Understanding diversity–stability relationships: towards a unified model of portfolio effects

A major ecosystem effect of biodiversity is to stabilise assemblages that perform particular functions. However, diversity–stability relationships (DSRs) are analysed using a variety of different population and community properties, most of which are adopted from theory that makes several restrictive assumptions that are unlikely to be reflected in nature. Here, we construct a simple synthesis and generalisation of previous theory for the DSR. We show that community stability is a product of two quantities: the synchrony of population fluctuations, and an average species‐level population stability that is weighted by relative abundance. Weighted average population stability can be decomposed to consider effects of the mean‐variance scaling of abundance, changes in mean abundance with diversity and differences in species' mean abundance in monoculture. Our framework makes explicit how unevenness in the abundances of species in real communities influences the DSR, which occurs both through effects on community synchrony, and effects on weighted average population variability. This theory provides a more robust framework for analysing the results of empirical studies of the DSR, and facilitates the integration of findings from real and model communities.     

Synchrony in small mammal community dynamics across a forested landscape

Long‐term studies at local scales indicate that fluctuations in abundance among trophically similar species are often temporally synchronized. Complementary studies on synchrony across larger spatial extents are less common, as are studies that investigate the subsequent impacts on community dynamics across the landscape. We investigate the impact of species population fluctuations on concordance in community dynamics for the small mammal fauna of the White Mountain National Forest, USA. Hierarchical open population models, which account for imperfect detection, were used to model abundance of the most common species at 108 sites over a three year period. Most species displayed individualistic responses of abundance to forest type and physiographic characteristics. However, among species, we found marked synchrony in population fluctuations across years, regardless of landscape affinities or trophic level. Across the region, this population synchrony led to high within‐year concordance of community composition and aggregate properties (e.g. richness and diversity) independent of forest type and low among‐year similarity in communities, even for years with similar species richness. Results suggest that extrinsic factors primarily drive abundance fluctuations and subsequently community dynamics, although local community assembly may be modified by species dispersal abilities and biotic interactions. Concordant community dynamics across space and over time may impact the stability of regional food webs and ecosystem functions.     

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.     

Distance decay of similarity,effects of environmental noise and ecological heterogeneity among species in the spatio‐temporal dynamics of a dispersal‐limited community

The joint spatial and temporal fluctuations in community structure may be due to dispersal, variation in environmental conditions, ecological heterogeneity among species and demographic stochasticity. These factors are not mutually exclusive, and their relative contribution towards shaping species abundance distributions and in causing species fluctuations have been hard to disentangle. To better understand community dynamics when the exchange of individuals between localities is very low, we studied the dynamics of the freshwater zooplankton communities in 17 lakes located in independent catchment areas, sampled at end of summer from 2002 to 2008 in Norway. We analysed the joint spatial and temporal fluctuations in the community structure by fitting the two‐dimensional Poisson lognormal model under a two‐stage sampling scheme. We partitioned the variance of the distribution of log abundance for a random species at a random time and location into components of demographic stochasticity, ecological heterogeneity among species, and independent environmental noise components for the different species. Non‐neutral mechanisms such as ecological heterogeneity among species (20%) and spatiotemporal variation in the environment (75%) explained the majority of the variance in log abundances. Overdispersion relative to Poisson sampling and demographic stochasticity had a small contribution to the variance (5%). Among a set of environmental variables, lake acidity was the environmental variable that was most strongly related to decay of community similarity in space and time.     

Consumer‐driven nutrient dynamics in freshwater ecosystems: from individuals to ecosystems

