodiversity and across-site productivity–diversity patterns have also been clarified. The local effects of diversity on ecosystem processes are expected to be masked by the effects of varying environmental parameters in across-site comparisons. A major reappraisal of the paradigm that has dominated during the last decades seems necessary if we are to account for long-term effects of biodiversity on ecosystem functioning. The classical deterministic, equilibrium approaches to stability do not explain the reduced temporal variability of aggregate ecosystem properties that has been observed in more diverse systems. On the other hand, stochastic, nonequilibrium approaches do show two types of biodiversity effects on ecosystem productivity in a fluctuating environment: (1) a buffering effect, i.e., a reduction in the temporal variance; and (2) a performance-enhancing effect, i.e., an increase in the temporal mean. The basic mechanisms involved in these long-term insurance effects are very similar to those that operate in short-term biodiversity effects: temporal niche complementarity, and selection of extreme trait values. The ability of species diversity to provide an insurance against environmental fluctuations and a reservoir of variation allowing adaptation to changing conditions may be critical in a long-term perspective. These recent theoretical developments in the area of biodiversity and ecosystem functioning suggest that linking community and ecosystem ecology is a fruitful avenue, which paves the way for a new ecological synthesis. [ABSTRACT FROM AUTHOR]

This paper reports current research concerning Oribatid populations occurring in three ecosystems (an oak forest, a meadow and a poplar plantation). The main criterion used in the faunistic analysis is the relative density; this is expressed as a percentage and conventionally represented by ten classes (Fig. 1). The specific components of each habitat are compared at first wholly (Fig. 2) and then on the more rational basis or the Oribatid mites group peculiar to each milieu or in common with all the habitats (Tab. 1). The study of the relative density sums indicates the interspecifics quantitative connections, but also the similarities and the divergences between the three habitats (Tab. 2, Fig. 3). In the same way, the relationships between the three communities are established on the basis of the species list; those results show not only the connecting species but also the discriminating species (Fig. 4). The conclusions obtained are validated by considering various concepts such as the diversity, the constraint index and the specificity. [ABSTRACT FROM AUTHOR]

Over evolutionary time, the number of species in a community reflects the balance between the rate of speciation and the rate of extinction. Over shorter time-scales local species richness is also affected by how often species move into and out of the local community. These processes are at the heart of Hubbell's ‘unified neutral theory of biodiversity’ ( Hubbell 2001 ). Hubbell's spatially implicit, dispersal-limited neutral model is the most widely used of the many implementations of neutral theory and it provides an estimate of the rate of speciation in a metacommunity (if metacommunity size is known) and the rate at which species migrate into the local community from the wider metacommunity. Recently, this neutral model has been used to compare rates of speciation and migration in the species-rich fynbos of South Africa and in neotropical forests. Here we use new analytical methods for estimating the neutral model's parameters to infer speciation and dispersal rates for three sites in species-rich sclerophyll shrublands (equivalent to fynbos) in Western Australia (WA). Our estimates suggest that WA shrublands are intermediate between fynbos and tropical rainforest in terms of speciation and dispersal. Although a weak test, the model predicts species abundance distributions and species accumulation curves similar to those observed at the three sites. The neutral model's predictions also remain plausible when confronted with independent data describing: (1) known edaphic relationships between sites, (2) estimates of metacommunity species richness and (3) rates of speciation among resprouters and nonsprouters. Two of the site pairs, however, show species turnovers significantly different from those predicted by the spatially implicit form of the neutral model that we use. This suggests that non-neutral processes, in this case probably edaphic specialisation, are important in the WA shrubland metacommunity. The neutral model predicts similar rates of speciation in resprouter and sprouter taxa, a finding supported by recent molecular phylogenies. Finally, when converted into temporally scaled speciation rates and species longevities, the estimates produced by the neutral model seem implausible. The apparent departure from neutrality in the turnover of species between some sites and the implausible temporal dynamics may be due to the particular model chosen and does not reduce the significance of our other results, which confirm that local dispersal limitation, coupled with broader scale edaphic fidelity, combine to structure this biodiverse metacommunity. [ABSTRACT FROM AUTHOR]

The loss of a predator from an ecological community can cause large changes in community structure and ecosystem processes, or have very little consequence for the remaining species and ecosystem. Understanding when and why the loss of a predator causes large changes in community structure and ecosystem processes is critical for understanding the functional consequences of biodiversity loss. We used experimental microbial communities to investigate how the removal of a large generalist predator affected the extinction frequency, population abundance and total biomass of its prey. We removed this predator in the presence or absence of an alternative, more specialist, predator in order to determine whether the specialist predator affected the outcome of the initial species removal. Removal of the large generalist predator altered some species’ populations but many were unaffected and no secondary extinctions were observed. The specialist predator, though rare, altered the response of the prey community to the removal of the large generalist predator. In the absence of the specialist predator, the effects of the removal were only measurable at the level of individual species. However, when the specialist predator was present, the removal of the large generalist predator affected the total biomass of prey species. The results demonstrate that the effect of species loss from high trophic levels may be very context-dependent, as rare species can have disproportionately large effects in food webs. [ABSTRACT FROM AUTHOR]

We used an individual-based spatially-explicit model to assess the role of facilitation and plant strategies in shaping the ‘community biomass–species richness’ relationship. Facilitation had few impacts on community's richness under both the most benign (high community biomass) and the most severe (low community biomass) environments where its intensity was weak. From medium to high environmental severity, facilitation increased community richness, because all plant strategies were facilitated. In contrast, from low to medium environmental severity facilitation decreased community richness, because only the most competitive species were facilitated, which induced a decrease in the richness of the stress-tolerant species overwhelming the increase in richness of the competitive species. Above all, our simulations show how ‘strategy-dependent’ interactions among species combine to shape the humped-back biomass–species richness relationship. It also demonstrates that facilitative effects might have long-term negative effects on species richness, which result is not included in current facilitation models. [ABSTRACT FROM AUTHOR]

