Does Tropical Forest Fragmentation Increase Long-Term Variability of Butterfly Communities?

Habitat fragmentation is a major driver of biodiversity loss. Yet, the overall effects of fragmentation on biodiversity may be obscured by differences in responses among species. These opposing responses to fragmentation may be manifest in higher variability in species richness and abundance (termed hyperdynamism), and in predictable changes in community composition. We tested whether forest fragmentation causes long-term hyperdynamism in butterfly communities, a taxon that naturally displays large variations in species richness and community composition. Using a dataset from an experimentally fragmented landscape in the central Amazon that spanned 11 years, we evaluated the effect of fragmentation on changes in species richness and community composition through time. Overall, adjusted species richness (adjusted for survey duration) did not differ between fragmented forest and intact forest. However, spatial and temporal variation of adjusted species richness was significantly higher in fragmented forests relative to intact forest. This variation was associated with changes in butterfly community composition, specifically lower proportions of understory shade species and higher proportions of edge species in fragmented forest. Analysis of rarefied species richness, estimated using indices of butterfly abundance, showed no differences between fragmented and intact forest plots in spatial or temporal variation. These results do not contradict the results from adjusted species richness, but rather suggest that higher variability in butterfly adjusted species richness may be explained by changes in butterfly abundance. Combined, these results indicate that butterfly communities in fragmented tropical forests are more variable than in intact forest, and that the natural variability of butterflies was not a buffer against the effects of fragmentation on community dynamics.     

The Scaling of Host Density with Richness Affects the Direction,Shape, and Detectability of Diversity-Disease Relationships

Pathogen transmission responds differently to host richness and abundance, two unique components of host diversity. However, the heated debate around whether biodiversity generally increases or decreases disease has not considered the relationships between host richness and abundance that may exist in natural systems. Here we use a multi-species model to study how the scaling of total host community abundance with species richness mediates diversity-disease relationships. For pathogens with density-dependent transmission, non-monotonic trends emerge between pathogen transmission and host richness when host community abundance saturates with richness. Further, host species identity drives high variability in pathogen transmission in depauperate communities, but this effect diminishes as host richness accumulates. Using simulation we show that high variability in low richness communities and the non-monotonic relationship observed with host community saturation may reduce the detectability of trends in empirical data. Our study emphasizes that understanding the patterns and predictability of host community composition and pathogen transmission mode will be crucial for predicting where and when specific diversity-disease relationships should occur in natural systems.     

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.     

Population and community variability in randomly fluctuating environments

The prediction that environmental fluctuations may destabilise populations and yet stabilise aggregate community properties has remained largely untested. We examined population and community stability under constant and fluctuating temperatures in simple planktonic assemblages of differing algal richness. Temperature dependent resource competition produced a highly asymmetric community structure where algal community biomass was dominated by one species. For a given level of species richness, temperature fluctuations induced lower community covariance and thus stabilised community biomass. However, increasing algal species richness increased the variability of population abundance and growth rates, as well as population and community variability. Consumer dynamics were directly destabilised by environmental fluctuations. These results confirm recent theoretical studies suggesting a stabilising effect of environmental fluctuations at the community level. However, they also support the theoretical prediction that increasing species richness may be of limited value for community stability, most especially in asymmetric communities, when competition directly affects population variability.     

Maintenance of Positive Diversity-Stability Relations along a Gradient of Environmental Stress

Background

Environmental stress is widely considered to be an important factor in regulating whether changes in diversity will affect the functioning and stability of ecological communities.

