A trade-off between latitude and elevation contributes to explain range segregation of broadly distributed cave-dwelling spiders

A fundamental goal in spatial ecology is to understand how the distribution of species varies along latitudinal and elevational gradients. This stems from the understanding that latitude and elevation are primary drivers affecting temperature variations on Earth's surface, and, in turn, that temperature plays a critical ecological role. These spatial gradients have been primarily documented using highly dispersive surface species--butterflies, birds, and plants--whereas studies on subterranean organisms remain scattered. The orb-web cave spiders Meta bourneti and M. menardi are ubiquitous inhabitants of European caves whose distributions stretches over a continental distance. They share a similar ecological niche, which should translate into competitive exclusion in co-occurring areas. Therefore, it can be predicted that there should be an effective spatial segregation between the two species along broadscale spatial gradients. Using a dataset of >3,000 georeferenced records, we show that the two species are primarily segregated along the latitudinal gradient, with M. menardi progressively becoming more frequent and M. bourneti rarer from south to north. In their overlapping range (36.5-53.4° latitude), the two species are secondarily separated along an elevational gradient, with M. menardi occupying, on average, sites at higher elevations than M. bourneti. However, in the northernmost part of its range and in the absence of its competitor, M. menardi inhabits caves at lower elevations. This clear pattern provides a textbook example of the trade-off between latitude and elevation in determining habitat segregation of broadly distributed competing species.