Ecological coupling by material exchanges or dispersal between spatially distinct communities has important impacts on ecological processes, such as diversity–stability relationships, ecosystem function, and food web dynamics. One important mode of coupling between ecosystems occurs via organisms with complex life histories, which often switch between distinct ecosystems during their life cycle, and so can be channels of material exchanges between these ecosystems. Some organisms with complex life histories (e.g. frogs, dragonflies) can be abundant and effective predators during one or more life stages, and so provide conduits for strong direct and indirect interactions across ecosystem boundaries, linking the dynamics of discrete and often quite dissimilar community types. We present simple models and a case study (tailored to pond ecosystems), to explore how interactions within larval habitats can indirectly impact ecological interactions in adult habitats. Using our case study as a springboard, we propose that cohorts of predators emerging from natal habitats (e.g. ponds) cast ‘predation shadows’ on the surrounding adult (e.g. terrestrial) landscape. Trophic interactions within ponds, and the distribution of ponds on the landscape, can thus affect the spatial pattern in the strength of these predation shadows, creating strong spatial patterning in terrestrial trophic cascades. Our findings emphasize the importance of organisms with complex life histories as generators of strong links across ecosystem boundaries, and as potential sources of spatial variation in the strength and indirect impacts of interspecific interactions. [ABSTRACT FROM AUTHOR]

The effect of biodiversity on ecosystem functioning has proven variable both within and among manipulative studies. Species richness is the most commonly used measure of biodiversity in such studies, but the range of species’ functional traits (functional diversity), not the number of species per se, likely underpins a key mechanistic link between species richness and ecosystem functioning. However, the majority of experiments that have examined the effect of functional diversity have manipulated functional group richness, an approach recognised to suffer numerous limitations. Continuous measures of functional diversity avoid many of these limitations, but the relationship between continuous functional diversity and the magnitude of ecosystem processes has been largely untested. Using one vs two-species mixtures of rock pool macroalgae as a model, we conducted a field experiment to determine the effect of a continuous measure of functional diversity (functional attribute diversity, FAD, the degree of functional differentiation based on four functional traits) on the magnitude of net primary productivity and overyielding, based upon two alternative null-models. The total magnitude of productivity was largely determined by the identity of species present, not FAD. However, FAD proved to be a good predictor of overyielding (variation in productivity after the dominant effects of species identity had been accounted for). Furthermore, despite differences in the mean magnitude of the effect of combining species, the positive relationship between FAD and overyielding was consistent according to both additive and substitutive null-models. Our findings imply that whilst knowledge of species’ independent contributions remains indispensable in the prediction of biotic effects on ecosystem functioning within a trophic level, continuous measures of functional diversity should be used as a supplementary tool to predict the magnitude of overyielding, thereby refining predictions. [ABSTRACT FROM AUTHOR]

Most studies investigating the relationship between plant diversity and ecosystem functioning lasted only a few years. These studies generally showed a positive relationship between diversity and productivity that strengthened with time. This pattern suggests the experimental communities have not yet reached maturity, which raises the question whether a positive relationship between diversity and productivity will remain if communities approach equilibrium conditions. Here, we analyze eight years of data from a plant diversity experiment without legumes. A positive relationship between plant diversity and productivity arose in the second year and increased until the fifth year. This relationship persisted throughout the remainder of the experiment, but the slope of the relationship remained constant at approximately 50 g/m2/log2[richness]. The positive relationship between diversity and productivity was caused by strong complementarity effects, which initially increased but remained constant after the fourth year of the experiment. Our results show a positive relationship between diversity and productivity is a long-term phenomenon, driven by complementary interactions. [ABSTRACT FROM AUTHOR]

Liebig's law of the minimum, which states that only one element limits the growth of organisms at any given time, is widely used in ecology. This principle is routinely applied to organisms, populations and communities, but can it really be applied indistinguishably across these different scales? Here we show, by prediction of a resource ratio conceptual model and with an experimental test carried out in microcosms with bacteria that, unlike single species, communities are likely to adjust their stoichiometry to that of their resources. This adjustment results from competitive exclusion and coexistence mechanisms, and is sensitive to the overall diversity of species in the community. It guaranties co-limitation, i.e. simultaneous limitation by multiple resources, at the community scale and optimal use of resources and maximization of community biomass for wide ranges of resource ratios. These results question the applicability of the Liebig's law of the minimum at the community level, and the relevance of ecosystem models relying on this principle. [ABSTRACT FROM AUTHOR]

Applying ideas of statistical mechanics in ecology have recently received quite some attention. The entropy maximization (EM) formalism looks particularly attractive, as it provides a simple algorithm to infer detailed system variables from a limited number of constraints. However, we point out that a blind application of this formalism can easily lead to wrong conclusions. To illustrate this, we reanalyze an ecological data set that has been used to claim the good performance of EM in predicting species abundances from trait measurements. We show that these results are entirely due to the restrictive constraints, and do not provide any support for the applicability of EM in ecology. By comparing with a simple example from physics, we indicate which characteristic mechanism of EM, and of statistical mechanics in general, is missing for the ecological example. This analysis introduces a series of methods to evaluate future attempts to apply EM in ecology. [ABSTRACT FROM AUTHOR]

Destabilization of one predator–one prey systems with an increase in nutrient input has been viewed as a paradox. We report that enrichment can damp population cycles by a food-web structure that balances inflexible and flexible interaction links (i.e. specialist and generalist predators). We modeled six predator–prey systems involving three or four species in which the predators practice optimal foraging based on prey profitability determined by handling time. In all models, the balance of interaction links simultaneously decreased the amplitude of population oscillations and increased the minimum density with increasing enrichment, leading to a potential theoretical resolution of the paradox of enrichment in non-equilibrium dynamics. The stabilization mechanism was common to all of the models. Important previous studies on the stability of food webs have also demonstrated that a balance of interaction strengths stabilizes systems, suggesting a general rule of ecosystem stability. [ABSTRACT FROM AUTHOR]

