Opposing mechanisms drive diversity patterns of core and occasional species

Background/Question/Methods

Ecological communities can be separated into two types of species: core species that maintain populations within the community continuously through time; and occasional species that are infrequent visitors and do not sustain viable populations. Ecologists have recognized that different mechanisms may constrain numerical abundance in core versus occasional species, with biological factors being more important determinants of core species’ abundances, and mass effects and source-sink dynamics being more important for occasional species’ abundances. Less studied are the factors that determine the number of core and occasional species within a particular community. We hypothesize that, if occasional species occur in communities due to mass effects, occasional species richness will be highest in areas where multiple habitats are in close proximity, but that richness will not be strongly affected by the actual quality of the local habitat. However, our predictions for core species richness are exactly the opposite: local environmental factors will be more important than landscape heterogeneity, since higher quality habitat conducive to larger populations should be able to support more core species. We test this hypothesis in avian communities using 497 sites across the United States surveyed from 1996-2010 by the North American Breeding Bird Survey. Species are categorized as core or occasional based on temporal occupancy, the proportion of years that a species was observed at a site. 

Results/Conclusions

We find that environmental variables known to be strong predictors of avian richness (NDVI, summer precipitation) have strong positive relationships with core species richness, but no relationship or negative relationships with occasional species richness. Conversely, mean elevation, which is correlated with environmental heterogeneity, has a strong positive relationship with occasional richness, but no relationship with core richness. Occasional species richness is highest in Colorado, Utah, and New Mexico; but core species richness is comparatively low there. Core species richness is highest in the northeastern United States and Wisconsin, where occasional species richness is moderate to low. These simple results clearly suggest that, in avian communities, the diversity of core and occasional species is determined by different and potentially opposing mechanisms. The accuracy of species richness models could be improved by separately modeling core and occasional species, especially in mountainous regions, along ecotones, and in other areas of high landscape heterogeneity.