Takehiro Sasaki

Response diversity determines the resilience of ecosystems to environmental change.

A growing body of evidence highlights the importance of biodiversity for ecosystem stability and the maintenance of optimal ecosystem functionality. Conservation measures are thus essential to safeguard the ecosystem services that biodiversity provides and human society needs. Current anthropogenic threats may lead to detrimental (and perhaps irreversible) ecosystem degradation, providing strong motivation to evaluate the response of ecological communities to various anthropogenic pressures. In particular, ecosystem functions that sustain key ecosystem services should be identified and prioritized for conservation action. Traditional diversity measures (e.g. ‘species richness’) may not adequately capture the aspects of biodiversity most relevant to ecosystem stability and functionality, but several new concepts may be more appropriate. These include ‘response diversity’, describing the variation of responses to environmental change among species of a particular community. Response diversity may also be a key determinant of ecosystem resilience in the face of anthropogenic pressures and environmental uncertainty. However, current understanding of response diversity is poor, and we see an urgent need to disentangle the conceptual strands that pervade studies of the relationship between biodiversity and ecosystem functioning. Our review clarifies the links between response diversity and the maintenance of ecosystem functionality by focusing on the insurance hypothesis of biodiversity and the concept of functional redundancy. We provide a conceptual model to describe how loss of response diversity may cause ecosystem degradation through decreased ecosystem resilience. We explicitly explain how response diversity contributes to functional compensation and to spatio‐temporal complementarity among species, leading to long‐term maintenance of ecosystem multifunctionality. Recent quantitative studies suggest that traditional diversity measures may often be uncoupled from measures (such as response diversity) that may be more effective proxies for ecosystem stability and resilience. Certain conclusions and recommendations of earlier studies using these traditional measures as indicators of ecosystem resilience thus may be suspect. We believe that functional ecology perspectives incorporating the effects and responses of diversity are essential for development of management strategies to safeguard (and restore) optimal ecosystem functionality (especially multifunctionality). Our review highlights these issues and we envision our work generating debate around the relationship between biodiversity and ecosystem functionality, and leading to improved conservation priorities and biodiversity management practices that maximize ecosystem resilience in the face of uncertain environmental change.

A global meta‐analysis of the relative extent of intraspecific trait variation in plant communities

Recent studies have shown that accounting for intraspecific trait variation (ITV) may better address major questions in community ecology. However, a general picture of the relative extent of ITV compared to interspecific trait variation in plant communities is still missing. Here, we conducted a meta-analysis of the relative extent of ITV within and among plant communities worldwide, using a data set encompassing 629 communities (plots) and 36 functional traits. Overall, ITV accounted for 25% of the total trait variation within communities and 32% of the total trait variation among communities on average. The relative extent of ITV tended to be greater for whole-plant (e.g. plant height) vs. organ-level traits and for leaf chemical (e.g. leaf N and P concentration) vs. leaf morphological (e.g. leaf area and thickness) traits. The relative amount of ITV decreased with increasing species richness and spatial extent, but did not vary with plant growth form or climate. These results highlight global patterns in the relative importance of ITV in plant communities, providing practical guidelines for when researchers should include ITV in trait-based community and ecosystem studies.

Dominant species, rather than diversity, regulates temporal stability of plant communities

A growing body of empirical evidence suggests that the temporal stability of communities typically increases with diversity. The counterview to this is that dominant species, rather than diversity itself, might regulate temporal stability. However, empirical studies that have explicitly examined the relative importance of diversity and dominant species in maintaining community stability have yielded few clear-cut patterns. Here, using a long-term data set, we examined the relative importance of changes in diversity components and dominance hierarchy following the removal of a dominant C4 grass, Bouteloua gracilis, in stabilizing plant communities. We also examined the relationships between the variables of diversity and dominance hierarchy and the statistical components of temporal stability. We found a significant negative relationship between temporal stability and species richness, number of rare species, and relative abundance of rare species, whereas a significant positive relationship existed between temporal stability and relative abundance of the dominant species. Variances and covariances summed over all species significantly increased with increasing species richness, whereas they significantly decreased with increasing relative abundance of dominant species. We showed that temporal stability in a shortgrass steppe plant community was controlled by dominant species rather than by diversity itself. The generality of diversity–stability relationships might be restricted by the dynamics of dominant species, especially when they have characteristics that contribute to stability in highly stochastic systems. A clear implication is that dominance hierarchies and their changes might be among the most important ecological components to consider in managing communities to maintain ecosystem functioning.

