The statistical inevitability of stability-diversity relationships in community ecology

In this article, we explain an often overlooked process that may significantly contribute to positive correlations between measures of species diversity and community stability. Empirical studies showing positive stability-diversity relationships have, for the most part, used a single class of stability (or, more accurately, instability) measures: the temporal variation in aggregate community properties such as biomass or productivity. We show that for these measures, stability will essentially always rise with species diversity because of the statistical averaging of the fluctuations in species' abundances. This simple probabilistic process will operate in the absence of any strong species interactions, although its strength is driven by the relative abundances of species, as well as by the existence of positive or negative correlations in the fluctuations of species. To explore the possible importance of this effect in real communities, we fit a simple simulation model to Tilman's grassland community. Our results indicate that statistical averaging might play a substantial role in explaining stability-diversity correlations for this and other systems. Models of statistical averaging can serve as a useful baseline for predictions of community stability, to which the influences of both negative and positive species interactions may then be added and tested.     

Species richness, environmental fluctuations, and temporal change in total community biomass

Theory and empirical results suggest that high biodiversity should often cause lower temporal variability in aggregate community properties such as total community biomass. We assembled microbial communities containing 2 to 8 species of competitors in aquatic microcosms and found that the temporal change in total community biomass was positively but insignificantly associated with diversity in a constant temperature environment. There was no evidence of any trend in variable temperature environments. Three non-exclusive mechanisms might explain the lack of a net stabilising effect of species richness on temporal change. (1) A direct destabilising effect of diversity on population level variances caused some populations to vary more when embedded in more diverse communities. (2) Similar responses of the different species to environmental variability might have limited any insurance effect of increased species richness. (3) Large differences in the population level variability of different species (i.e., unevenness) could weaken the relation between species richness and community level stability. These three mechanisms may outweigh the stabilising effects of increases in total community biomass with diversity, statistical averaging, and slightly more negative covariance in more diverse communities. Our experiment and analyses advocate for further experimental investigations of diversity-variability relations.     

How enrichment,ecosystem size,and their effects on species richness co‐determine the stability of microcosm communities

Nutrient enrichment, ecosystem size, and richness each may directly affect the stability of both populations and communities. Alternatively, nutrient enrichment and ecosystem size each may directly affect richness, which in turn may affect stability. No previous studies, however, have tested empirically how these three factors interact and co‐determine stability. We manipulated nutrient input and ecosystem size in replicate microcosms containing a diverse bacterial flora, and a range of green algae and heterotrophic protozoa, and used these manipulations and the resulting variation in species richness to measure their combined effects on temporal stability of both populations and communities. Results showed that nutrient enrichment and ecosystem size controlled protist richness, and their effects on stability could be mediated by richness. In addition, both community‐level and population‐level stability increased with protist richness. Furthermore, mean species evenness and mean species richness was negatively related. Effects of statistical averaging, overyielding, and component population stability were identified as possible mechanisms involved explaini ng the stabilizing effects of richness on community stability. Their relative strength in influencing stability, however, is likely to change as mean evenness decreased with increasing richness. This decrease in evenness would tend to weaken the strength of the statistic averaging effect, but increase the strength of the other two mechanisms due to relatively lower population variability (component population stability) and higher mean biovolumes of dominant protists (overyielding).     

Soil heterogeneity buffers community response to climate change in species‐rich grassland

Climate change impacts on vegetation are mediated by soil processes that regulate rhizosphere water balance, nutrient dynamics, and ground‐level temperatures. For ecosystems characterized by high fine‐scale substrate heterogeneity such as grasslands on poorly developed soils, effects of climate change on plant communities may depend on substrate properties that vary at the scale of individuals (<m²), leading to fine‐scale shifts in community structure that may go undetected at larger scales. Here, we show in a long‐running climate experiment in species‐rich limestone grassland in Buxton, England (UK), that the resistance of the community to 15‐year manipulations of temperature and rainfall at the plot scale (9 m²) belies considerable community reorganization at the microsite (100 cm²) scale. In individual models of the abundance of the 25 most common species with respect to climate treatment and microsite soil depth, 13 species exhibited significant soil depth affinities, and nine of these have shifted their position along the depth gradient in response to one or more climate treatments. Estimates of species turnover across the depth gradient reviewed in relation to measurements of water potential, nitrogen supply, pH, and community biomass suggest that communities of shallow microsites are responding directly to microenvironmental changes induced by climate manipulation, while those of the deepest microsites are shifting in response to changes in competitive interference from more nutrient‐demanding species. Moreover, for several species in summer drought and winter heated treatments, climate response in deep microsites was opposite that of shallow microsites, suggesting microsite variation is contributing to community stability at the whole‐plot level. Our study thus demonstrates a strong link between community dynamics and substrate properties, and suggests ecosystems typified by fine‐scale substrate heterogeneity may possess a natural buffering capacity in the face of climate change.     

