Plant Species Richness in Riparian Wetlands—a Test of Biodiversity Theory

In this study, flood frequency, productivity, and spatial heterogeneity were correlated with plant species richness (SR) among wetlands on a coastal island in southeast Alaska. Studies of 16 sites in or near the Kadashan River basin demonstrated nonlinear, unimodal relations between flood frequency and SR, productivity and SR, and linear relations between SR and the spatial variation of flood frequencies (SVFF) within a site. SVFF is caused by microtopographic variation in elevation. A nonlinear regression model relating SR to flood frequency and SVFF explained much of the variation in SR between wetland communities. Sites with intermediate flood frequencies and high SVFF were species-rich, while sites frequently, rarely, or permanently flooded and with low SVFF were species-poor. The data suggest that small-scale spatial variation can dramatically alter the impact of disturbances. The data also support Michael Huston’s dynamic-equilibrium model of species diversity, which predicts the effects of productivity and disturbance on diversity patterns. Species-rich sites had low to intermediate levels of productivity and intermediate flood frequencies, and species-poor sites had very low or high flood frequencies and low productivity, supporting the model’s predictions. The model was tested at contrasting spatial scales (1000 m2 and 1 m2). At the 1000-m2 scale, Huston’s model predicted 78% of the variation in SR. At the microplot scale, relationships between SR and flood frequency were weaker, and the dynamic-equilibrium model predicted only 36% of the variation in SR.