Most characterizations of ecological communities from survey data include transient species that are poorly suited to the site but are present due to immigration from neighboring source areas. Transient species are expected to interact with their biotic and abiotic environments differently than core species since they do not maintain viable populations and are not necessarily well adapted to the environment in which they are sometimes found. As such, some ecological theories of distribution, abundance and diversity may not actually apply to transient species even though such species are typically included in species inventories and diversity estimates. Here, we use a unique data compilation spanning a wide range of taxonomic groups and ecosystems to first evaluate the prevalence of transient species across these different groups. Next, we ask how the exclusion of transient species affects four classic macroecological patterns: species abundance distributions, richness-environment relationships, species-area relationships, and temporal turnover.
Results/Conclusions
We identified 89 datasets with community time series data (minimum of 5 temporal samples) for 1,290 communities spanning terrestrial, marine, and aquatic ecosystems. 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. Taxa with the largest proportion of transient species included plants and plankton, while taxa with the smallest proportion of transients included benthic invertebrates and birds. The proportion of transient species in a community depended strongly on scale, both in terms of absolute area (p < 0.01) and in terms of community size measured in number of individuals (p < 0.0001). The exclusion of transient species from community analyses led to species abundance distributions better fit by log-normal versus log-series, richness patterns that were better predicted by remotely sensed estimates of primary productivity, shallower species-area slopes, and lower temporal turnover. Assessing generalities in these ecological patterns may require that ecologists more carefully consider the role of transient species in community datasets.