Natural‐Channel‐Design Restorations That Changed Geomorphology Have Little Effect on Macroinvertebrate Communities in Headwater Streams

Stream restorations that increase geomorphic stability can improve habitat quality, which should benefit selected species and local aquatic ecosystems. This assumption is often used to define primary restoration goals; yet, biological responses to restoration are rarely monitored or evaluated methodically. Macroinvertebrate communities were inventoried at 6 study reaches within 5 Catskill Mountain streams between 2002 and 2006 to characterize their responses to natural‐channel‐design (NCD) restoration. Although bank stability increased significantly at most restored reaches, analyses of variation showed that NCD restorations had no significant effect on 15 of 16 macroinvertebrate community metrics. Multidimensional scaling ordination indicated that communities from all reach types within a stream were much more similar to each other within any given year than they were in the same reaches across years or within any type of reach across streams. These findings indicate that source populations and watershed‐scale factors were more important to macroinvertebrate community characteristics than were changes in channel geomorphology associated with NCD restoration. Furthermore, the response of macroinvertebrates to restoration cannot always be used to infer the response of other stream biota to restoration. Thus, a broad perspective is needed to characterize and evaluate the full range of effects that restoration can have on stream ecosystems.     

Spatial heterogeneity reduces temporal variability in stream insect communities

Although all natural systems are heterogeneous, the direct influence of spatial heterogeneity on most ecological variables is unknown. In many systems, spatial heterogeneity is positively correlated with both microhabitat refugia and species richness. Both an increased number of microhabitat refugia and the effects of statistical averaging via increased species richness should lead to an inverse relationship between spatial heterogeneity and variability in community composition. To test this prediction, I measured diversity and temporal variability of invertebrate communities in a northern New Hampshire stream along a natural gradient of spatial heterogeneity formed by variation in stream substrates. On average, there was a 42% decrease in community variability along a gradient of increasing heterogeneity. This pattern was robust to changes in metrics of both heterogeneity and community variability. There was also a significant positive relationship between taxon richness and spatial heterogeneity with predicted taxon richness increasing c. 1.5× along the heterogeneity gradient. By resampling community abundance data, I estimated that statistical averaging accounted for only 4% of the observed decrease in community variability in this study. I concluded that the remaining decrease was very likely explained by a greater number of refugia from predation and/or flooding in high‐heterogeneity habitats. The results of this study suggest that maximizing heterogeneity in ecological restoration programmes may promote temporally stable and diverse communities and may aid in responsible management of aquatic resources.     

Biodiversity acts as insurance of productivity of bacterial communities under abiotic perturbations

Anthropogenic disturbances are detrimental to the functioning and stability of natural ecosystems. Critical ecosystem processes driven by microbial communities are subjected to these disturbances. Here, we examine the stabilizing role of bacterial diversity on community biomass in the presence of abiotic perturbations such as addition of heavy metals, NaCl and warming. Bacterial communities with a diversity gradient of 1–12 species were subjected to the different treatments, and community biomass (OD₆₀₀) was measured after 24 h. We found that initial species richness and phylogenetic structure impact the biomass of communities. Under abiotic perturbations, the presence of tolerant species in community largely contributed in community biomass production. Bacterial diversity stabilized the biomass across the treatments, and differential response of bacterial species to different perturbations was the key reason behind these effects. The results suggest that biodiversity is crucial for maintaining the stability of ecosystem functioning and acts as ecological insurance under abiotic perturbations. Biodiversity in natural ecosystems may also uphold the ecosystem functioning under anthropogenic disturbance.     

A macroecological perspective of trait patterns in stream communities

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Diversity-stability relationships in community ecology: re-examination of the portfolio effect

In plant communities, the portfolio effect, also called "statistical averaging effect", expresses the fact that stability in aggregate community properties such as biomass productivity generally rises with species diversity, simply because of the statistical averaging of the fluctuations in species' properties. This paper essentially upgrades the previous formulations of the portfolio effect, first developed by Doak and collaborators and then by Tilman. It uses a theoretical approach based on simple statistical relationships and some simplifying assumptions proposed by these authors. The new formulation presented extends and improves the previous relationships in the sense that it takes into account simultaneously a varying scaling power of the variance, the interaction effect between species, the heterogeneity in species productivity and interspecies correlated responses to the environment. It appears that the simple statistical averaging, as inferred from this formulation, does not necessarily lead to a positive correlation between species diversity and community stability.     

