What are the relative effects of resource availability and heterogeneity on the slope of the species-area curve along a floodplain-upland gradient?
Réserve Léon-Provancher, Québec, Canada.
The mean and coefficient of variation of several environmental variables (soil organic matter, N and pH; irradiance; elevation) were estimated in 70 plots along a floodplain-upland gradient. The slope of the species-area curve was calculated from subplots nested within each plot. Path analysis was used to determine the effects of the environmental variables on the slope of the species-area curve.
Spatial variation in flooding intensity created a complex gradient along which resource availability, but not resource heterogeneity, varied. Together, the environmental variables explained 73% of the variance in the slope of the species-area curve. Elevation, soil variables (but not light) and heterogeneity in soil fertility (but not in light) were all significantly associated with the slope of the species-area curve. Resource availability explained three times as much of the variance as resource heterogeneity.
The expected positive relationships between resource availability, resource heterogeneity and diversity were obscured by effects of flooding stress and species dominance. Flooding intensity varies inversely with elevation and restricts the pool of species that can occupy the more fertile/heterogeneous section of the gradient, i.e. near the river. The few species combining flooding stress tolerance and competitive ability use the abundant resources near the river to grow quickly and utilize space, thereby enhancing the negative effect of the flooding stress on diversity.