The role of animals in modulating nutrient cycling [hereafter, consumer‐driven nutrient dynamics (CND)] has been accepted as an important influence on both community structure and ecosystem function in aquatic systems. Yet there is great variability in the influence of CND across species and ecosystems, and the causes of this variation are not well understood. Here, we review and synthesize the mechanisms behind CND in fresh waters. We reviewed 131 articles on CND published between 1973 and 1 June 2015. The rate of new publications in CND has increased from 1.4 papers per year during 1973–2002 to 7.3 per year during 2003–2015. The majority of investigations are in North America with many concentrating on fish. More recent studies have focused on animal‐mediated nutrient excretion rates relative to nutrient demand and indirect impacts (e.g. decomposition). We identified several mechanisms that influence CND across levels of biological organization. Factors affecting the stoichiometric plasticity of consumers, including body size, feeding history and ontogeny, play an important role in determining the impact of individual consumers on nutrient dynamics and underlie the stoichiometry of CND across time and space. The abiotic characteristics of an ecosystem affect the net impact of consumers on ecosystem processes by influencing consumer metabolic processes (e.g. consumption and excretion/egestion rates), non‐CND supply of nutrients and ecosystem nutrient demand. Furthermore, the transformation and transport of elements by populations and communities of consumers also influences the flow of energy and nutrients across ecosystem boundaries. This review highlights that shifts in community composition or biomass of consumers and eco‐evolutionary underpinnings can have strong effects on the functional role of consumers in ecosystem processes, yet these are relatively unexplored aspects of CND. Future research should evaluate the value of using species traits and abiotic conditions to predict and understand the effects of consumers on ecosystem‐level nutrient dynamics across temporal and spatial scales. Moreover, new work in CND should strive to integrate knowledge from disparate fields of ecology and environmental science, such as physiology and ecosystem ecology, to develop a comprehensive and mechanistic understanding of the functional role of consumers. Comparative and experimental studies that develop testable hypotheses to challenge the current assumptions of CND, including consumer stoichiometric homeostasis, are needed to assess the significance of CND among species and across freshwater ecosystems.     

Temporal variability of forest communities: empirical estimates of population change in 4000 tree species

Long‐term surveys of entire communities of species are needed to measure fluctuations in natural populations and elucidate the mechanisms driving population dynamics and community assembly. We analysed changes in abundance of over 4000 tree species in 12 forests across the world over periods of 6–28 years. Abundance fluctuations in all forests are large and consistent with population dynamics models in which temporal environmental variance plays a central role. At some sites we identify clear environmental drivers, such as fire and drought, that could underlie these patterns, but at other sites there is a need for further research to identify drivers. In addition, cross‐site comparisons showed that abundance fluctuations were smaller at species‐rich sites, consistent with the idea that stable environmental conditions promote higher diversity. Much community ecology theory emphasises demographic variance and niche stabilisation; we encourage the development of theory in which temporal environmental variance plays a central role.     

The Effects of Interannual Rainfall Variability on Tree–Grass Composition Along Kalahari Rainfall Gradient

Precipitation variability has been predicted to increase in a global warmer climate, and is expected to greatly affect plant growth, interspecies interactions, plant community composition, and other ecosystem processes. Although previous studies have investigated the effect of intra-annual rainfall variability on plant growth and ecosystem dynamics, the impacts of interannual rainfall variability remain understudied. This paper uses satellite data and develops a new mechanistic model to investigate the response of tree–grass composition to increasing interannual rainfall variability in arid to sub-humid ecosystems along the Kalahari Transect in Southern Africa. Both satellite data and model results show that increasing interannual rainfall fluctuations favor deep-rooted trees over shallow-rooted grasses in drier environments (that is, mean annual rainfall, MAP < 900–1000 mm) but favor grasses over trees in wetter environments (that is, MAP > 900–1000 mm). Trees have a competitive advantage over grasses in dry environments because their generally deeper root systems allow them to have exclusive access to the increased deep soil water resources expected to occur in wet years as a result of the stronger interannual rainfall fluctuations. In relatively wet environments, grasses are favored because of their high growth rate that allows them to take advantage of the window of opportunity existing in years with above average precipitation and thus increase fire-induced tree mortality. Thus, under increasing interannual rainfall fluctuations both direct effects on soil water availability and indirect effects mediated by tree–grass interactions and fire dynamics are expected to play an important role in determining changes in plant community composition.     

Grasshopper (Orthoptera: Acrididae) communities respond to fire,bison grazing and weather in North American tallgrass prairie: a long-term study