Fishing degrades size structure of coral reef fish communities

Fishing pressure on coral reef ecosystems has been frequently linked to reductions of large fishes and reef fish biomass. Associated impacts on overall community structure are, however, less clear. In size‐structured aquatic ecosystems, fishing impacts are commonly quantified using size spectra, which describe the distribution of individual body sizes within a community. We examined the size spectra and biomass of coral reef fish communities at 38 US‐affiliated Pacific islands that ranged in human presence from near pristine to human population centers. Size spectra ‘steepened’ steadily with increasing human population and proximity to market due to a reduction in the relative biomass of large fishes and an increase in the dominance of small fishes. Reef fish biomass was substantially lower on inhabited islands than uninhabited ones, even at inhabited islands with the lowest levels of human presence. We found that on populated islands size spectra exponents decreased (analogous to size spectra steepening) linearly with declining biomass, whereas on uninhabited islands there was no relationship. Size spectra were steeper in regions of low sea surface temperature but were insensitive to variation in other environmental and geomorphic covariates. In contrast, reef fish biomass was highly sensitive to oceanographic conditions, being influenced by both oceanic productivity and sea surface temperature. Our results suggest that community size structure may be a more robust indicator than fish biomass to increasing human presence and that size spectra are reliable indicators of exploitation impacts across regions of different fish community compositions, environmental drivers, and fisheries types. Size‐based approaches that link directly to functional properties of fish communities, and are relatively insensitive to abiotic variation across biogeographic regions, offer great potential for developing our understanding of fishing impacts in coral reef ecosystems.     

Microbial leaf degraders in boreal streams: bringing together stochastic and deterministic regulators of community composition

1. Leaves that fall into the water represent a new habitat for microorganisms to colonise in streams, providing an opportunity to study colonisation and the subsequent regulation of community structure. We explored community composition of bacteria and fungi on decomposing alder leaves in nine streams in central Sweden, and describe their relationship with environmental variables. Succession of the microbial community was studied in one of the streams for 118 days. Microbial community composition was examined by denaturing gradient gel electrophoresis on replicate samples of leaves from each stream. 2. During succession in one stream, maximum taxon richness was reached after 34 days for bacteria and 20 days for fungi respectively. Replicate samples within this stream differed between each other earlier in colonisation, while subsequently such variation among replicate communities was low and remained stable for several weeks. Replicate samples taken from all the nine streams after 34 days of succession showed striking similarities in microbial communities within‐streams, although communities differed more strongly between streams. 3. Canonical analysis of microbial communities and environmental variables revealed that water chemistry had a significant influence on community composition. This influence was superimposed on a statistical relationship between the properties of stream catchments and microbial community composition. 4. The catchment regulates microbial communities in two different ways. It harbours the species pool from which the in‐stream microbial community is drawn and it governs stream chemistry and the composition of organic substrates that further shape the communities. We suggest that there is a random element to colonisation early in succession, whereas other factors such as species interactions, stream chemistry and organic substrate properties, result in a more deterministic regulation of communities during later stages.     

The stability–diversity relationship in stream macroinvertebrates: influences of sampling effects and habitat complexity

1. Theory predicts that the stability of a community should increase with diversity. However, despite increasing interest in the topic, most studies have focused on aggregate community properties (e.g. biomass, productivity) in small‐scale experiments, while studies using observational field data on realistic scales to examine the relationship between diversity and compositional stability are surprisingly rare. 2. We examined the diversity–stability relationship of stream invertebrate communities based on a 4‐year data set from boreal headwater streams, using among‐year similarity in community composition (Bray–Curtis coefficient) as our measure of compositional stability. We related stability to species richness and key environmental factors that may affect the diversity–stability relationship (stream size, habitat complexity, productivity and flow variability) using simple and partial regressions. 3. In simple regressions, compositional stability was positively related to species richness, stream size, productivity and habitat complexity, but only species richness and habitat complexity were significantly related to stability in partial regressions. There was, however, a strong relationship between species richness and abundance. When abundance was controlled for through re‐sampling, stability was unrelated to species richness, indicating that sampling effects were the predominant mechanism producing the positive stability–diversity relationship. By contrast, the relationship between stability and habitat complexity (macrophyte cover) became even stronger when the influence of community abundance was controlled for. Habitat complexity is thus a key factor enhancing community stability in headwater streams.     