Indicator species and functional groups as predictors of proximity to ecological thresholds in Mongolian rangelands

We focused on responses to grazing by individual species and functional groups in relation to ecological thresholds in Mongolian rangelands, with repeated measures from the same ecological sites to account for rainfall variability. At all sites, even under rainfall fluctuations, there were robust combinations of indicator species that could be used to forewarn managers to take action to minimize the probability of crossing ecological thresholds. Depending on the landscape condition of each site, the cover of functional groups, which shared traits of perennial life history, grass or forb growth form, linear leaf shape, and alternate leaf attachment, or the cover of functional groups of woody shrubs dramatically decreased before an ecological threshold was crossed. Thus, across all sites, the responses of certain functional groups to grazing appeared to predict the crossing of an ecological threshold. The ecological indicators derived in this study should help to improve land managers’ ability to prevent adverse changes in states before ecological thresholds are reached.

Can edaphic factors demonstrate landscape-scale differences in vegetation responses to grazing?

We focused on land units as landscape characteristics and selected seven typical land units on a land catena comprising two areas of southern Mongolia. Hierarchical analysis was used to test the hypothesis that a land unit’s edaphic factors could explain the differences in vegetation responses to grazing. We established the survey sites at increasing distances from a livestock camp or water point within each land unit, then analysed patterns of change in floristic and functional compositions, vegetation volume and soil properties within each land unit to reveal differences in vegetation responses to grazing. We also examined the variations in floristic and functional compositions across land units to identify the edaphic factors that may underlie these differences. Changes in vegetation and soil properties at increasing distances from a camp or water point within each land unit were into three different patterns. Ordination techniques consistently indicated that land unit groups categorised using edaphic factors corresponded to those categorised using response patterns. Our study revealed that edaphic factors were responsible for the observed landscape-scale differences in vegetation responses to grazing in the study areas. In addition, the mechanisms underlying vegetation responses to grazing may have been primarily determined by edaphic factors.

Sustaining ecosystem functions in a changing world: a call for an integrated approach

With ever-increasing human pressure on ecosystems, it is critically important to predict how ecosystem functions will respond to such human-induced perturbations. We define perturbations as either changes to abiotic environment (e.g. eutrophication, climate change) that indirectly affects biota, or direct changes to biota (e.g. species introductions). While two lines of research in ecology, biodiversity-ecosystem function (BDEF) and ecological resilience (ER) research, have addressed this issue, both fields of research have nontrivial shortcomings in their abilities to address a wide range of realistic scenarios. We outline how an integrated research framework may foster a deeper understanding of the functional consequences of perturbations via simultaneous application of (i) process-based mechanistic predictions using trait-based approaches and (ii) detection of empirical patterns of functional changes along real perturbation gradients. In this context, the complexities of ecological interactions and evolutionary perspectives should be integrated into future research. Synthesis and applications. Management of human-impacted ecosystems can be guided most directly by understanding the response of ecosystem functions to controllable perturbations. In particular, we need to characterize the form of a wide range of perturbation-function relationships and to draw connections between those patterns and the underlying ecological processes. We anticipate that the integrated perspectives will also be helpful for managers to derive practical implications for management from academic literature. Management of human-impacted ecosystems can be guided most directly by understanding the response of ecosystem functions to controllable perturbations. In particular, we need to characterize the form of a wide range of perturbation-function relationships and to draw connections between those patterns and the underlying ecological processes. We anticipate that the integrated perspectives will also be helpful for managers to derive practical implications for management from academic literature.

Robustness of trait distribution metrics for community assembly studies under the uncertainties of assembly processes

Numerous studies have revealed the existence of nonrandom trait distribution patterns as a sign of environmental filtering and/or biotic interactions in a community assembly process. A number of metrics with various algorithms have been used to detect these patterns without any clear guidelines. Although some studies have compared their statistical powers, the differences in performance among the metrics under the conditions close to actual studies are not clear. Therefore, the performances of five metrics of convergence and 16 metrics of divergence under alternative conditions were comparatively analyzed using a suite of simulated communities. We focused particularly on the robustness of the performances to conditions that are often uncertain and uncontrollable in actual studies; e.g., atypical trait distribution patterns stemming from the operation of multiple assembly mechanisms, a scaling of trait-function relationships, and a sufficiency of analyzed traits. Most tested metrics, for either convergence or divergence, had sufficient statistical power to distinguish nonrandom trait distribution patterns without uncertainty. However, the performances of the metrics were considerably influenced by both atypical trait distribution patterns and other uncertainties. Influences from these uncertainties varied among the metrics of different algorithms and their performances were often complementary. Therefore, under the uncertainties of an assembly process, the selection of appropriate metrics and the combined use of complementary metrics are critically important to reliably distinguish nonrandom patterns in a trait distribution. We provide a tentative list of recommended metrics for future studies.

