Core versus transient species as a general framework for thinking about ecological assemblages
Determining the processes governing community assembly and diversity is crucial to understanding and managing ecological systems. However, most studies investigating patterns of species richness fail to recognize a critical insight made 30 years ago by Shmida and Wilson: species at a site typically fall into two distinct groups. Core species maintain self-sustaining populations at a site, while transient species are poorly suited to the site but are present due to immigration from neighboring source areas. In birds, we have shown that most species may be reliably designated at a site based on the frequency with which they are observed at individual sites. Furthermore, we have shown that diversity of these two groups is driven by different processes, implying that accurate inference about the importance of those processes requires the explicit consideration of each group separately. Here, we test the generality of the core-transient distinction across communities spanning plants, mammals, birds, insects, fishes, plankton, and marine invertebrates. We use data from a wide range of taxonomic groups and ecosystems to determine how core-transient patterns vary across taxa, dispersal mode, and spatial scale.
We identified 119 datasets with community time series data (minimum of 5 temporal samples) for 35,014 communities. At each site, we examined the frequency of core and transient species, and conducted tests of bimodality of the temporal occupancy distribution. Across all taxonomic groups and sites, the vast majority of species could be categorized as core species (occurring > 67% of the time) or transient species (<33% of the time) with only 12% of species on average with intermediate temporal frequency. Bimodality tests yielded similar results (only 17% of sites with non-bimodal occupancy distributions). Taxa with the largest proportion of core species included benthic invertebrates and mammals, while taxa with the smallest proportion of core species included plants and terrestrial arthropods. We found a strong effect of spatial scale as a predictor of the percentage of core species (p < 0.0001), and illustrate how scaling core-transient patterns by the total number of individuals in the community sample may facilitate comparisons across such radically different organisms and ecosystems. The core-transient distinction is a general phenomenon and incorporating this distinction should improve the predictive ability of biodiversity models.