Neutrality is Size-Dependent in Tropical Understory Trees

The relative influence of random processes and species niche differences on community structure may depend on the intensity of ecological interactions between individuals. The space-filling crowns and extensive root systems of large trees can severely reduce the availability of light and nutrients for trees growing underneath them. The relative intensity of such competition increases as individuals grow because resource use is proportional to tree size. We here assess whether tree size influences the structure of a community of tropical trees in Costa Rica. To do so, we represent the community as a species association network. Such a network, as we define it here, is a graph whose nodes are species. A directed edge points from species V to species W if the understory of at least one large tree of species V harbors a smaller tree of species W, such that their crowns overlap by at least 50 percent. We compare the structure of species association networks representing the empirically observed community with that of randomized communities in which we reshuffled the geographical coordinates of individual trees. We find that a measure of network connectedness (the characteristic path length) and a measure of spatial clustering of species (the clustering coefficient), indicate non-random community organization for several size classes of understory trees. We also find that how one analyzes community structure is critical to the detection of non-random community organization.