Ecosystems around the world are experiencing unprecedented rates of extinction and species decline. The question of how community disassembly--the ongoing process of nonrandom species losses and declines--affects ecosystem functions, including those that influence persistence of other species, is addressed. The order in which species disappear from a community depends on their vulnerability to specific stressors and on traits associated with inherent susceptibility to decline. Information on species characteristics associated with vulnerability (response traits) is synthesized, and it is asked whether they are associated with characteristics that underpin significant contributions to ecosystem functioning (effect traits). Direct evidence that community disassembly affects ecosystem functioning comes from a variety of sources, ranging from documentation of long-term changes following the loss of an initial species or fragmentation of a landscape, to modeling and manipulative experiments that simulate species losses and observe their consequences. The usefulness to conservation and restoration practice of community disassembly as a concept is evaluated, and it is asked whether and how community disassembly can provide guidance about species loss order, its consequences, what each of these depends on, and whether a positive link exists between vulnerability and contribution to function--a link that would exacerbate the consequences of the ongoing extinction crisis.

Variation in organic matter inputs caused by differences in plant community composition has been shown to affect microbial activity, although the mechanisms controlling these effects are not entirely understood. In this study we determine the effects of variation in substrate composition, quantity, and diversity on soil extracellular enzyme activity and respiration in laboratory microcosms. Microbial respiration responded predictably to substrate composition and quantity and was maximized by the addition of labile substrates and greater substrate quantity. However, there was no effect of substrate diversity on respiration. Substrate composition significantly affected enzyme activity. Phosphatase activity was maximized with addition of C and N together, supporting the common notion that addition of limiting resources increases investment in enzymes to acquire other limiting nutrients. Chitinase activity was maximized with the addition of chitin, suggesting that some enzymes may be stimulated by the addition of the substrate they degrade. In contrast, activities of glucosidase and peptidase were maximized by the addition of the products of these enzymes, glucose and alanine, respectively, for reasons that are unclear. Substrate diversity and quantity also stimulated enzyme activity for three and four of the six enzymes assayed, respectively. We found evidence of complementary (i.e., non-additive) effects of additions of different substrates on activity for three of the six enzymes assayed; for the remaining enzymes, effects of adding a greater diversity of substrates appeared to arise from the substrate-specific effects of those substrates included in the high-diversity treatment. Finally, in a comparison of measures of microbial respiration and enzyme activity, we found that labile C and nutrient-acquiring enzymes, not those involved in the degradation of recalcitrant compounds, were the best predictors of respiration rates. These results suggest that while composition, quantity, and diversity of inputs to microbial communities all affect microbial enzyme activity, the mechanisms controlling these relationships are unique for each particular enzyme.

Summary1. The majority of experiments examining effects of species diversity on ecosystem functioning have randomly manipulated species richness. More recent studies demonstrate that realistic species losses have dramatically different effects on ecosystem functioning than those of randomized losses, but these results are based primarily on microcosm experiments or modelling efforts. 2. We conducted a field-based experiment directly comparing the consequences of realistic and randomized plant species losses on invasion resistance and productivity in a native-dominated serpentine grassland in California, USA. The realistic species loss order was based on nested subset analysis of long-term presence ⁄ absence data from our research site and reflects differing species sensitivities to prolonged drought. 3. Biomass of exotic invasive plant species was inversely related to native species richness in the realistic loss order. In contrast, invader biomass was consistently low across species richness levels in the randomized species loss order, with no effect of native species richness on invader biomass among randomized assemblages. Although total above-ground plant biomass increased with soil depth (a proxy for resource availability) in both realistic and randomized assemblages, soil depth influenced invader biomass only in the randomized assemblages. 4. Synthesis. Our results illustrate that the functional consequences of realistic species losses can differ distinctly from those of randomized species losses and that incorporation of realistic species loss scenarios can increase the relevance of experiments linking biodiversity and ecosystem functioning to conservation in the face of anthropogenic global change.

Maintaining native biodiversity in grasslands requires management and mitigation of anthropogenic changes that have altered resource availability, grazing regimes, and community composition. In California (USA), high levels of atmospheric nitrogen (N) deposition have facilitated the invasion of exotic grasses, posing a threat to the diverse plant and insect communities endemic to serpentine grasslands. Cattle grazing has been employed to mitigate the consequences of exotic grass invasion, but the ecological effects of grazing in this system are not fully understood. To characterize the effects of realistic N deposition on serpentine plant communities and to evaluate the efficacy of grazing as a management tool, we performed a factorial experiment adding N and excluding large herbivores in California's largest serpentine grassland. Although we observed significant interannual variation in community composition related to climate in our six-year study, exotic cover was consistently and negatively correlated with native plant richness. Sustained low-level N addition did not influence plant community composition, but grazing reduced grass abundance while maintaining greater native forb cover, native plant diversity, and species richness in comparison to plots excluding large herbivores. Furthermore, grazing increased the temporal stability of plant communities by decreasing year-to-year variation in native forb cover, native plant diversity, and native species richness. Taken together, our findings demonstrate that moderate-intensity cattle grazing can be used to restrict the invasive potential of exotic grasses and maintain native plant communities in serpentine grasslands. We hypothesize that the reduced temporal variability in serpentine plant communities managed by grazing may directly benefit populations of the threatened Edith's Bay checkerspot butterfly (Euphydryas editha bayensis).

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