Natural disturbances are key processes in the vast majority of ecosystems and a range of ecological theories have been developed in an attempt to predict biotic responses to them. However, empirical support for these theories has been inconsistent and considerable additional work remains to be done to better understand the response of biodiversity to natural disturbance. We tested predictions from the intermediate disturbance hypothesis and the habitat accommodation model of succession for reptile responses to fire history and a single major fire event. We focused our work on a broad range of vegetation types spanning sedgeland to temperate rainforest located within a national park in south-eastern Australia. We found no significant relationships between reptile species richness and the number of fires over the past 35 years, the time since the last fire, or the severity of a major fire in 2003. Thus, we found no strong evidence to support the intermediate disturbance hypothesis. A correspondence analysis of reptile assemblages revealed a gradient in species responses to fire history. However, we found limited evidence for an ordered succession of reptiles. Nor could the responses of individual species be readily predicted from life history attributes. Thus, our findings were generally not consistent with predictions from the habitat accommodation model of succession. A possible explanation for the absence of a predictable sequence of recovery following disturbance might be the rapidity of post-fire recovery of many components of native vegetation cover that were found to be important for reptiles (e.g. the extent of grass cover). This would have limited the time for early successional conditions to prevail and limited opportunities for species associated with such conditions. We found that most reptile species responses were much more strongly linked to vegetation type than fire variables, emphasizing a need to understand relationships with vegetation before being able to understand possible fire effects (if and where they exist). We found the disturbance concepts we examined were limited in their ability to accurately predict reptile responses to past fire history or the impacts of a single major fire in 2003. Practical management might be best guided not by disturbance theory, but by carefully setting objectives to meet conservation goals for particular individual species of reptiles. [ABSTRACT FROM AUTHOR]

Diversity in one group of species or genotypes is often correlated with diversity in a second group – prominent examples including native vs exotic species, and genetic diversity in a focal species vs species diversity in the rest of the community. I used simulation models to investigate the roles of competition and facilitation among species or genotypes in creating diversity–diversity relationships, with a focus on facilitation, which has received little theoretical attention. When competitive interactions dominate, increasing diversity in one group reduces diversity in the second group via filling of available niche space. Facilitation can create positive diversity–diversity relationships via a sampling effect, whereby a strong facilitator of the second group is more likely to be present as diversity increases in the first group, and also via one group acting as a source of biotic heterogeneity (i.e. diversifying selection) on the second group. However, the biotic heterogeneity effect is expected only under restricted conditions – with asymmetric facilitation, only during a transient period, or only over a small range of species diversity levels – and therefore seems unlikely to operate within trophic levels in natural communities. More generally, the simultaneous operation of competition and facilitation results in several different diversity–diversity relationships and underlying mechanisms. The results clarify the potential roles of positive and negative interactions in creating diversity–diversity relationships, and in determining the outcome of community dynamics in general. This study also highlights some important difficulties in incorporating facilitation into ecological theory for communities with many species. [ABSTRACT FROM AUTHOR]

Chalk streams are among the most species-rich and productive of all temperate ecosystems. Despite this, a few keystone species have the potential to exert disproportionately powerful effects on community structure and ecosystem processes. Two of these are the bullhead Cottus gobio, a small benthic fish that is an extremely abundant, voracious predator, and the freshwater shrimp Gammarus pulex, which dominates the prey assemblage and is the principal detritivore. Field experiments detected a bullhead– Gammarus–detritus trophic cascade, with detrital processing rates slowed dramatically in the presence of the predator. In addition, survey data also revealed strong negative density-dependence between bullhead and brown trout, adding a further link in the cascade. However, although bullhead also depressed the abundance of a dominant grazer, the snail Potamopyrgus antipodarum, there was no cascading effect upon algal production, suggesting that autochthonous inputs were not controlled by top–down effects. This skewed effect of the predator upon autochthonous versus allochthonous basal resources stresses the need to consider both pathways of energy flux into the food web, whereas many previous studies have potentially overemphasized the importance of predator–herbivore–primary producer cascades. The wider community food web contained 142 species and 1383 feeding links. This complex network exhibited “small world” properties, such as high clustering (unlike many other food webs) and shortest path lengths between species were small (in common with many other food webs). In particular, each of the four members of the detrital cascade could be connected to any other species by three links or fewer. Our data revealed that powerful cascading effects can be imbedded within even very complex ecological networks. [ABSTRACT FROM AUTHOR]

Migration is a widespread phenomenon in many ecosystems. Most often, studies on migration have focused on how migration strategies are dependent on ecological parameters, but little attention has been paid to the top-down effect of migration on ecosystem processes. Cyprinid fish in many European lakes undergo partial migration, where a part of the population leaves the lake and enters streams for the winter. In this study, we model the effect of partial migration by fish on lower trophic levels in a lake ecosystem. Our results suggest that spring phyto- and zooplankton dynamics, including occurrences of clear-water phases, can be related to the timing and magnitude of partial migration of planktivorous fish. From our results we conclude that partial migration can influence the dynamics of lower trophic levels in the ecosystem. Furthermore, we hypothesize that partial migration may affect the stability of alternative stable states and transitions between them. [ABSTRACT FROM AUTHOR]

