Arthropod distribution on an alpine elevational gradient: the relationshipwith preferred temperature and cold tolerance

The distribution of arthropod species on a 400 in elevational gradient (equ ivalent to a temperature decrease of 2.5 degreesC) on Snowdon, North Wales, was examined and compared with the British distribution. Preferred tempera ture, an indication of optimal body temperature (T-b), and supercooling poi nt (SCP), an indication of cold tolerance, of several species on the gradie nt were determined experimentally. The alpine beetle species Patrobus assim ilis and Nebria rufescens had low preferred T-b, of 5.6 and 7.1 degreesC re spectively, whereas the more widespread upland species had higher preferred T-b, between 12.9 and 15.5 degreesC. The SCP of both alpine and widespread beetles were similar, being between -6.9 and -5.8 degreesC. The alpine spe cies, which were smaller, were freeze intolerant, whereas the widespread sp ecies, which were larger, were freeze tolerant. On the national scale there was significant correlation between preferred T-b and species elevation, b ut no correlation with SCP. It is concluded that the alpine species survive on Snowdon because their optimal T-b is close to the ambient temperature a t the time of day and year when they are active and because they are able t o tolerate winter temperatures, by a combination of cold tolerance and shel ter. Although a species' optimal niche will tend to shift upwards as mean t emperatures rise with global climatic change, complex microclimatic and bio tic factors make changes in distribution difficult to predict.