MONOTONIC OR UNIMODAL DIVERSITY PRODUCTIVITY GRADIENTS - WHAT DOES COMPETITION THEORY PREDICT

This article discusses two types of proposed relationships between res ource productivity and the diversity of coexisting Consumer species. M onotonically increasing curves have recently been dismissed on both em pirical and theoretical grounds, while unimodal (''hump-shaped'') curv es have been supported. Unimodal curves have been attributed to increa sed competitive exclusion, usually as the result of decreased heteroge neity in limiting resources at high productivities. This article argue s that: (1) there are many viable mechanisms that can produce monotoni c curves in the presence of competition; (2) there is little empirical support for any of the major variants of the hypothesis that producti vity decreases heterogeneity, which increases competitive exclusion; a nd (3) there are alternative reasons for unimodal curves, some or all of which are consistent with previously observed productivity-diversit y relationships. Additional theoretical and empirical work is required to understand what relationships follow from different mechanisms of competition, and what relationships are most frequently observed under different observational protocols.