The lack of predictability in litter-mix studies may result from the low correlation between species number and the traits that drive the processes under observation. From the standpoint of litter-quality-dependent ecological processes, we propose that litter chemical qualities are functional traits and introduce a multivariate index of chemical diversity (CDQ) based on Rao's quadratic entropy to describe the compositional heterogeneity of litter and foliar mixtures. Using published data from temperate and tropical forest systems to illustrate the relationship between species richness and chemical diversity, we show the variation of chemical diversity based on profiles of total nutrient concentrations (N, P, K, Ca and Mg) with species richness. We discuss how this behavior may explain the idiosyncratic responses exhibited in litter-mix experiments and how it may contribute to the observed dominance of species identity over species diversity. As a summary of resource heterogeneity relevant to detritivore and microbial processes, the chemical diversity index is potentially a better predictor of diversity effects on nutrient dynamics than species richness. Finally, we propose the use of infrared spectroscopy techniques for a rapid and more comprehensive determination of foliar and litter chemical composition to provide a more information-rich index. [ABSTRACT FROM AUTHOR]

Pollinators may generate selective pressures that affect covariation patterns of multiple traits as well as the mean values of single floral morphological traits. Berg predicted that flowers pollinated by animals whose morphology closely matches the flower's shape will be phenotypically more integrated (tighter correlation of flower traits) than will flowers pollinated by animals not closely fitting the floral morphology. We tested this hypothesis by comparing, in the Strait of Gibraltar region (south Spain, northern Morocco), populations of Narcissus papyraceus that have geographical differences in pollinator faunas. Long-tongued, nectar-feeding moths dominate the pollinator faunas of those populations close to the Strait of Gibraltar, whereas short-tongued, pollen-feeding syrphid flies dominate in peripheral populations farther from the Strait. Populations pollinated by moths and flies differed in the mean values of several floral traits, consistent with the evolution of regional pollination ecotypes. Populations pollinated by moths showed stronger intercorrelation (floral integration) than populations pollinated by hoverflies. Moth-pollinated populations also showed less variation in flower traits than vegetative traits, and this difference was stronger than in fly-pollinated populations. Thus, the pattern of differences in the phenotypic architecture of the Narcissus flowers is consistent with the hypothesis that populations have responded to different selective pressures generated by different pollinators. These data also supported most of the specific predictions of Berg's hypotheses about integration and modularity. [ABSTRACT FROM AUTHOR]

In this study, we investigate community structure in day geckos (genus Phelsuma) in the Indian Ocean. Much of what we know about communities of diurnal arboreal lizards comes from studies of Caribbean Anolis. Phelsuma in the Indian Ocean are ecologically similar to Anolis but not closely related. Using field observations and an experiment, we test three hypotheses for Phelsuma communities, all derived from work on Anolis: (1) Phelsuma species richness will be correlated with the diversity of available perches, (2) sympatric species will partition their habitat use, shifting their habitat use depending on which other species of Phelsuma are present, and (3) experimentally removing individuals of one species will lead to changes in the microhabitat use and/or abundance of sympatric congeners. We find support for all three hypotheses. We also describe some unique aspects of Phelsuma communities, such as partitioning of palm vs. non-palm trees. This study identifies some potentially general features of diurnal arboreal lizard communities, and suggests that some aspects of community assembly might be repeatable. [ABSTRACT FROM AUTHOR]

Large-scale field patterns are a fundamental source of inferences on processes responsible for variation in species richness among habitats. We examined species richness of larval amphibian communities in 37 ponds over seven years on the Univ. of Michigan's E. S. George Reserve. Ordination of the community incidence matrix indicated a strong major axis of variation in species associations that was correlated with pond hydroperiod, surface area and forest canopy cover. Communities were significantly nested with those species found in ponds with high canopy cover, small area and short hydroperiod being nested subsets of those found in ponds with contrasting characteristics. Presence of fish had strong negative effects on species richness; relaxation of this effect also was apparent when fish were extirpated from ponds by drought. We employed a model selection analysis to identify the most appropriate statistical model for predicting the long-term average species richness of these ponds from local abiotic and biotic (predator and competitor density) factors. A model including only the abiotic factors was overwhelmingly superior for the anurans; hierarchical partitioning indicated that area and canopy cover alone accounted for over 70% of the independent effects of predictor variables. The global model including both abiotic and biotic factors was the best supported model for the caudates, and correspondingly hierarchical partitioning suggested that area, hydroperiod, invertebrate predators and caudate biomass all accounted for 9–16% of the independent effects. Overall, biotic factors accounted for much less of the variation in species richness than abiotic factors. The patterns in larger, open-canopy ponds provided little evidence of competitive effects on species richness, though there were patterns consistent with competitive effects in small, closed-canopy ponds. The unusual temporal and spatial extent of these data enabled us to critically evaluate ideas regarding patterns in larval amphibian communities, and the effects of area, disturbance (hydroperiod) and productivity (canopy cover) on species richness of these communities. These results have important implications to the conservation of amphibian species richness in freshwater wetlands, which are among the most threatened ecosystems worldwide. [ABSTRACT FROM AUTHOR]

The relationship between above and belowground species composition has been researched in forests, grasslands, and wetlands to understand what mechanisms control community composition. I thoroughly reviewed 108 articles published between 1945 and 2006 that summarized and provided specific values on similarities between above and belowground communities to identify common trends among ecosystems. Using Sørenson's index of similarity, I found that standing vegetation and its associated seed bank was the least similar in forest ecosystems, most similar in grasslands, and of intermediate similarity in wetlands. I also discovered that species richness was not related to seed bank – vegetation similarity in any of the three ecosystems. Disturbances were a common mechanism driving community composition in all ecosystems, where similarity decreased with time since disturbance in forest and wetland ecosystems and increased with time since disturbance in grasslands. Knowing the relationships between seed bank and standing vegetation may help conservationists to manage against exotic species, plan for community responses to disturbances, restore diversity, and better understand the resilience of an ecosystem. [ABSTRACT FROM AUTHOR]

The article reports on the significance of individual foraging specialization towards ecology, conservation, and evolution. According to the author, there are various studies which identified as well as investigated individual foraging specialization; however, the results of said investigations failed to come with a common definition of specialization. The author further argues that there is a need to clearly identify characteristics of niches that feature information about individual foraging specialization in order to remedy the complexity brought about by the diverse approaches.

