The effect of topographic complexity on species range size estimation and its conservation implications: a subtle oversight

Estimates of species range sizes are essential for reporting species extinction risk status or other conservation traits. However, they are usually quantified in planimetric terms which assume that one square kilometer on a map represents the same area of land regardless of topograpy. Consideration of relief could represent a more accurate approach to calculation of true ranges, especially for species associated with steeply sloping landscapes. We calculated the entire area of Mexico using two methods, four digital elevation models, and two different projection systems, as well as considering study area size at sites with different topographic features. We then determined the differences between planimetric and topographic estimates of species range sizes of 21 Mexican endemic bird species, finding that their range sizes varied by 0.09% to 10.24% depending on the approach taken to area estimation. Consideration of variability in terrain slope therefore allows a more refined approach to reporting species range sizes, making them more commensurate among species. Including slope in area calculations can improve not only extinction risk estimates but also the management of important conservation areas by helping identify places with the greatest surface area in which to maintain population viability.