Size‐based ecological interactions drive food web responses to climate warming

Predicting climate change impacts on animal communities requires knowledge of how physiological effects are mediated by ecological interactions. Food‐dependent growth and within‐species size variation depend on temperature and affect community dynamics through feedbacks between individual performance and population size structure. Still, we know little about how warming affects these feedbacks. Using a dynamic stage‐structured biomass model with food‐, size‐ and temperature‐dependent life history processes, we analyse how temperature affects coexistence, stability and size structure in a tri‐trophic food chain, and find that warming effects on community stability depend on ecological interactions. Predator biomass densities generally decline with warming – gradually or through collapses – depending on which consumer life stage predators feed on. Collapses occur when warming induces alternative stable states via Allee effects. This suggests that predator persistence in warmer climates may be lower than previously acknowledged and that effects of warming on food web stability largely depend on species interactions.     

Nutrient enrichment weakens the stabilizing effect of species richness

With global freshwater biodiversity declining at an even faster rate than in the most disturbed terrestrial ecosystems, understanding the effects of changing environmental conditions on relationships between biodiversity and the variability of community and population processes in aquatic ecosystems is of significant interest. Evidence is accumulating that biodiversity loss results in more variable communities; however, the mechanisms underlying this effect have been the subject of considerable debate. We manipulated species richness and nutrients in outdoor aquatic microcosms composed of naturally occurring assemblages of zooplankton and benthic invertebrates to determine how the relationship between species richness and variability might change under different nutrient conditions. Temporal variability of populations and communities decreased with increasing species richness in low nutrient microcosms. In contrast, we found no relationship between species richness and either population or community variability in nutrient enriched microcosms. Of the different mechanisms we investigated (e.g. overyielding, statistical averaging, insurance effects, and the stabilizing effect of species richness on populations) the only one that was consistent with our results was that increases in species richness led to more stable community abundances through the stabilizing effect of species richness on the component populations. While we cannot conclusively determine the mechanism(s) by which species richness stabilized populations, our results suggest that more complete resource-use in the more species-rich low nutrient communities may have dampened population fluctuations.     

Nitrogen addition does not reduce the role of spatial asynchrony in stabilising grassland communities

While nitrogen (N) amendment is known to affect the stability of ecological communities, whether this effect is scale‐dependent remains an open question. By conducting a field experiment in a temperate grassland, we found that both plant richness and temporal stability of community biomass increased with spatial scale, but N enrichment reduced richness and stability at the two scales considered. Reduced local‐scale stability under N enrichment arose from N‐induced reduction in population stability, which was partly attributable to the decline in local species richness, as well as reduction in asynchronous local population dynamics across species. Importantly, N enrichment did not alter spatial asynchrony among local communities, which provided similar spatial insurance effects at the larger scale, regardless of N enrichment levels. These results suggest that spatial variability among local communities, in addition to local diversity, may help stabilise ecosystems at larger spatial scales even in the face of anthropogenic environmental changes.     

Intraspecific variation in a predator drives cascading variation in primary producer community composition

Predation has important cascading impacts on primary producer biomass and community composition in many ecosystems. While most studies have focused on the consequences of interspecific or density differences in predators, it is recognized that phenotypic variation within species can have strong and cascading community and ecosystem consequences at lower trophic levels. In coastal New England lakes, both the presence and life history form of the zooplanktivorous fish alewife, Alosa pseudoharengus, have strong influence on the biomass, size structure and community composition of crustacean zooplankton communities. Here we test the hypothesis that alewife presence and life history will have cascading impacts on phytoplankton biomass and community composition in a mesocosm experiment that previously reported strong biomass and compositional differences of crustacean zooplankton communities among alewife treatments. We show that alewife life history led to small but statistically significant differences in phytoplankton community composition among treatments. This compositional difference was driven primarily by an increase in the density of two edible phytoplankton genera associated with lower zooplankton biomass in the anadromous alewife treatment. Our results show that intraspecific variation in a predator can have cascading effects on primary producer communities. However we did not observe significant differences in total algal biomass.     