Food chain length is a fundamental ecosystem property, and plays a central role in determining ecosystem functioning. Recent advances in the field of stable isotope ecology allow the estimation of food chain length (FCL) from stable nitrogen isotope (δ15N) data. We conducted a global literature synthesis and estimated FCL for 219 lake, stream, and marine ecosystems. Streams had shorter food chains (∼3.5 trophic levels) than marine and lake ecosystems (∼4.0 trophic levels). In marine systems, inclusion of marine mammals increased FCL by 2/3 of a trophic level. For each ecosystem type, estimates of FCL were normally distributed and spanned two full trophic levels. Comparison with published connectance food webs revealed similar mean FCL values, though stable isotope-derived FCL estimates were less variable. At the global scale, FCL showed weak or no relationships with ecosystem size, mean annual air temperature, or latitude. Our study highlights the utility of stable isotopes for quantifying among-system food web variability, and the application of this approach for assessing global-scale patterns of food chain length. [ABSTRACT FROM AUTHOR]

Body size is a major phenotypic trait of individuals that commonly differentiates co-occurring species. We analyzed inter-specific competitive interactions between a large consumer and smaller competitors, whose energetics, selection and giving-up behaviour on identical resource patches scaled with individual body size. The aim was to investigate whether pure metabolic constraints on patch behaviour of vagile species can determine coexistence conditions consistent with existing theoretical and experimental evidence. We used an individual-based spatially explicit simulation model at a spatial scale defined by the home range of the large consumer, which was assumed to be parthenogenic and semelparous. Under exploitative conditions, competitive coexistence occurred in a range of body size ratios between 2 and 10. Asymmetrical competition and the mechanism underlying asymmetry, determined by the scaling of energetics and patch behaviour with consumer body size, were the proximate determinant of inter-specific coexistence. The small consumer exploited patches more efficiently, but searched for profitable patches less effectively than the larger competitor. Therefore, body-size related constraints induced niche partitioning, allowing competitive coexistence within a set of conditions where the large consumer maintained control over the small consumer and resource dynamics. The model summarises and extends the existing evidence of species coexistence on a limiting resource, and provides a mechanistic explanation for decoding the size-abundance distribution patterns commonly observed at guild and community levels. [ABSTRACT FROM AUTHOR]

Mutualists have been suggested to play an important role in the assembly of many plant and animal communities, but it is not clear how this depends on environmental factors. Do, for instance, natural disturbances increase or decrease the role of mutualism? We focused on entire guilds of mutualists, studying seed-dispersing ants and ant-dispersed plants along gradients of inundation disturbances. We first studied how abundance and richness of the mutualists, relative to non-mutualists, change along 35 small-scale gradients of inundation disturbances. We found that at disturbed sites, mutualistic plant species, those that reproduce by seeds dispersed by ants, increased in abundance and in consequences in richness, relative to other herbaceous plants. In contrast, we found that among the epigeic arthropods the abundance of mutualists declined, even more so than other arthropods. Correspondingly, distributions of plant and animal mutualists became increasingly discordant at disturbed sites: most plant mutualists were spatially separated from most animal mutualists. We finally found that high abundances of plant mutualists did not translate into a high nutrition service rendered to ants: at disturbed sites, many of the plants of ant-dispersed species did not produce seeds, which coincided with a decline in seed dispersal by ants and a changing searching behavior of the ants. Overall, the small-scale natural disturbances we studied were correlated to a major change in the assembly of mutualist guilds. However, the correlation was often opposite between interacting plant and animal mutualist guilds and may thus reduce the potential interaction between them. [ABSTRACT FROM AUTHOR]

Explaining the variance of local communities in a spatial-environmental matrix is one of the core interests of ecology today. Recent progress in metacommunity theory has made a substantial contribution to this field, however good empirical data in support of available theories are still relatively scarce. In this study we sampled a cluster of 36 temporary rock pools four times during one season to assess invertebrate metacommunity structure and dynamics and to search for steering processes and variables. Both Mantel tests and redundancy models indicate that local abiotic factors were dominant over spatial factors in explaining community structure and both were acting independently. Spatial variables were only important for passive dispersers and significantly explained 11% of variation in this community component. Pools connected by temporary overflows hosted more similar communities of passive dispersers than unconnected ones while community dissimilarity significantly increased with inter-pool distance. A negative curvilinear relation was discovered between taxon richness and isolation in passive dispersers, providing some support for existing theoretical models of Mouquet and Loreau. Of different metacommunity perspectives, a combination of species sorting and mass effects best explains the observed patterns. Additionally, priority effects and monopolization may buffer against the homogenising effects of dispersal and contribute to the distinctness of isolated communities. This is one of the first studies to present evidence for spatial patterns in aquatic communities on such a small spatial scale (a rock ledge of ±9000 m2). Bridging the gap between theory and observed patterns in natural systems is one of the main challenges for future metacommunity research. Small aquatic habitats such as pitcher plants and freshwater rock pools may well have an important role to play as model systems to study ecological processes in a natural spatially explicit environment. [ABSTRACT FROM AUTHOR]

Patterns and changes in functional diversity can inform about spatial and temporal variation in trait diversity, about the processes that drive assembly, and whether assemblages are likely to contain redundant species. We recently provided a new measure (termed FD) and detailed its advantages over previous ones. Since then an increasing amount of research effort has been directed towards both developing appropriate measures of functional diversity and critiquing previous ones, including FD. Podani and Schmera (2006) attempt to do both, though here we argue that they accomplish neither. First, they suggest that a particular distance measure and clustering method are appropriate. We suggest that this is not the case, and show that they may have little effect on quantitative patterns in FD. Second, they suggest that values of functional diversity must be insensitive to the number of functional traits used. We do not agree because we can envisage no relevant ecological question. Third, they observe that we originally defined an FD of zero for an empty assemblage, whereas it is more appropriate for single species assemblages to have FD of zero. We agree. Their solution, however, is to create a measure of functional diversity which violates set monotonicity. Our solution is a revised version of FD for which single species assemblages have FD=0, and which does not violate set monotonicity. In conclusion, we are confident that FD behaves appropriately and note that it remains the measure of functional diversity with greatest power to explain variation in ecosystem functioning. [ABSTRACT FROM AUTHOR]

