Relative importance of niches versus immigration in driving the diversity of intertidal communities

Background/Question/Methods

A central goal in ecology is to understand what maintains species diversity 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) and Chisholm & Fung (2021) 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.

Results/Conclusions

In total, we recorded 10,156 individuals of 64 different species in the tropical intertidal communities on the experimental tiles. We found that species richness on each tile stabilized after 6 months. We found, for the first time, experimental evidence of the biphasic relationship of species richness to immigration rate, indicating a transition from niche- to dispersal-assembled communities. Our results help reveal the conditions under which niches and dispersal structure communities (low versus high immigration respectively), and to estimate the niche diversity of tropical intertidal communities (roughly five niches). Our novel experimental approach can be adapted for other systems and be used as a “niche detector” and a tool for assessing when communities are niche- versus dispersal-assembled.