Upsides and downsides: contrasting topographic gradients in species richness and associated scenarios for climate change

Aim We tested whether montane butterflies exhibit similar responses to elevation in two adjacent mountain ranges in the Great Basin; whether surface water availability, canyon depth and canyon width vary predictably with elevation; and whether those factors act in conjunction with elevation to generate the observed gradients in species richness. These variables might reasonably be expected to affect species richness of butterflies and can be derived readily for most landscapes. We also explored how climate change may affect species distributions in both mountain ranges. Location Field research was conducted in the Toiyabe and Toquima ranges, Lander and Nye counties, Nevada, USA. Methods We obtained data for 102 locations in the Toiyabe Range and 49 locations in the Toquima Range. These locations covered an elevational gradient of c.1350 m in the Toiyabe Range and 900 m in the Toquima Range. Species richness data were based upon comprehensive field inventories. Elevation was measured by differentially corrected global positioning systems. Values for the remaining predictive variables were derived using geographic information systems. Availability of surface water was quantified as the distance from the centre of the inventory route to the nearest permanent source of running or standing water. Results In the Toiyabe Range, species richness decreases as elevation increases. Water availability and canyon depth and width vary predictably with elevation, but do not interact with elevation to affect species richness patterns. In the Toquima range, in contrast, species richness increases as elevation increases, and no other predictive variable covaries with elevation. Main conclusions We suggest that range-specific gradients in climatic severity contribute to the distinct elevational gradients in species richness. In the face of climate change, species richness patterns in the Toquima range will likely remain similar The elevational gradient in species richness in the Toiyabe range may flatten, or the patterns in the two ranges may converge.