There has been much debate about the relative merits of single-species vs ecosystem-oriented research for conservation. This debate has become increasingly important in recent times as resource managers and policy makers in some jurisdictions focus on ecosystem-level problems. We highlight the potential strengths and limitations of both kinds of research, discuss their complementarity and highlight problems that may arise where competition occurs between the two kinds of research. While a combination of approaches is ideal, a scarcity of funding, time, and expertise means it is impossible to study and manage each species, ecological process, or ecological pattern separately. Making decisions about priorities for the kinds of research, priorities for the kinds of conservation management, and associated allocation of scarce funds is a non-trivial task. We argue for an approach whereby limited resources for conservation research are targeted at projects most likely to close important knowledge gaps, while also promoting ongoing synergies between single-species and ecosystem-oriented research. [ABSTRACT FROM AUTHOR]

Mechanistic and phenomenological dispersal modelling of organisms has long been an area of intensive research. Recently, there has been an increased interest in intermediate models between the two. Intermediate models include major mechanisms that affect dispersal, in addition to the dispersal curve of a phenomenological model. Here we review and describe the mathematical and statistical framework for phenomenological dispersal modelling. In the mathematical development we describe modelling of dispersal in two dimensions from a point source, and in one dimension from a line or area source. In the statistical development we describe applicable observation distributions, and the procedures of model fitting, comparison, checking, and prediction. The procedures are also demonstrated using data from dispersal experiments. The data are hierarchically structured, and hence, we fit hierarchical models. The Bayesian modelling approach is applied, which allows us to show the uncertainty in the parameter estimates and in predictions. Finally, we show how to account for the effect of wind speed on the estimates of the dispersal parameters. This serves as an example of how to strengthen the coupling in the modelling between the phenomenon observed in an experiment and the underlying process – something that should be striven for in the statistical modelling of dispersal. [ABSTRACT FROM AUTHOR]

Since the introduction of the term ecosystem engineering by Jones et al. many studies have focused on positive, facilitative interactions caused by ecosystem engineering. Much less emphasis has been placed on the role of ecosystem engineering in causing negative interactions between species. Here, we report on negative interactions between two well known ecosystem engineers occurring at the interface of salt marsh and intertidal flat (i.e. common cordgrass Spartina anglica and lugworms Arenicola marina), via modification of their joint habitat. A field survey indicated that, although both species share a common habitat, they rarely co-occur on small spatial scales (<1 m). Experiments in the field and in mesocosms reveal that establishment of small Spartina plants is inhibited in Arenicola-dominated patches because of low sediment stability induced by the lugworms. In turn, Arenicola establishment in Spartina-dominated patches is limited by high silt content, compactness and dense rooting of the sediment caused by Spartina presence. Our results show that negative interactions by modification of the environment can result in rapid mutual exclusion, particularly if adverse effects of habitat modification are strong and if both species exhibit positive feedbacks. This illustrates the potential for negative interactions via the environment to affect community composition. [ABSTRACT FROM AUTHOR]

The article presents a discussion on alternative stable states (ASS) in ecology. It argues the theories of R. K. Didham and colleagues regarding ASS of ecological communities. It also cites the three major criteria for ASS such as it must be shown to occur in the same environment, the perturbation which causes the change in species composition must be applied as a pulse and must not cause a prolonged change in the environment, and it must be self sustaining. ASS has been discussed mainly for semi-arid rangelands.

Hubbell's neutral theory of biodiversity has generated much debate over the need for niches to explain biodiversity patterns. Discussion of the theory has focused on its neutrality assumption, i.e. the functional equivalence of species in competition and dispersal. Almost no attention has been paid to another critical aspect of the theory, the assumptions on the nature of the speciation process. In the standard version of the neutral theory each individual has a fixed probability to speciate. Hence, the speciation rate of a species is directly proportional to its abundance in the metacommunity. We argue that this assumption is not realistic for most speciation modes because speciation is an emergent property of complex processes at larger spatial and temporal scales and, consequently, speciation rate can either increase or decrease with abundance. Accordingly, the assumption that speciation rate is independent of abundance (each species has a fixed probability to speciate) is a more natural starting point in a neutral theory of biodiversity. Here we present a neutral model based on this assumption and we confront this new model to 20 large data sets of tree communities, expecting the new model to fit the data better than Hubbell's original model. We find, however, that the data sets are much better fitted by Hubbell's original model. This implies that species abundance data can discriminate between different modes of speciation, or, stated otherwise, that the mode of speciation has a large impact on the species abundance distribution. Our model analysis points out new ways to study how biodiversity patterns are shaped by the interplay between evolutionary processes (speciation, extinction) and ecological processes (competition, dispersal). [ABSTRACT FROM AUTHOR]

