Water availability drives habitat quality for the butterfly Plebejus argus in a Mediterranean sand dune landscape

For terrestrial species, habitat is frequently defined as physical patches of a certain land cover type in a matrix of non-habitat. However, the appropriate definition of habitat quality requires more detailed information on the resources and conditions that are necessary for the maintenance of the species (‘resource-based habitat approach’). In arthropods, their life cycle comprises several clearly differentiated stages that may depend on different resources and conditions for successful development; in turn, males and females may have different habitat requirements due to their different behaviours. Here we examine the habitat quality for males, females and eggs of the butterfly Plebejus argus in a continuous sand dune landscape in southern Spain. For both sexes, abundance was related to host plant density (negatively) and distance to heathland vegetation (a surrogate for higher water availability; negatively for males and hump-shaped for females); and, for males only, positively with flower presence. Egg abundance was positively related to mutualistic ant frequency and flower presence, and negatively to distance to heathland. Hence, female and male adults and eggs of P. argus partly differed in their dependence on resources and conditions, although distance to wetter areas of heathland was the common variable for all models. This suggests that soil humidity is the key variable for P. argus habitat quality, probably through both direct effects on individuals and indirect effects on interacting species (mutualistic ants and nectar sources). As a result, we found a high spatial congruence in habitat quality between males and females, and a moderate congruence between adults and eggs.