Ulrich Sommer

Consumer versus resource control of species diversity and ecosystem functioning

A key question in ecology is which factors control species diversity in a community. Two largely separate groups of ecologists have emphasized the importance of productivity or resource supply, and consumers or physical disturbance, respectively. These variables show unimodal relationships with diversity when manipulated in isolation. Recent multivariate models, however, predict that these factors interact, such that the disturbance–diversity relationship depends on productivity, and vice versa. We tested these models in marine food webs, using field manipulations of nutrient resources and consumer pressure on rocky shores of contrasting productivity. Here we show that the effects of consumers and nutrients on diversity consistently depend on each other, and that the direction of their effects and peak diversity shift between sites of low and high productivity. Factorial meta-analysis of published experiments confirms these results across widely varying aquatic communities. Furthermore, our experiments demonstrate that these patterns extend to important ecosystem functions such as carbon storage and nitrogen retention. This suggests that human impacts on nutrient supply and food-web structure have strong and interdependent effects on species diversity and ecosystem functioning, and must therefore be managed together.

Ecology: Competition and coexistence

A long-standing ecological puzzle is why plant communities dont consist of a single competitively dominant species. A new model shows that plankton diversity can be maintained by the population dynamics that result from species competing for resources.

Synthesis: Back to Santa Rosalia or, no wonder there are so many species

Modern competition research started with G.E. Hutchinson’s, Homage to Santa Rosalia, and his now-famous question “why are there so many species?” (Hutchinson 1959,1961). This confronted observed species richness with the competitive exclusion principle, a principle that had been derived from theory and from highly artificial experiments. It would always have been easy to point at the “artificial” character of the competitive exclusion principle. Indeed many researchers have refused to deal with Hutchinson’s question because they considered it a pseudo-problem, which arose from a contradiction between overly simplified theory and complicated reality. However, those who took Hutchinson’s challenge seriously have gained fundamental insights into how competition plays out in nature, how species coexist, and how communities function. In this final chapter we attempt to synthesize these insights as they have been presented in this book. We focus on six key topics: n n- Identification of major trade-off axes (Sect. 8.1) n- Confirmation of the “intermediate disturbance hypothesis”, and detection of interactions among competition, resource supply, predation and disturbance in field experiments (Sect. 8.2) n- The interplay of space colonization, dispersal and neighborhood competition in sessile communities (Sect. 8.3) n- Potential for chaotic, self-generated heterogeneity in communities (Sect. 8.4) n- Role of exclusive resources in competition among mobile animals (Sect. 8.5) n- Coexistence by slow exclusion (Sect. 8.6)