Non-random species extinction and plant production: implications for ecosystem functioning

1. Understanding ecosystem responses to plant species loss is essential for the optimal management of grasslands. Recent studies have examined the effects of simulated random species loss in experimental plant communities but not those of realistic non-random species loss resulting from transient extinction pressures in semi-natural grasslands. 2. To investigate the potential effects of non-random species loss on grassland productivity, we established mesocosms with mixed communities comprising 15 plant species, and exposed them to 2 years of high-intensity management (an extinction phase) followed by 2 years of low-intensity management (a restoration phase) allowing recolonization from differentially managed neighbouring plots. In addition, monocultures of each component species were subject to the same extinction–restoration phases. 3. During the extinction phase, species with high monoculture biomass had lower extinction probabilities in the mixed community than species with low monoculture biomass, but there was also species-specific variation. The species that were most productive or most persistent during the extinction phase were not the same as those performing best in the restoration phase. 4. No consistent effects of spontaneous recolonization from neighbouring communities on species richness or productivity of the focal communities were observed during the restoration phase. 5. We estimated that extinction of all but the species with the lowest extinction risk reduced biomass productivity by 42–49%; loss of all but the four species with the lowest extinction risk reduced it by 2–35%. Identical calculations for a random extinction scenario yielded reductions of 52% and 26–54%, respectively. 6. Synthesis and applications. Prediction of the effects of species loss on plant production and on other aspects of ecosystem functioning in semi-natural grasslands must account for both specific non-random extinction processes and post-extinction conditions. For European mesic grasslands experiencing a shift from high-intensity to low-intensity management, our results suggest that recolonization by 'missing' species must be actively assisted if high production is a management objective

Functional diversity: back to basics and looking forward

Functional diversity is a component of biodiversity that generally concerns the range of things that organisms do in communities and ecosystems. Here, we review how functional diversity can explain and predict the impact of organisms on ecosystems and thereby provide a mechanistic link between the two. Critical points in developing predictive measures of functional diversity are the choice of functional traits with which organisms are distinguished, how the diversity of that trait information is summarized into a measure of functional diversity, and that the measures of functional diversity are validated through quantitative analyses and experimental tests. There is a vast amount of trait information available for plant species and a substantial amount for animals. Choosing which traits to include in a particular measure of functional diversity will depend on the specific aims of a particular study. Quantitative methods for choosing traits and for assigning weighting to traits are being developed, but need much more work before we can be confident about trait choice. The number of ways of measuring functional diversity is growing rapidly. We divide them into four main groups. The first, the number of functional groups or types, has significant problems and researchers are more frequently using measures that do not require species to be grouped. Of these, some measure diversity by summarizing distances between species in trait space, some by estimating the size of the dendrogram required to describe the difference, and some include information about species' abundances. We show some new and important differences between these, as well as what they indicate about the responses of assemblages to loss of individuals. There is good experimental and analytical evidence that functional diversity can provide a link between organisms and ecosystems but greater validation of measures is required. We suggest that non-significant results have a range of alternate explanations that do not necessarily contradict positive effects of functional diversity. Finally, we suggest areas for development of techniques used to measure functional diversity, highlight some exciting questions that are being addressed using ideas about functional diversity, and suggest some directions for novel research