Methodology/Principal Findings

We investigated the effects of a major environmental stressor (a decrease in water volume) on diversity-abundance and diversity-stability relations in laboratory microcosms composed of temperate multi-trophic rock pool communities to identify differences in community and functional group responses to increasing functional group richness along a gradient of environmental stress (low, medium, and high water volume). When a greater number of functional groups were present, communities were less temporally variable and achieved higher abundances. The stabilizing effect of increased functional group richness was observed regardless of the level of environmental stress the community was subjected too. Despite the strong consistent stabilizing effect of increased functional group richness on abundance, the way that individual functional groups were affected by functional group richness differed along the stress gradient. Under low stress, communities with more functional groups present were more productive and showed evidence of strong facilitative interactions. As stress increased, the positive effect of functional group richness on community abundance was no longer observed and compensatory responses became more common. Responses of individual functional groups to functional group richness became increasing heterogeneous are stress increased, prompting shifts from linear diversity-variability/abundance relations under low stress to a mix of linear and non-linear responses under medium and high stress. The strength of relations between functional group richness and both the abundances and temporal variability of functional groups also increased as stress increased.

Conclusions/Significance

While stress did not affect the relation between functional group richness and stability per se, the way in which functional groups responded to changes in functional group richness differed as stress increased. These differences, which include increases in the heterogeneity of responses of individual functional groups, increases in compensatory dynamics, and increases in the strength of richness-abundance and richness-variability relations, may be critical to maintaining stability under increasingly stressful environmental conditions.     

Removing the confounding effect of habitat specialization reveals the stabilizing contribution of diversity to species variability

Earlier studies have found that diversity, S, stabilizes the relative variability of combined biomass or abundance of species making up a community. However, the effect of S on variability of constituent species has been elusive. We hypothesize that the proportion of specialists increases with S and, because specialists are more variable, this shift in composition will mask the stabilizing effect of S on populations of species making up a community. The test uses data on variability and ecological specialization of species in 49 natural rock pool invertebrate communities. Initial analyses produced inconclusive results similar to earlier studies. However, when variability owing to species' specialization was factored out, S reduced species' abundance variability, although not in all communities. Our study explains why the stabilizing effect of diversity on populations has not been found earlier.     

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).     

Drought negatively affects communities on a foundation tree: growth rings predict diversity

Understanding how communities respond to extreme climatic events is important for predicting the impact of climate change on biodiversity. The plant vigor and stress hypotheses provide a theoretical framework for understanding how arthropods respond to stress, but are rarely tested at the community level. Following a record drought, we compared the communities of arthropods on pinyon pine (Pinus edulis) that exhibited a gradient in physical traits related to environmental stress (e.g., growth rate, branch dieback, and needle retention). Six patterns emerged that show how one of the predicted outcomes of climate change in the southwestern USA (i.e., increased drought severity) alters the communities of a foundation tree species. In accordance with the plant vigor hypothesis, increasing tree stress was correlated with an eight to tenfold decline in arthropod species richness and abundance. Trees that were more similar in their level of stress had more similar arthropod communities. Both foliage quantity and quality contributed to arthropod community structure. Individual species and feeding groups differed in their responses to plant stress, but most were negatively affected. Arthropod richness (r ² = 0.48) and abundance (r ² = 0.48) on individual trees were positively correlated with the tree’s radial growth during drought. This relationship suggests that tree ring analysis may be used as a predictor of arthropod diversity, which is similar to findings with ectomycorrhizal fungi. A contrast of our findings on arthropod abundance with published data on colonization by mutualistic fungi on the same trees demonstrates that at low stress these two communities respond differently, but at high stress both are negatively affected. These results suggest that the effect of extreme climatic events such as drought on foundation tree species are likely to decrease multi-trophic diversity and shift arthropod community composition, which in turn could cascade to affect other associated taxa.     

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.     