Despite the importance of community-structuring processes operating at both local and regional scales, there is relatively little work examining both forces within a single system. I used a combination of observational and experimental approaches to examine the processes structuring larval dragonfly distributions in lentic habitats that encompass a gradient of both permanence and top predator type. I compared the relative vulnerability of species to predators from different portions of this gradient to assess the role of predation as a local force structuring communities. I also assessed the role of regional processes on species’ distributions by examining species’ propensity to disperse to and colonize artificial ponds distributed across a landscape. In both studies I contrasted habitat specialist species, which had larvae restricted to permanent lakes, with habitat generalist species, which had larvae that occur broadly across the habitat permanence and top predator transition. Results from this work suggest that dispersal and colonization behavior were critical mechanisms restricting the distributions of habitat specialist species, but that predation may act to reinforce this pattern. The habitat specialists dispersed less frequently, colonized artificial ponds less often when they did reach them, and most moved shorter distances than the habitat generalist species. Habitat specialists were also more vulnerable than habitat generalists to an invertebrate top predator with which they do not co-exist. Results from these studies suggest that species distributions can be shaped by processes operating at both regional and local spatial scales. The role of dispersal and recruitment limitation may be generally underestimated as a force shaping species distributions and community structure across habitat gradients in which there is a transition in both the biotic interactions and the disturbance interval across that gradient. [ABSTRACT FROM AUTHOR]

Understanding variability of population abundances is of central concern to theoretical and applied evolutionary ecology, yet quantifying the conceptually simple idea has been substantially problematic. Standard statistical measures of variability are particularly biassed by rare events, zero counts and other ‘non-Gaussian’ behaviour, which are often inappropriately weighted or excluded from analysis. I conjecture that these problems are primarily a function of calculating variation as deviation from an average abundance, while the average may not be static, nor actually reflect abundance at any point in the time series. Here I describe a simple metric (population variability PV) that quantifies variability as the average percent difference between all combinations of observed abundances. Zero counts can be included if desired. Similar to standard metrics, variability is measured on a proportional scale, facilitating comparative applications. Standard metrics are based on Gaussian distributions, are over-sensitive to rare events and heavy tailed behaviour, and can inappropriately indicate ‘more time-more variation’ effects (reddened spectrum). Here I demonstrate that, while PV behaves similarly for ‘normal’ time series, it is independent of deviation from mean abundance for heavy tailed distributions, its robustness to non-Gaussian behaviour resolves artificial reddened spectrum issues, and variability calculated using PV from short time series is substantially more accurate at estimating known long term variability than standard metrics. PV therefore provides common ground for evaluating the variability of populations undergoing different dynamics, and with different statistical distributions of abundance, and can be easily generalized to a variety of contexts and disciplines. [ABSTRACT FROM AUTHOR]

We investigated if dragonfly larvae community composition and species abundance curves are sensitive to variation in predation intensity, and whether the fit to a particular niche partitioning model could be used to make inferences about mechanisms structuring communities. The approach taken was to compare communities in lakes either having or lacking fish predation. Dragonfly species classified as active, strongly dominated the dragonfly communities in fishless lakes, and low active species dominated fishless lakes. As activity level is known to correlate with susceptibility to fish predation this indicates that these communities are structured by fish predation. Fitting relative abundance data to five niche partitioning models showed that the same model fitted data from both types of habitats (fish/no fish). This means that the observed differences in relative abundances were substitutive, i.e. the relative abundance of a rank stayed constant, even though the identity of the species having this rank changed. The best fit to data from both types of lakes was found for the random assortment model, which is usually interpreted as an indication that the community is not structured by within-guild interactions. This interpretation for fishless lakes did not seem to agree with other community measures (i.e. lowered diversity and evenness and no relationship between species richness and dragonfly biomass), which indicate that the community is structured by within-guild interactions. Moreover, a detail in the fitting procedure, the number of species included in the analysis, affected which model that fitted data best. Thus, we question if fitting niche partitioning models to data can provide mechanistic understanding of how resources are partitioned in natural communities. [ABSTRACT FROM AUTHOR]

T. Fukami and W. G. Lee argue that the logical expectation from ecological theory is that competitively-structured assemblages will be more likely to exhibit alternative stable states than abiotically-structured assemblages. We suggest that there are several important misinterpretations in their arguments, and that the substance of their hypothesis has both a weak basis in ecological theory and is not supported by empirical evidence which shows that alternative stable states occur more frequently in natural systems subject to moderate- to harsh abiotic extremes. While this debate is founded in ecological theory, it has important applied implications for restoration management. Sound theoretical predictions about when to expect alternative stable states can only aid more effective restoration if theoretical expectations can be shown to translate into predictable empirical outcomes. If strongly abiotically- or disturbance-structured systems are more likely to exhibit catastrophic phase shifts in community structure that can be resilient to management efforts, then restoration ecologists will need to treat these systems differently in terms of the types of management inputs that are required. [ABSTRACT FROM AUTHOR]

Didham et al. suggested that abiotically structured communities showing trait under-dispersion are more likely to exhibit alternative stable states than are competitively structured communities showing trait over-dispersion. We argue that the opposite is the logical expectation, and discuss implications for ecological restoration at local and regional scales. [ABSTRACT FROM AUTHOR]

The geographic area hypothesis advances area as the primary cause of latitudinal gradients in diversity. The greater area of tropical zones, it suggests, stimulates speciation, inhibits extinction, and leads to increased species richness compared to the situation in smaller temperate and boreal zones. Because bats exhibit exceptionally strong latitudinal gradients of richness at multiple spatial scales in the New World, they are an appropriate system with which to test the geographic area hypothesis. We used range maps for 250 species of New World bats to estimate species richness in biogeographic zones at two hierarchical spatial scales: biome types and provinces. We then conducted a series of regression analyses to evaluate the ability of area to account for latitudinal gradients in species richness. However, spillover (zonal bleeding) of tropical species into extra-tropical zones may mask the species-area relationship and alter perceptions of the latitudinal gradient. To address this issue, we conducted additional analyses excluding tropical species, using a series of increasingly inclusive definitions of tropical ranges. Ecogeographic zones of the New World are not larger at tropical versus extra-tropical latitudes. Moreover, spillover of tropical species into ecogeographic zones within extra-tropical regions generally does not diminish the association between richness and area. Nonetheless, the latitudinal gradient of species richness is strong and significant at both ecogeographic scales. Clearly, area does not drive the latitudinal gradient of bat species richness in the New World. In fact, area represents a source of noise rather than a dominant signal at the focal scale of biome types and provinces in the Western Hemisphere. [ABSTRACT FROM AUTHOR]

