Linking biodiversity and ecosystems: towards a unifying ecological theory

Abstract

Community ecology and ecosystem ecology provide two perspectives on complex ecological systems that have largely complementary strengths and weaknesses. Merging the two perspectives is necessary both to ensure continued scientific progress and to provide society with the scientific means to face growing environmental challenges. Recent research on biodiversity and ecosystem functioning has contributed to this goal in several ways. By addressing a new question of high relevance for both science and society, by challenging existing paradigms, by tightly linking theory and experiments, by building scientific consensus beyond differences in opinion, by integrating fragmented disciplines and research fields, by connecting itself to other disciplines and management issues, it has helped transform ecology not only in content, but also in form. Creating a genuine evolutionary ecosystem ecology that links the evolution of species traits at the individual level, the dynamics of species interactions, and the overall functioning of ecosystems would give new impetus to this much-needed process of unification across ecological disciplines. Recent community evolution models are a promising step in that direction.

Figure 1.

Ecology has traditionally regarded biodiversity as an epiphenomenon driven by the abiotic environment and ecosystem functioning (solid arrows). Recent biodiversity and ecosystem functioning research has focused on the reverse effect of biodiversity on ecosystem functioning (thick dashed arrow). The ecosystem engineering and niche construction concepts further seek to capture biological feedbacks on the abiotic environment (thin dashed arrows).

Figure 2.

Effects of plant species richness (a) on annual above-ground plant biomass production in the BIODEPTH experiment and (b) on annual total plant biomass production in the Cedar Creek experiment. Average biomass production increased with plant diversity across the eight sites in the BIODEPTH experiment, an effect that became stronger through time in the Cedar Creek experiment. In (a) lines are regression slopes and symbols (staggered for clarity) are richness level means and standard errors: filled squares = Germany, line 1; filled circles = Portugal, line 2; filled triangles = Switzerland, line 3; filled diamonds = Greece, line 4; open squares = Ireland, line 5; open circles = Sweden, line 6; open diamonds = Sheffield (UK), line 7 and Silwood Park (UK), line 8. Modified from Hector et al. (2002) and Tilman et al. (2001).

Figure 3.

The general mechanism that generates the stabilizing effect of diversity on aggregate community or ecosystem properties. (b) When species have asynchronous responses to environmental fluctuations (species have independent environmental responses), their population sizes (thin lines) also fluctuate asynchronously, which reduces the variability of community size (the sum of population sizes, thick lines). Increasing the number of species generally increases the potential for species asynchrony, and hence the stabilization of community properties. (a) When species have perfectly synchronous environmental responses, however, increasing the number of species does not contribute to stabilize community size. Modified from Loreau (in press). (a,b) (i), one species; (a,b) (ii), two species; (a,b) (iii), eight species.

Figure 4.

First steps of the evolutionary emergence of a size-structured food web in a community evolution model. The upper panel shows the trait composition of the community through time, while the lower panel details the different steps of the emergence. The simulation starts with a single species that consumes inorganic nutrient (A). Once in a while, mutants appear (here, larger than the resident) and replace their parent (B, in which the grey morph goes to extinction). After several replacements, an evolutionary branching happens, leading to coexistence of the mutant and the resident (C). A rapid diversification then occurs in which several morphs are able to coexist (D). These morphs are then selected to yield differentiated trophic levels (E). Reprinted from Loeuille & Loreau (in press). A, initial condition; B, replacement; C, coexistence, D, diversification; E, divergence.