Because both intrinsic and extrinsic factors influence insect population dynamics, operating at a range of temporal and spatial scales, it is difficult to assess their contributions. Long-term studies are ideal for assessing the relative contributions of multiple factors to abundance and community dynamics. Using data spanning 25 years, we investigate the contributions of weather at annual and decadal scales, fire return interval, and grazing by bison to understand the dynamics of abundance and community composition in grasshopper assemblages from North American continental grassland. Each of these three primary drivers of grassland ecosystem dynamics affects grasshopper population and community dynamics. Negative feedbacks in abundances, as expected for regulated populations, were observed for all feeding guilds of grasshoppers. Abundance of grasshoppers did not vary in response to frequency of prescribed burns at the site. Among watersheds that varied with respect to controlled spring burns and grazing by bison, species composition of grasshopper assemblages responded significantly to both after 25 years. However, after more than 20 years of fire and grazing treatments, the number of years since the last fire was more important than the managed long-term fire frequency per se. Yearly shifts in species composition (1983–2005), examined using non-metric multidimensional scaling and fourth-corner analysis, were best explained by local weather events occurring early in grasshopper life cycles. Large-scale patterns were represented by the Palmer Drought Severity Index and the North Atlantic Oscillation (NAO). The NAO was significantly correlated with annual mean frequencies of grasshoppers, especially for forb- and mixed-feeding species. Primary grassland drivers—fire, grazing and weather—contributing both intrinsic and extrinsic influences modulate long-term fluctuations in grasshopper abundances and community taxonomic composition. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Hydrologic variability in dryland regions: impacts on ecosystem dynamics and food security

Research on ecosystem and societal response to global environmental change typically considers the effects of shifts in mean climate conditions. There is, however, some evidence of ongoing changes also in the variance of hydrologic and climate fluctuations. A relatively high interannual variability is a distinctive feature of the hydrologic regime of dryland regions, particularly at the desert margins. Hydrologic variability has an important impact on ecosystem dynamics, food security and societal reliance on ecosystem services in water-limited environments. Here, we investigate some of the current patterns of hydrologic variability in drylands around the world and review the major effects of hydrologic fluctuations on ecosystem resilience, maintenance of biodiversity and food security. We show that random hydrologic fluctuations may enhance the resilience of dryland ecosystems by obliterating bistable deterministic behaviours and threshold-like responses to external drivers. Moreover, by increasing biodiversity and the associated ecosystem redundancy, hydrologic variability can indirectly enhance post-disturbance recovery, i.e. ecosystem resilience.     

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.     

Population fluctuations, regulation and limitation in stream-living brown trout

Determining causes of variation in population size and identifying factors responsible for fluctuations in species abundance are crucial questions both in theoretical and applied ecology. Based on the analysis of abundance time series, many studies have concluded that population dynamics of the stream-living brown trout ( Salmo trutta L.) are mainly driven by year-to-year variation in the discharge level during emergence. Endogenous regulatory processes have often been considered as weak explanations for these fluctuations. This led some authors to consider that brown trout was able to persist in time with no operation of density-dependent processes. Using a model of population dynamics, we studied the influence of both discharge level during emergence and density-dependent regulatory processes on population limitation and fluctuations. We show that density-dependent and density-independent processes can act together on population density and stability at equilibrium (limitation process). We also show that the effects of internal feedbacks regulating population may often be invisible when analyzing abundance fluctuations at the interannual scale. Our results question the accuracy of studies based on the analysis of interannual fluctuations in abundance to identify processes driving population density at equilibrium.     

Seasonal and interannual variation in abundance of Calanus finmarchicus (Gunnerus) and Calanus helgolandicus (Claus) in inshore waters (west coast of the Isle of Man) in the central Irish Sea

The seasonal and interannual variability in abundance of Calanusfinmarchicus and Calanus helgolandicus in the Irish Sea wasexamined on the west side of the Isle of Man (central IrishSea) between 1995 and 2001. There was considerable interannualvariability in abundance of both species that was greater thanthat seen in adjacent seas. In general, maximum abundance occurredbetween May and July each year with C. finmarchicus tendingto occur earlier in the year than C. helgolandicus. There wasa significant difference in interannual maximum abundances inthe area (14–18 stations treated as replicates) with mostyears having a low abundance and one year a relatively highabundance. The maximum abundance of C. finmarchicus (1996) coincidedwith a low (negative) index value of the North Atlantic Oscillation(NAO), and with a high abundance of young gadoids (cod, haddockand whiting) in the water column and a large settlement of plaice.A high abundance of C. helgolandicus coincided with a relativelyhigh (positive) index value of the NAO in 2000. The magnitudeof interannual and seasonal variability in abundance was inclose agreement with data from an inshore area (Port Erin Bay,Isle of Man) between 1907 and 1920. As with many Calanus populations,the adults in both species were predominantly female. As shownpreviously, Calanus is not one of the numerically dominant copepodspecies in the Irish Sea. However, the presence of young stagesof Calanus within the central Irish Sea suggests that it mustsupport a breeding population.     

