Empirical results

Tilman and Downing [15] suggest that primary productivity in more diverse plant communities is more resistant to, and recovers more fully from, a major drought.

• 207 plots of prairie grasslands differing in species richness from 1 to 26.

• Measured resistance as relative rate of community biomass change from 1986, the year before a drought, to 1988, the peak of the drought.

• Drought resistance is an increasing function of community diversity.

• Saturates at about 10-15 species.

• More diverse communities are more resistant⁷ than less diverse communities, but they don't have to be very diverse.

• Recent results on the relationship between bacterial species diversity and community respiration (a measure of total microbial activity) show that there are diminishing returns as the number of species in the bacterial community increases [1]. The strong diminishing returns associated with increases in species diversity are likely to be a general feature of relationships between ecosystem processes and species richness.

In a similar experiment Tilman et al. [14] found that plant cover is an increasing function of species richness and lower concentrations of inorganic soil nitrogen, presumably because of greater nitrogen uptake in more diverse communities.

Experiments on mycorrhizal diversity [16] suggest that plant species composition and community structure are more sensitive to the present or absence of particular mycorrhizal associates when the diversity of mycorrhizal fungi is low. Similarly, plant species diversity, nutrient capture, and productivity are increasing functions of mycorrhizal diversity.

But there are two possible explanations for patterns like these:

1. More diverse communities could increase the chances that at least one species within them is highly productive - a selection effect.

2. More diverse communities may be able to tap resources more effectively because the differ in strategies for resource acquisition - a complementarity effect.

Cardinale et al. [2] perform a meta-analysis of 111 field, greenhouse, and laboratory studies that manipulated species diversity to determine its effect on abundance and biomass. They found that

• Decreasing diversity is, on average, associated with descreased abundance, decreased biomass, or both.

• The standing biomass of the richest polyculture tends to be no different from that of the most productive monoculture.

``Collectively [their] analyses suggest that the average species loss does indeed affect the functioning of a wide variety of organisms and ecosystems, but the magnitude of these effects is ultimately determined by the identity of species that are going extinct'' [2, p. 989]. Using a somewhat different approach Grace et al. reach a similar conclusion: ``[T]he influence of small-scale diversity on productivity in natural systems is a weak force, both in absolute terms and relative to the effects of other controls on productivity'' [7, p. 680]

More recently Raymundo et al. [12] present evidence that when coral reefs are associated with diverse fish populations, the coral are less susceptible to disease. In looking more closely at their data they found that the abundance of chaetodontid butteflyfishes was positively associated with the extent of disease. Butterflyfishes were more abundant in less diverse communities because they are released from competition - a selection effect in the opposite direction from what we would normally think of.