A central question at the intersection of ecology and biogeography is what determines the diversity of plant and animal communities at different spatial scales. In terrestrial habitats, vegetation characteristics, such as biomass, productivity and structural complexity, set the stage for the local diversity of animal species. Greater vegetation biomass and productivity provide the physical space and resources for a greater number of individuals, whereas greater structural complexity should provide a diversity of niches to support different life forms. Although some studies have looked at how each of these variables influences animal diversity, few have considered how they interact to determine the number of species an ecosystem can harbour. As a first step towards addressing this question, we have used a combination of land surveys and digital photographs to characterize how vegetation volume, density, and structural complexity covary along a 3500 m elevation gradient on the eastern slopes of the tropical Andes.
We show that whereas canopy height decreased monotonically with elevation, the total amount of vegetation structures available for biodiversity peaked between 1500 – 2000m. This pattern was the result of vegetation fill increasing monotonically with elevation, when assessed at randomly selected points in the forest, or peaking at intermediate elevations, when assessed around randomly selected trees. The diversity of vegetation forms and textures also peaked at intermediate elevations, as did measures of vegetation complexity at the microhabitat scale. Vegetation structural complexity at the landscape level, assessed from the pattern of hue, saturation, and value in digital photographs, on the other hand, was flat with elevation. Thus, despite vegetation volume decreasing with elevation, several measures of vegetation fill and structural complexity peaked at intermediate elevations, potentially explaining documented peaks in the diversity of a variety of animal groups. To assess the extent to which different animal taxa are sensitive to scale differences in the structure and complexity of the vegetation as a function of their body size, habitat range, and trophic level, we are in the process of sampling the diversity of vertebrate and invertebrate taxa along the gradient using a combination of traditional and next generation sampling techniques.