川西平原农田啮齿动物群落动态：年间变动和季节变动

为了研究农田啮齿动物群落的动态规律,用标志重捕的方法给出了川西平原农田的啮齿动物群落的物种数、结合种群密度、生物量、物种多样性的Shannon指数与Simpson指数等5个变量的时间序列资料,并分析了这些时间序列的年间变动和季节变动的特点。结果表明：（1）5个变量总是处于不断的变化之中,特别是年间交替升降十分明显;（2）群落的全部8个物种从未同时出现过,同一个月最多有5个物种,最少只有1个物种;（3）5个变量季节变动的幅度均较大并各具有1至3个不等的明显的峰值,而年间变动的幅度则相对较小;（4）5个变量的最大值均出现在夏季,最小值则出现在春季、夏季或冬季;（5）5个变量的季节性均不强;（6）优势种大足鼠（Rattus nitidus)的种群密度分别与群落结合种群密度、生物量的年间变动和季节变动均具有相似的变动规律;此外,优势种大足鼠的种群密度、生物量分别占群落结合种群密度、生物量的比例均较高,而且这两个比例季节变动的幅度都较大而年间变动则较为稳定。     

Mean conditions predict salt marsh plant community diversity and stability better than environmental variability

Environmental variability and the frequency of extreme events are predicted to increase in future climate scenarios; however, the role of fluctuations in shaping community composition, diversity and stability is not well understood. Identifying current patterns of association between measures of community stability and climatic means and variability will help elucidate the ways in which altered variability and mean conditions may change communities in the future. Salt marshes provide essential ecosystem services and are increasingly threatened by sea‐level rise, land‐use change, eutrophication and predator loss, yet the effects of temporal environmental variation on salt marshes remain unknown. We synthesized long‐term plant community monitoring data from 11 sites on both coasts of the United States. We used an information‐theoretic approach and linear models to determine the associations among long‐term mean conditions, interannual environmental variability, and plant community stability and diversity. We found that salt marsh community stability and diversity were more strongly related to long‐term means of temperature and precipitation than to interannual variation. Warm and wet environments had fewer species and less turnover among years. Our results suggest that communities in cool, dry environments may be more resilient to climate warming due to greater species richness and turnover. Mean conditions are sufficient to predict contemporary patterns of salt marsh plant community dynamics, but environmental variability may have stronger impacts as it increases with climate change.     

Bottom-up effects of species diversity on the functioning and stability of food webs

1. The importance of species diversity for the stability of populations, communities and ecosystem functions is a central question in ecology. 2. Biodiversity experiments have shown that diversity can impact both the average and variability of stocks and rates at these levels of ecological organization in single trophic-level ecosystems. Whether these impacts hold in food webs and across trophic levels is still unclear. 3. We asked whether resource species diversity, community composition and consumer feeding selectivity in planktonic food webs impact the stability of resource or consumer populations, community biomass and ecosystem functions. We also tested the relative importance of resource diversity and community composition. 4. We found that resource diversity negatively affected resource population stability, but had no effect on consumer population stability, regardless of the consumer's feeding selectivity. Resource diversity had positive effects on most ecosystem functions and their stability, including primary production, resource biomass and particulate carbon, nitrogen and phosphorus concentrations. 5. Community composition, however, generally explained more variance in population, community and ecosystem properties than species diversity per se. This result points to the importance of the outcomes of particular species interactions and individual species' effect traits in determining food web properties and stability. 6. Among the stabilizing mechanisms tested, an increase in the average resource community biomass with increasing resource diversity had the greatest positive impact on stability. 7. Our results indicate that resource diversity and composition are generally important for the functioning and stability of whole food webs, but do not have straightforward impacts on consumer populations.     

Differing contributions of density dependence and climate to the population dynamics of three eruptive herbivores