Impact of exotic invasion on the temporal stability of natural annual plant communities

Much work in ecology has focused on understanding how changes in community diversity and composition will affect the temporal stability of communities (the degree of fluctuations in community abundance or biomass over time). While theory suggests diversity and dominant species can enhance temporal stability, empirical work has tended to focus on testing the effect of diversity, often using synthetic communities created with high species evenness. We use a complementary approach by studying the temporal stability of natural plant communities invaded by a dominant exotic, Erodium cicutarium. Invasion was associated with a significant decline in community diversity and change in the identity of the dominant species allowing us to evaluate predictions about how these changes might affect temporal stability. Community temporal stability was not correlated with community richness or diversity prior to invasion. Following invasion, community stability was again not correlated with community richness but was negatively correlated with community diversity. Before and after invasion, community stability was positively correlated with the stability of the most dominant species in the community, even though the identity of the dominant species changed from a native (prior to invasion) to an exotic species. Our results demonstrate that invasion by a dominant exotic species may reduce diversity without negatively affecting the temporal stability of natural communities. These findings add support to the idea that dominant species can strongly affect temporal stability, independent of community diversity.     

Robustness to thermal variability differs along a latitudinal gradient in zooplankton communities

Determining how thermal variability will affect the structure, stability, and function of ecological communities is becoming increasingly important as global warming is predicted to affect not only average temperatures but also increase the frequency of long runs of high temperatures. Latitudinal differences in the responses of ecological communities to changes in their thermal regimes have also been predicted based on adaptations over evolutionary time to different thermal environments. We conducted an experiment to determine whether variability in temperature leads to consistent changes in community structure, temporal dynamics, and ecosystem functioning in laboratory analogues of natural freshwater supralittoral rock pool communities inhabited by meiofauna and zooplankton collected from sub‐Arctic, temperate, and tropical regions. Thermal variability of +4 °C around mean temperature led to increased extinction frequency, decreases in consumer abundance, increases in temporal variability of consumer abundance, and shifts from predominately negative interactions observed under constant temperature to positive interactions in the temperate and tropical communities but not in the sub‐Arctic communities. That sub‐Arctic zooplankton communities may be more robust to thermal variability than temperate or tropical communities’ supports recent studies on macrophysiological adaptations of species along latitudinal gradients and suggests that increasing thermal variability may have the greatest effects on community structure and function in tropical and temperate regions.     

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.     

Native versus exotic community patterns across three scales: Roles of competition,environment and incomplete invasion

Three fundamental, interrelated questions in invasion ecology are: (1) to what extent do exotic species outcompete natives; (2) are native and exotic communities functionally similar or different; and (3) are differences in biogeographic patterns in native and exotic communities due to incomplete invasions among exotics? These questions are analogous to general questions in community ecology regarding the relative roles of competition, environmental response and dispersal limitation in community assembly. We addressed each of these questions for plant communities in discrete meadow patches, using analyses at three scales ranging from the landscape to microsites. A weak positive relationship between native and exotic species richness in microsites, and a predominance of positive correlations in abundance among native and exotic species pairs suggest that competition has been less important than other factors in determining native versus exotic abundance and community composition. In contrast, models of species richness and community compositional change across scales suggest native versus exotic community patterns are largely determined by a mix of scale-dependent concordant (shared positive or negative) and discordant relationships with environmental variables. In addition, detailed analyses of species-area and species-abundance relationships suggest ongoing expansion of exotic species populations, indicating that the assembly of the exotic community is in its early stages. Thus, while competition does not appear to strongly affect native versus exotic abundances and compositions at present, it may intensify in the future. Our results indicate that synoptic patterns in native versus exotic richness that have been previously attributed to a single cause may in fact be due to a complex mix of concordant and discordant responses to environmental factors across scales. They also suggest that conservation efforts aimed at promoting natives and reducing exotics should focus on the factors and scales for which such a response (i.e., promotion of high native and low exotic richness) can be expected.     

