Species that dominate spatial turnover can be of (almost) any abundance

An ongoing quest in ecology is understanding how species commonness influences compositional change. While each species' contribution to beta diversity (SCBD) depends both on its abundance and how widespread it is (e.g. occupancy) a general expectation for these influences is lacking. Using published data for 9924 species across 177 metacommunities, we modelled relative SCBD as a function of abundance and occupancy using both correlative and mechanistic regression models (the latter derived from population demographic theory). Although the correlative model provided a superior fit to the data, both results suggest it is species with infrequent combinations of abundance and occupancy (high abundance and mid-high occupancy) that make the dominant contribution to beta diversity. The nature of their interaction is most apparent when depicted in abundance-occupancy sample space, which shows the probability of making a dominant contribution to beta diversity is a concave-up function of abundance. Species found in an intermediate number of sites (0.56) required the smallest share of total abundance (0.05) to make a top-decile contribution. Simulations varying evenness of abundance and conspecific spatial patterns support the main findings and show that it is variations in the strength of aggregation that predominantly result in the observed relationship between the abundance and occupancy of a species and its contribution to beta diversity. The abundance-occupancy sample space illustrates how empirical abundance-SCBD relationships can be linear or unimodal and provides a general framework to understand global change processes. To preserve compositional turnover, species of infrequent abundance and occupancy should be prioritized.