Dr Lynette H. L. Loke1, Dr Ryan A. Chisholm2
1Macquarie University, Sydney, Australia, 2National University of Singapore, , Singapore
One of the central goals in ecology has long been to understand what drives and maintains the diversity of species in local communities. Niche-assembly theory proposes that local species richness can be explained in terms of stable coexistence attributable to properties of the local environment, whereas dispersal-assembly theory proposes that local species richness is attributable to immigration from the regional scale. To resolve these perspectives, Chisholm et al. (2016) presented a unified theory positing that as immigration increases, communities undergo a transition from niche-assembled regime to a dispersal-assembled regime. The overall relationship of species richness to immigration rate is predicted to be biphasic, with the niche diversity revealed where immigration is low (but non-zero, to offset occasional stochastic extinctions). We tested this prediction for the first time. In a manipulative field experiment, we varied the level of immigration across custom-built tiles placed on intertidal seawalls in tropical Singapore. Species richness and abundances on tiles were censused monthly over a year. A mechanistic model of niche and immigration processes was fitted to the data. In total, we recorded 10,156 individuals of 64 different species in the tropical intertidal communities on the experimental tiles. We found, for the first time, experimental evidence of the biphasic transition from niche-structured to immigration-structured communities. Our results help reveal the conditions under which niches and dispersal structure communities, and to estimate the niche diversity of tropical intertidal communities.