Wild bee community change over a 26‐year chronosequence of restored tallgrass prairie

Restoration efforts often focus on plants, but additionally require the establishment and long‐term persistence of diverse groups of nontarget organisms, such as bees, for important ecosystem functions and meeting restoration goals. We investigated long‐term patterns in the response of bees to habitat restoration by sampling bee communities along a 26‐year chronosequence of restored tallgrass prairie in north‐central Illinois, U.S.A. Specifically, we examined how bee communities changed over time since restoration in terms of (1) abundance and richness, (2) community composition, and (3) the two components of beta diversity, one‐to‐one species replacement, and changes in species richness. Bee abundance and raw richness increased with restoration age from the low level of the pre‐restoration (agricultural) sites to the target level of the remnant prairie within the first 2–3 years after restoration, and these high levels were maintained throughout the entire restoration chronosequence. Bee community composition of the youngest restored sites differed from that of prairie remnants, but 5–7 years post‐restoration the community composition of restored prairie converged with that of remnants. Landscape context, particularly nearby wooded land, was found to affect abundance, rarefied richness, and community composition. Partitioning overall beta diversity between sites into species replacement and richness effects revealed that the main driver of community change over time was the gradual accumulation of species, rather than one‐to‐one species replacement. At the spatial and temporal scales we studied, we conclude that prairie restoration efforts targeting plants also successfully restore bee communities.     

Population synchrony decreases with richness and increases with environmental fluctuations in an experimental metacommunity

Fluctuations of local but connected populations may show correlation or synchrony whenever they experience significant dispersal or correlated environmental biotic and abiotic variability. Synchrony may be an important variable in multispecies systems, but its nature and implications have not been explicitly examined. Because the number of locally coexisting species (richness) affects the population variability of community members, we manipulated richness under different regimes of environmental fluctuation (EF). We predicted that the temporal synchrony of populations in a species should decline with increasing richness of the metacommunity they live in. Additionally, we predicted that specialist species that are sensitive to a specific environmental factor would show higher synchronization when EF increases. We thus created experimental communities with varied richness, EF, and species specialization to examine the synchronizing effects of these factors on three aquatic invertebrate species. We created four levels of richness and three levels of EF by manipulating the salinity of the culture media. Monocultures exhibited higher population synchrony than metacommunities of 2–4 species. Furthermore, we found that species responded differently to EF treatments: high EF enhanced population synchrony for the specialist and intermediate species, but not for the generalist species. Our findings emphasize that the magnitude of EF and species richness both contribute to determine population synchrony, and importantly, our results suggest that biotic diversity may actually stabilize metacommunities by disrupting synchrony.     

Ecological realism and mechanisms by which diversity begets stability

We investigated how ecological realism might impact the outcome of three experimental manipulations of species richness to determine whether the patterns and the mechanisms underlying richness–variability relationships differ as ecological communities are increasingly exposed to external forces that may drive richness–variability patterns in nature. To test for such an effect, we conducted experiments using rock pool meio‐invertebrate communities housed in three experimental venues: controlled laboratory microcosms, artificially constructed rock pools in the field, and naturally occurring rock pools in the field. Our results showed that experimental venue can have a strong effect on the outcome of richness manipulation experiments. As ecological realism increased, the strength of the relationship between species richness and community variability declined from 32.9% in the laboratory microcosms to 16.8% in the artificial pools to no effect of species richness on community variability in the natural rock pools. The determinants of community variability also differed as ecological realism increased. In laboratory microcosms, community variability was driven solely by mechanisms related to increasing species richness. In artificial rock pools, community variability was driven by a combination of direct and indirect environmental factors as well as mechanisms related to increasing species richness. In the natural rock pools community variability was independent of species richness and was only related to environmental factors. In summary, we found that stabilizing mechanisms associated with species interactions were influential in establishing species richness–variability relations only in the less realistic experimental venues (the laboratory microcosms and the artificial rock pools in the field), and that these mechanisms diminished in importance as ecological realism and complexity of the experimental venue increased. Our results suggest that the effects of diversity might be more difficult to detect in natural systems due to the combined effects of biotic and abiotic forcing, which can mask our ability to detect richness effects.     