Ecologists have proposed that tree species may coexist by specialising on light environments associated with gaps of different sizes. Remarkably few community-level studies, however, have actually examined juvenile tree distributions along light availability gradients. Here we describe distributions of juvenile trees in relation to canopy openness in a temperate rainforest, and test the hypothesis that competitive sorting causes coexisting species to overlap less in light environment occupancy than would be expected by chance. Average overlap of species’ interquartile ranges on the canopy openness gradient was tested against a bounded domain null model of community structure which used range-size criteria to constrain random placement of species optima. Microsite availability was strongly skewed towards low light, with 43% of microsites occurring at <5% canopy openness. We therefore transformed canopy openness values to ranks, so that equal intervals on the transformed gradient represented equal areas of microsite availability. We then calculated the interquartile range (25–75%) of sample ranks occupied by juveniles of each species. About half the assemblage was non-randomly distributed in relation to canopy openness, providing evidence of niche expression. Average overlap of species’ interquartile ranges did not depart significantly from that predicted by the bounded null model, indicating that community structure in relation to canopy openness was mainly explained by a mid-domain effect. As predicted by the null model, species’ interquartile range mid-points were concentrated in the centre of the rank-transformed gradient, and species richness (overlap of interquartile ranges) peaked close to the median light environment. Most species therefore had intermediate light requirements. The apparent lack of constraints on pairwise overlap suggest that differences in light use are not a prerequisite for tree species coexistence. As far as we are aware, this is the first study to identify a mid-domain effect on a resource availability axis. [ABSTRACT FROM AUTHOR]

Models commonly used in population ecology have somewhat vague and often misleading connections with physical and biological processes connecting organisms with their environments. The metaphysiological modelling approach offers a conceptual framework that is consistent with basic laws and enables integration across levels of organization from cells to ecosystems. This perspective accommodates the intrinsically disequilibrial nature of biological systems in changing environments, as well as adaptive responses to these changes. More attention needs to be paid to factors governing mortality losses rather than the functional or intake response alone. Models need to be based more firmly on underlying biophysical processes in order to serve as reliable guides to conservation action in novel environments. [ABSTRACT FROM AUTHOR]

In natural biological communities the disappearance of one species can have knock-on effects causing extinction of further species from the food web. To investigate these effects we used an evolutionary model to assemble many independent simulated food webs, and studied their dynamical behaviour when one species was deleted. On average, only 2.1% of the remaining species went extinct as a result of the deletion. However, the probability of extinction of predators and indirect predators (more than one link up the chain) of the deleted species was several times larger than for an average species. The model allows predators to adapt their choice of prey in response to changing frequencies of the prey. It was found that the larger the proportion of the deleted species in the predator's diet, the greater its probability of extinction. The probability of extinction of prey of the deleted species was also significantly higher than for an average species. This is due to increased competition between prey species after removal of their predator. The effect was largest for prey species that formed an intermediate fraction of the diet of the deleted species. The number of further extinctions increased significantly with the number of links in the food web to the deleted species prior to deletion, and was also correlated with the bottom-up and top-down keystone species indices. We also considered which properties of the web as a whole influenced its robustness to species deletion. This revealed a significant correlation between ecosystem redundancy and deletion stability, but no clear relationship with complexity. [ABSTRACT FROM AUTHOR]

Ecologists continue to debate the roles of deterministic versus stochastic (or neutral) processes in the assembly of ecological communities. The debate often hinges on issues of temporal and spatial scale. Resolution of the competing views depends on a detailed understanding of variation in the structure of local communities through time and space. Analyses of twelve years of data on a diverse assemblage of 13 boreal small mammal species revealed both deterministic and stochastic patterns. Stochastic membership in the overall community created unique assemblages of species in both time and space. But the relative abundances of the two codominant species were much less variable, and suggest a significant role for strong interactions that create temporal and spatial autocorrelation in abundance. As species wax and wane in abundance, they are nevertheless subject to probabilistic rules on local assembly. At the scales I report on here, poorly understood large scale processes influence the presence and absence of the majority of (sparse) species in the assembly. But the overall pool of species nevertheless obeys local rules on their ultimate stochastic assembly into groups of interacting species. [ABSTRACT FROM AUTHOR]

Plant species effects on ecosystem processes are mediated by traits such as litter quality and exudation. These same traits also influence the activity and distribution of animals that play key roles in regulating ecosystem dynamics. We planted monocultures of eight plant species commonly found in California grasslands to investigate the relative importance of plant species direct effects on nitrogen cycling, versus their indirect effects mediated by plant interactions with gophers. Plant species differed in their litter C:N ratio, which closely related to species effects on rates of net mineralization and nitrification in undisturbed soil. However, the effect of selective gopher disturbance on N cycling greatly altered these species effects.Plant species differed in their effects on the type and timing of gopher disturbance. Small feeding holes were formed in late spring in plots containing species with high tissue quality. These feeding holes minimally disturbed the soil and did not alter N cycling rates over the short term. Large gopher mounds were formed in the winter and early spring, primarily in plots containing the grass,Aegilops triuncialis, and to a lesser extent in plots containingAvena barbata. These large mounds significantly disturbed the soil and greatly increased net nitrification rates, but had no consistent effects on net N mineralization. In undisturbed soil,Aegilopshad the highest litter C:N ratio and one of the lowest rates of net nitrification. However, gophers preferentially built large mounds inAegilopsplots. Once the effects of gopher burrowing were considered,Aegilopshad one of the highest rates of net nitrification, indicating that the indirect effects of plant species on N cycling can be more important than the direct effects alone. This experiment indicates that it is vital to consider interactions between plants and other organisms in order to predict the ecosystem effects of plant communities. [ABSTRACT FROM AUTHOR]