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 elev ation in two adjacent mountain ranges in the Great Basin; whether surface w ater availability, canyon depth and canyon width vary predictably with elev ation; and whether those factors act in conjunction with elevation to gener ate the observed gradients in species richness. These variables might reaso nably be expected to affect species richness of butterflies and can be deri ved readily for most landscapes. We also explored how climate change may af fect species distributions in both mountain ranges. Location Field research was conducted in the Toiyabe and Toquima ranges, La nder and Nye counties, Nevada, USA. Methods We obtained data for 102 locations in the Toiyabe Range and 49 loca tions 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 r ichness data were based upon comprehensive field inventories. Elevation was measured by differentially corrected global positioning systems. Values fo r the remaining predictive variables were derived using geographic informat ion systems. Availability of surface water was quantified as the distance f rom the centre of the inventory route to the nearest permanent source of ru nning or standing water. Results In the Toiyabe Range, species richness decreases as elevation incre ases. Water availability and canyon depth and width vary predictably with e levation, but do not interact with elevation to affect species richness pat terns. In the Toquima range, in contrast, species richness increases as ele vation increases, and no other predictive variable covaries with elevation. Main conclusions We suggest that range-specific gradients in climatic sever ity contribute to the distinct elevational gradients in species richness. I n the face of climate change, species richness patterns in the Toquima rang e will likely remain similar The elevational gradient in species richness i n the Toiyabe range may flatten, or the patterns in the two ranges may conv erge.