Niche overlap and host specificity in parasitic Maculinea butterflies (Lepidoptera: Lycaenidae) as a measure for potential extinction risks under climate change

Species depending on specific biotic interactions are particularly threatened by environmental changes. Therefore, host dependency in species living in parasitic relationships is acknowledged as a crucial factor increasing climatic susceptibility and species decline. In Maculinea butterflies and Myrmica ants, a complex form of social parasitism brings the butterflies into dependency of some few associated ant species for reproductive success. We evaluated to what extent these relations can be attributed to similarity in their climatic niches and whether alterations in niche overlap might foster the climatic susceptibility in Maculinea butterflies under future climate change. We show that the niches of ants and butterflies differ significantly more than expected at random corresponding to the observed flexibility in host exploitation. Moreover, a generally low degree of niche overlap and similar niche overlap scores between main and secondary host associations argue for a low host specificity of Maculinea butterflies. Pronounced range shifts and habitat retractions in all species will result in severe spatial limitations of Maculinea species by the presence of their hosts. No tightening in any of the host-parasite associations was detected under the pressure of range restrictions. Niche overlap scores remain comparably low in future scenarios and call for the possibility of extensive host shifts between all related species. Therefore, the absence of niche constraints might enable the parasites to extensively shift among hosts to allow the co-occurrence of several Maculinea species, in principle using the same host, and to maintain viable populations under the pressure of limited distributions and changing environments.