Forces driving the composition of butterfly assemblages in Andorra

(ENG) Despite the impact that human presence has on the area, Andorra in the eastern Pyrenees still harbours a rich butterfly fauna and is a potentially excellent area for studying the effects of global change on biodiversity. The aim of this study was to identify and understand the factors that are inducing observed patterns of butterfly richness in Andorra. We used data collected between 2006 and 2010 from six transects of the Andorran Butterfly Monitoring Scheme that lie at heights from 1,000 to 2,400 m a.s.l. These transects are divided into 44 discrete sections and during the study period 18,603 individuals belonging to 126 butterfly species were recorded. The effects of elevation and habitat composition on species richness and abundance were analyzed, as was the presence of spatial structure in the butterfly assemblages. We found a clear tendency for species richness to decrease as elevation increased and also identified a major faunal turnover. Habitat composition seems to have little effect on species richness and butterfly abundance. A spatial structure was observed in the dataset, with a positive spatial autocorrelation at section scale that reflects a clear effect of altitudinal gradient on species assemblages. Finally, a cluster analysis enabled us to define two main faunistic groups, corresponding to lower (generally in closed habitats) and higher sites (generally in subalpine meadows and grasslands). We thus conclude that the elevation gradient is the principal factor driving butterfly distribution and abundance in Andorra.