Vulnerability of moorland plant communities to environmental change: consequences of realistic species loss on functional diversity

Understanding the consequences of realistic species loss on the functioning and persistence of vulnerable ecosystems is key to devising conservation strategies when environmental changes are immediate threats. Yet, few studies have provided direct evidence for conservation prioritization and decision-making. We incorporated the quantification of functional diversity based on the Rao index of diversity (FDrao) into this applied context and examined the consequences of realistic species loss on functional diversity in moorland plant communities widely interspersed within a subalpine zone in northern Japan. The realistic order of species loss was derived from the nested subset pattern in the moorland communities, which was corroborated by selective species tolerance and selective extinction. We analysed the relationships between the FDrao half-life, as an index of each moorlands vulnerability to species loss, and a range of environmental variables describing the moorlands. We then mapped this index across the entire landscape. At most sites, ordered species loss caused a relatively small decline in FDrao until a certain number of species was lost and an accelerating decline thereafter, suggesting relatively low initial vulnerability to species loss. At the other sites, however, ordered species loss caused an approximately proportional decrease in FDrao, suggesting relatively high vulnerability to species loss. The model indicated that sites with higher elevation, higher carrying capacities, or increasing isolation have a shorter FDrao half-life. The mapping of this index allowed us to identify the geographical distribution of sites of high conservation priority.Synthesis and applications. We performed fine-scale assessments of the vulnerability of moorland plant communities to species loss, which is likely to occur under future environmental conditions, by simulating the consequences of realistic species loss for functional diversity. The methods used here can provide urgently needed information to support the prioritization and decision-making involved in conserving ecosystems in the face of global biodiversity loss. We performed fine-scale assessments of the vulnerability of moorland plant communities to species loss, which is likely to occur under future environmental conditions, by simulating the consequences of realistic species loss for functional diversity. The methods used here can provide urgently needed information to support the prioritization and decision-making involved in conserving ecosystems in the face of global biodiversity loss.

Vegetation in a Post-Ecological Threshold State May Not Recover after Short-Term Livestock Exclusion in Mongolian Rangelands

In the present study, we tested the potential irreversibility of vegetation dynamics in Mongolian rangelands using well-studied plant communities that exist along grazing gradients, in which ecological thresholds (defined as the points or zones at which disturbance should be limited to prevent drastic changes in ecological conditions) exist in terms of the compositional changes along these gradients. To accomplish this, we removed livestock grazing impacts by establishing exclosures along a grazing gradient at two study sites located in Mandalgobi and Bulgan, Mongolia. Each exclosure was established in the summer of 2004 at a location with either a post-ecological threshold state or a pre-ecological threshold state. We examined general patterns of temporal change in vegetation for the permanent plots inside and outside each exclosure at each site between 2005 and 2010. The trajectories of floristic composition in the permanent plots outside and inside each exclosure were similar from 2005 to 2010, indicating that the trajectories were mainly associated with annual rainfall and annual phenological changes in the plant communities. Post-threshold states at both sites did not reach their respective target community for restoration, indicating the lack of restorability despite livestock exclusion. Moreover, ordination separated the trajectories of floristic composition for the permanent plots inside exclosure in the post-threshold state from those in the pre-threshold state. Thus, our results suggest that vegetation in a post-threshold state may not recover after short-term livestock exclusion in Mongolian rangelands.

Differential responses and mechanisms of productivity following experimental species loss scenarios

Experiments that simulate nonrandom species loss from natural communities can offer a fundamentally different understanding of the impacts of species loss on ecosystem function and their underlying mechanisms compared to seeding experiments where species are randomly assembled from a local species pool. We examined the mechanisms underlying changes in primary productivity following experimental species loss scenarios in Mongolian grassland. The range of species loss scenarios was based on natural patterns of species abundance that reflect the species’ contributions to ecosystem processes. We found a clear reduction in productivity due to species loss only when species were lost randomly. Grassland productivity was relatively robust following nonrandom species loss scenarios. Even in the context of density compensation, the decrease in dominant trait values for leaf height would explain the reduction in productivity with random species loss. In contrast, the maintenance of dominant trait values of key productivity traits such as leaf dry matter content and leaf height might contribute to the maintenance of productivity in response to nonrandom species loss. Our experiment demonstrated that the responses and mechanisms of primary productivity to species loss differ according to the scenarios of species loss in natural grassland communities. The effects of diversity on productivity might be weak in mature natural systems when species loss is nonrandom. Understanding the consequences of realistic species loss on ecosystem functioning based on field-based removal experiments will give insights into real conservation strategies in the face of global biodiversity change.