Elevated CO2 and water addition enhance nitrogen turnover in grassland plants with implications for temporal stability

Temporal variation in soil nitrogen (N) availability affects growth of grassland communities that differ in their use and reuse of N. In a 7‐year‐long climate change experiment in a semi‐arid grassland, the temporal stability of plant biomass production varied with plant N turnover (reliance on externally acquired N relative to internally recycled N). Species with high N turnover were less stable in time compared to species with low N turnover. In contrast, N turnover at the community level was positively associated with asynchrony in biomass production, which in turn increased community temporal stability. Elevated CO₂ and summer irrigation, but not warming, enhanced community N turnover and stability, possibly because treatments promoted greater abundance of species with high N turnover. Our study highlights the importance of plant N turnover for determining the temporal stability of individual species and plant communities affected by climate change.     

Plant functional traits improve diversity‐based predictions of temporal stability of grassland productivity

Synthesis The temporal stability of plant production is greater in communities with high than low species richness, but stability also may depend on species abundances and growth‐related traits. Annual precipitation varied by greater than a factor of three over 11 years in central Texas, USA leading to large variation in production. Stability was greatest in communities that were not dominated by few species and in which dominant species rooted shallowly, had dense leaves, or responded to the wettest year with a minimal increase in production. Stability may depend as much on species abundances and functional traits as on species richness alone. Aboveground net primary productivity (ANPP) varies in response to temporal fluctuations in weather. Temporal stability of community ANPP may be increased by increasing plant species richness, but stability often varies at a given richness level implying a dependence on abundances and functional properties of member species. We measured stability in ANPP during 11 years in field plots (Texas, USA) in which we varied the richness and relative abundances of perennial grassland species at planting. We sought to identify species abundance patterns and functional traits linked to the acquisition and processing of essential resources that could be used to improve richness‐based predictions of community stability. We postulated that community stability would correlate with abundance‐weighted indices of traits that influence plant responses to environmental variation. Annual precipitation varied by a factor of three leading to large inter‐annual variation in ANPP. Regression functions with planted and realized richness (species with > 1% of community ANPP during the final four years) explained 32% and 25% of the variance in stability, respectively. Regression models that included richness plus the fraction of community ANPP produced by the two most abundant species in combination with abundance‐weighted values of either the fraction of sampled root biomass at 20–45 cm depth, leaf dry matter content (LDMC), or response to greater‐than‐average precipitation of plants grown in monocultures explained 58–69% (planted richness) and 58–64% (realized richness) of the variance in stability. Stability was greatest in communities that were not strongly dominated by only two species and in which plants rooted shallowly, had high values of LDMC, or responded to the wettest year with a minimal increase in ANPP. Our results indicate that the temporal stability of grassland ANPP may depend as much on species abundances and functional traits linked to plant responses to precipitation variability as on species richness alone.     

Grazer diversity affects resistance to multiple stressors in an experimental seagrass ecosystem

When multiple stressors act simultaneously, their effects on ecosystems become more difficult to predict. In the face of multiple stressors, diverse ecosystems may be more stable if species respond differently to stressors or if functionally similar species can compensate for stressor effects on focal species. Many habitats around the globe are threatened by multiple stressors, including highly productive seagrass habitats. For example, in Chesapeake Bay, USA, regional climate change predictions suggest that elevated temperature and freshwater inputs are likely to be increasingly important stressors. Using seagrass mesocosms as a model system, we tested whether species richness of crustacean grazers buffers ecosystem properties against the impacts of elevated temperature and freshwater pulse stressors in a fully factorial experiment. Grazer species responded to pulsed salinity changes differently; abundance of Elasmopus levis responded negatively to freshwater pulses, whereas abundance of Gammarus mucronatus and Erichsonella attenuata responded positively or neutrally. Consistent with the hypothesis that biodiversity provides resistance stability, biomass of epiphytic algae that form the base of the food web was less affected by stressors in species‐rich grazer treatments than in single‐species grazer treatments. Stochastic (among‐replicate) variation of sessile invertebrate biomass within treatments was also reduced in more diverse grazer treatments. Therefore, grazer species richness tended to increase the resistance stability of both major components of the seagrass fouling community, algae and invertebrates, in the face of environmental stressors. Finally, in our model system, multi‐stressor impacts suggested a pattern of antagonism contrary to previous assumptions of synergistic stressor effects. Overall, our results confirm that invertebrate grazer species are functionally diverse in their response to environmental stressors, but are largely functionally redundant in their grazing effects leading to greater resistance stability of certain ecosystem properties in diverse grazer assemblages even when influenced by multiple environmental stressors.     

Environmental and spatial controls of taxonomic versus trait composition of stream biota

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