Neutral theory has sparked a renewed interest in the mechanisms of species coexistence. If species differ in fitness, then stabilizing mechanisms that increase per capita growth rates when rare are required for coexistence. Stabilizing mechanisms resulting from negative frequency-dependence have received comparatively little attention in the debate over niche and neutral processes. Negative frequency-dependence is direct evidence against the importance of neutral processes that assume functional equivalence among individuals. In my current work using species assembly experiments in Irvine, CA, I found empirical evidence for strong negative frequency-dependent per capita growth rates that act to stabilize species coexistence in the presence of strong fitness differences (Harpole & Suding 2007, Ecology Letters).
I also showed that niche mechanisms rather than neutral processes are the primary drivers of community assembly in Midwestern prairies (Harpole & Tilman 2006, Ecology Letters). I used a simple species trait associated with a plant's competitive ability for nitrogen to successfully predict species’ abundances using multiple data sets (Harpole & Tilman 2006, Ecology Letters). This trait-based approach constitutes one of only a few “strong tests” of neutral theory. Additionally I found that species’ responses to environmental change were not neutral but also correlated with their traits. Because neutral and niche theories make very different predictions about how species’ abundances should change along fertility and disturbance gradients, these results have important implications for global change issues such as nitrogen deposition, land use, and biological invasions.