Measuring the edges of species' geographic ranges

The fundamental unit of spatial ecology is a species range: the geographical area that it occupies. Species ranges are delineated by range edges (also known as boundaries or limits). Why range edges occur where they do and not elsewhere, and what makes them move, has been an active area of research since the 19th century. In the present day, range edge dynamics are an important metric of biodiversity's response to climate change, as species shift toward the poles to track their climatic niches. Yet there are many methods for measuring range edges and quantifying their displacement.

Here I describe common methods for describing range edge positions and apply them to example data for a bird species and a fish species, using some of the most popular datasets in climate biogeography: the Audubon Society Christmas Bird Count and a National Oceanic and Atmospheric Administration bottom trawl survey.

I show that the choice of range edge metric influences where range edge positions are estimated to occur; whether they are estimated to be shifting over time; and the estimated rate of shift. The lack of standardized metrics for range edges has likely shaped statistics reported in synthesis studies that measured overall biodiversity responses to climate change and global rates of range shifts. Through simulation, I found that reliably detecting range edge shifts may require decades of data or more, suggesting that many global change studies in this field are underpowered.

Pairing metrics to research questions, sharing raw data and code, and conducting power analyses before reporting statistically significant results will all help to minimize this issue. Going forward, the field of biogeography should confront the degree to which ad hoc methods have influenced our understanding of range edge dynamics and move towards standardized metrics.