Temporal patterns of diversity, abundance and evenness for invertebrate communities from coastal freshwater and brackish water rock pools

Aquatic invertebrate data were collected from 49 erosional, Jamaican,rock pools between 1989 and 1998 and used to describe temporal patterns ofspecies diversity. This unique series of pools on the north coast of Jamaica,classified as either brackish (31) or freshwater (18), was used to determinehowdiversity changes over time, whether there was a difference between poolclassifications, and the impacts of environmental variables. Mean communitymetrics (richness, diversity, evenness, abundance) were not significantlydifferent between freshwater and brackish pools. However, there weresignificantdifferences among the eight sampling dates and differences over time dependedonpool classification. Measures of diversity for freshwater pools were relativelyconstant over time, implying little change at the community level. Brackishpools showed significant differences over time in species richness, totalabundance, and evenness implying that community composition and structure werenot static but changed in response to either environmental or biotic changes(possibly initiated by environmental change).Some temporal changes in community metrics could be linked to temporal changesin environmental variables. In brackish water pools, a significant increase inpool salinity between January 1991 and January 1992 corresponded to an increasein species richness, likely due to an increase in marine fauna. Similarly,changes in abundance and evenness corresponded to changes in temperature,dissolved oxygen, and pH. In addition, physicochemical variables used in thisstudy were shown to affect community metrics and those relationships dependedonpool classification. Most relationships between community metrics andenvironmental variables were negative with the exception of Simpson's diversityindex for which positive relationships were found. This may indicate that, aspool conditions become less favorable, a few species flourish and dominate thecommunity.     

Partitioning the effects of species loss on community variability using multi‐level selection theory

The temporal variability of ecological communities may depend on species richness and composition due to a variety of statistical and ecological mechanisms. However, ecologists currently lack a general, unified theoretical framework within which to compare the effects of these mechanisms. Developing such a framework is difficult because community variability depends not just on how species vary, but also how they covary, making it unclear how to isolate the contributions of individual species to community variability. Here I develop such a theoretical framework using the multi‐level Price equation, originally developed in evolutionary biology to partition the effects of group selection and individual selection. I show how the variability of a community can be related to the properties of the individual species comprising it, just as the properties of an evolving group can be related to the properties of the individual organisms comprising it. I show that effects of species loss on community variability can be partitioned into effects of species richness (random loss of species), effects of species composition (non‐random loss of species with respect to their variances and covariances), and effects of context dependence (post‐loss changes in species’ variances and covariances). I illustrate the application of this framework using data from the Biodiversity II experiment, and show that it leads to new conceptual and empirical insights. For instance, effects of species richness on community variability necessarily occur, but often are swamped by other effects, particularly context dependence.     

Patterns of temporal community turnover are spatially synchronous across boreal lakes

1. Ecosystems are often exposed to broad‐scale environmental change, which can potentially synchronise community dynamics and biodiversity trends. Detection of temporal coherence may, however, depend on the metrics used and their sensitivity to detect change, requiring several lines of evidence to elucidate the full range of temporal responses to environmental change. 2. Here, we tested whether the patterns of synchrony among littoral invertebrate communities of Swedish lakes over 20 years (1988–2007) differed when analysed using univariate (taxon richness, evenness, Shannon diversity and total abundance) or multivariate (temporal turnover in community composition) metrics. We included both culturally acidified and circumneutral lakes to examine whether anthropogenic stress influenced the patterns of synchrony. 3. Average total abundance, taxon richness and temporal turnover in community composition changed monotonically with time, while evenness and Shannon diversity fluctuated around a long‐term mean. However, among‐lake variability was high, resulting in a weak temporal coherence. Only trends of temporal turnover changed synchronously across lakes, irrespective of their acidification history. 4. Spatially synchronous trends in turnover across lakes were correlated with increasing water colour and decreasing sulphate concentrations, showing the importance of regional drivers of spatiotemporal coherence. 5. Our results underpin an increasing body of evidence that the detection of diversity patterns varies among metrics that ignore (taxon richness, evenness, Shannon diversity) or consider (turnover) species identities. More generally, our results suggest that community‐level studies of synchrony are suitable for elucidating the role of intrinsic versus extrinsic factors in mediating complex community assembly processes in the long term. This, in turn, contributes to our understanding of temporal patterns of biodiversity.