Monotonic or Unimodal Diversity‐Productivity Gradients: What Does Competition Theory Predict?

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Abstract
This article discusses two types of proposed relationships between resource productivity and the diversity of coexisting consumer species. Monotonically increasing curves have recently been dismissed on both empirical and theoretical grounds, while unimodal ("hump—shaped") curves have been supported. Unimodal curves have been attributed to increased competitive exclusion, usually as the result of decreased heterogeneity in limiting resources at high productivities. This article argues that: (1) there are many viable mechanisms that can produce monotonic curves in the presence of competition; (2) there is little empirical support for any of the major variants of the hypothesis that productivity decreases heterogeneity, which increases competitive exclusion; and (3) there are alternative reasons for unimodal curves, some or all of which are consistent with previously observed productivity—diversity relationships. Additional theoretical and empirical work is required to understand what relationships are follow from different mechanisms of competition, and what relationships are most frequently observed under different observational protocols.

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