Predicting the response of communities to climate change is a major challenge for ecology. Communities may well not respond as entities but be disrupted, particularly if trophic levels respond differently, but as yet there is no evidence for differential responses from natural systems. We therefore analyzed unusually detailed plant and animal data collected over 20 years from two grassland communities to determine whether functional group climate sensitivity differed between trophic levels. We found that sensitivity increases significantly with increasing trophic level. This differential sensitivity would lead to community destabilization under climate change, not simple geographical shifts, and consequently must be incorporated in predictive ecological climate models.
Summary1. The consequences of globally declining biodiversity and climate change for ecosystem functions are intensively debated topics in ecological research. However, few studies have investigated potential interactions, or the combined effects of both scenarios, for ecosystem functioning. In the work presented here we tested the hypothesis that increasing plant diversity acts as insurance for ecosystem functions during extreme weather events which are predicted by climate change scenarios. 2. We measured the effect of plant diversity on above-and below-ground productivity in semi-natural grasslands following experimentally induced early summer drought. To test the insurance hypothesis directly, we determined in each community the range of δ 13 C values of individual plant species as drought stress indicators. 3. Increasing plant diversity significantly enhanced below-ground productivity as a consequence of simulated drought, while above-ground productivity was reduced independently of plant diversity. 4. Plants shifting carbon allocation to below-ground compartments during drought maintain various aspects of ecosystem services and functions. Although we were not able to detect physiological evidence for the insurance hypothesis, we conclude from our below-ground results that plant diversity is an essential entity of ecosystems for maintaining ecosystem functions in a changing climate.
Numerous recent studies have tested the effects of plant, pollinator, and predator diversity on primary productivity, pollination, and consumption, respectively. Many have shown a positive relationship, particularly in controlled experiments, but variability in results has emphasized the context-dependency of these relationships. Complementary resource use may lead to a positive relationship between diversity and these processes, but only when a diverse array of niches is available to be partitioned among species. Therefore, the slope of the diversity-function relationship may change across differing levels of heterogeneity, but empirical evaluations of this pattern are lacking. Here we examine three important functions/properties in different real world (i.e., nonexperimental) ecosystems: plant biomass in German grasslands, parasitism rates across five habitat types in coastal Ecuador, and coffee pollination in agroforestry systems in Indonesia. We use general linear and structural equation modeling to demonstrate that the effect of diversity on these processes is context dependent, such that the slope of this relationship increases in environments where limiting resources (soil nutrients, host insects, and coffee flowers, respectively) are spatially heterogeneous. These real world patterns, combined with previous experiments, suggest that biodiversity may have its greatest impact on the functioning of diverse, naturally heterogeneous ecosystems.
In the past years, a number of studies have used experimental plant communities to test if biodiversity influences ecosystem functioning such as productivity. It has been argued, however, that the results achieved in experimental studies may have little predictive value for species loss in natural ecosystems. Studies in natural ecosystems have been equivocal, mainly because in natural ecosystems differences in diversity are often confounded with differences in land use history or abiotic parameters. In this study, we investigated the effect of plant diversity on ecosystem functioning in semi-natural grasslands. In an area of 10x20 km, we selected 78 sites and tested the effects of various measures of diversity and plant community composition on productivity. We separated the effects of plant diversity on ecosystem functioning from potentially confounding effects of community composition, management or environmental parameters, using multivariate statistical analyses. In the investigated grasslands, simple measures of biodiversity were insignificant predictors of productivity. However, plant community composition explained productivity very well (R2=0.31) and was a better predictor than environmental variables (soil and site characteristics) or management regime. Thus, complex measures such as community composition and structure are important drivers for ecosystem functions in semi-natural grasslands. Furthermore, our data show that it is difficult to extrapolate results from experimental studies to semi-natural ecosystems, although there is a need to investigate natural ecosystems to fully understand the relationship of biodiversity and ecosystem functioning.
Functional groups (FG) are an useful generalization to investigate environmental change effects on biotic communities. Assigning species to FGs is a contextual task and carries an arbitrary element, regardless of whether the grouping is obtained a priori or by sophisticated numerical methods. Using two grassland community case studies, we show that even simple FG allocation based on growth form, architecture and longevity (plants and mosses), or foraging characteristics (above-ground invertebrates) can be useful to increase our understanding of community processes. For example, the sensitivity of organisms to climate change increases with trophic rank and is higher in disturbed than in undisturbed communities. Complexity of interaction webs (in terms of web connectance), however, is larger in undisturbed than in disturbed communities. A significant and important relationship is likely to exist between anthropogenic disturbances, community complexity and the ecosystem effects of climate change. Trophic interactions may be disrupted much easier by climate changes in disturbed than in undisturbed communities where complexity may be buffering these effects.
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