1. Although both endogenous and exogenous processes regulate populations, the current understanding of the contributions from density dependence and climate to the population dynamics of eruptive herbivores remains limited. 2. Using a 17‐year time series of three cereal aphid species [Rhopalosiphum padi L., Metopolophium dirhodum (Walker), and Diuraphis noxia (Kurdumov)] compiled from a trapping network spanning the northwestern U.S.A., temporal and spatial patterns associated with population fluctuations, and modelled density dependence in aphid abundances were tested. These models were used to analyse correlations between climate and aphid abundances in the presence and absence of residual variance as a result of density‐dependent effects. 3. The temporal dynamics of aphid population fluctuations indicated periodicity, with no clear evidence for a spatial pattern underlying population fluctuations. 4. Aphid abundances oscillated in a manner consistent with delayed density dependence for all three aphid species, although the strength of these feedbacks differed among species. 5. Diuraphis noxia abundances were negatively correlated with increasing temperatures in the absence of density‐dependent effects, whereas M. dirhodum abundances were positively correlated with increasing cumulative precipitation in the presence of density‐dependent effects; yet, R. padi abundances were unrelated to climate variables irrespective of population feedbacks. 6. Our analysis suggests that endogenous feedbacks differentially regulate aphid populations in the northwestern U.S.A., and these feedbacks may operate at an expansive spatial scale. It is concluded that the contributions of density dependence and climate to aphid population dynamics are species‐specific in spite of similar ecological niches, with implications for assessing species responses to climate variability.     

Reduced variability in range‐edge butterfly populations over three decades of climate warming

Populations at the high latitude edge of species’ geographical ranges are thought to show larger interannual population fluctuations, with subsequent higher local extinction risk, than those within the ‘core’ climatic range. As climate envelopes shift northward under climate warming, however, we would expect populations to show dampened variability. We test this hypothesis using annual abundance indices from 19 butterfly species across 79 British monitoring sites between 1976 and 2009, a period of climatic warming. We found that populations in the latter (warmer) half of the recording period show reduced interannual population variability. Species with more southerly European distributions showed the greatest dampening in population variability over time. Our results suggest that increases in population variability occur towards climatic range boundaries. British sites, previously existing at the margins of suitable climate space, now appear to fall closer to the core climatic range for many butterfly species.     

Interannual bumble bee abundance is driven by indirect climate effects on floral resource phenology

Climate change can influence consumer populations both directly, by affecting survival and reproduction, and indirectly, by altering resources. However, little is known about the relative importance of direct and indirect effects, particularly for species important to ecosystem functioning, like pollinators. We used structural equation modelling to test the importance of direct and indirect (via floral resources) climate effects on the interannual abundance of three subalpine bumble bee species. In addition, we used long‐term data to examine how climate and floral resources have changed over time. Over 8 years, bee abundances were driven primarily by the indirect effects of climate on the temporal distribution of floral resources. Over 43 years, aspects of floral phenology changed in ways that indicate species‐specific effects on bees. Our study suggests that climate‐driven alterations in floral resource phenology can play a critical role in governing bee population responses to global change.     

Analyzing spatial structure of communities using the two-dimensional poisson lognormal species abundance model

The joint spatial and temporal fluctuations in the community structure of tropical butterflies are analyzed by fitting the bivariate Poisson lognormal distribution to a large number of observations in space and time. By applying multivariate dependent diffusions for describing the fluctuations in the abundances, the environmental variance is estimated to be very large and so is the strength of local density regulation. The variance in the lognormal species abundance distribution is partitioned into components expressing the heterogeneity between the species, independent noise components for the different species, a demographic stochastic component, and a component due to overdispersion in the sampling. In disagreement with the neutral theory, the estimates show that the heterogeneity component is the dominating one, representing 81% of the total variance in the lognormal model. Different spatial components of diversity, the alpha, beta, and gamma diversity, are also estimated. The spatial scale of the autocorrelation function for the community is of order 1 km, while sampling of a quadrat would need to be 10 km on a side to yield the total diversity for the community.     

Population dynamical consequences of gregariousness in a size-structured consumer-resource interaction

Many animal species live in groups. Group living may increase exploitation competition within the group, and variation among groups in intra-group competition intensity could induce life-history variability among groups. Models of physiologically structured populations generally predict single generation cycles, driven by exploitation competition within and between generations. We expect that life-history variability and habitat heterogeneity induced by group living may affect such competition-driven population dynamics. In this study, we vary the gregariousness (the tendency to aggregate in groups) of a size-structured consumer population in a spatially explicit environment. The consumer has limited mobility, and moves according to a probabilistic movement process. We study the effects on the population dynamics, as mediated through the resource and the life-history of the consumer. We find that high gregariousness leads to large spatial resource variation, and highly variable individual life-history, resulting in highly stochastic population dynamics. At reduced gregariousness, life-history of consumers synchronizes, habitat heterogeneity is reduced, and single generation cycles appear. We expect this pattern to occur for any group living organism with limited mobility. Our results indicate that constraints set by population dynamical feedback may be an important aspect in understanding group living in nature.