Diagram showing how the ‘Portfolio-effect’ (A) and community evenness (B) may lead to a positive diversity-ecosystem functioning relationship.

The ‘Portfolio-effect’ compares communities with a stock portfolio (Tilman et al., 1998). Consider two different portfolios: one with 2 stocks (left hand side) and one with 4 stocks (right hand side) each with a total value of $100 (top panel). A stochastic event causes a 50% reduction in the value of stock A (bottom panel). In the portfolio with only 2 stocks this leads to an overall loss of $25, whereas only $12.5 are lost in the portfolio with 4 stocks. Similarly, a decrease in the abundance of one species during disturbance events will have a larger impact in species poor communities than species rich communities, and functioning in diverse communities are therefore likely to show a lower response. Moreover, the relative abundance of species within communities may also influence how ecosystem functioning responds to disturbances. Consider two communities with four species each, but one is dominated by one species (low evenness; left hand side) while all four species are evenly represented in the other (high evenness; right hand side). A disturbance event leads to a 50% decrease in the abundance of species 1 in both communities. In the community dominated by one species this leads to a 30% reduction in overall abundance but only a 12.5% reduction in the community with high evenness. This is likely to result in a greater reduction in functioning in the community with high dominance than in the community with high evenness. Thus communities with many species and high evenness are likely to show smaller responses to external stimuli than communities with few species and high dominance by single species.