Lack of congruence between fundamental and realised aridity niche in a lineage of water beetles

Abstract

Coping with aridity is a physiological challenge for all organisms, including freshwater ones. Aridity shapes distributions of aquatic species at fine and large geographical scales. Specifically, for aquatic beetles, the desiccation resistance of the adults is a potential constraint for the colonisation of arid regions. We assessed the congruence between the fundamental and realised aridity niche in eight species of a Palearctic lineage of water beetles (subgenus Lumetus, genus Enochrus, family Hydrophilidae). We also estimated the relative explanatory capacity of aridity-related versus other environmental variables in species distributions. Most of the species, even those most sensitive to desiccation stress in laboratory experiments, occur in areas with high aridity within the Palearctic region. Our results suggest a lack of association between the physiological (desiccation resistance) and environmental distance matrix (realised aridity niche), or between either of these and phylogenetic distances. Aridity-related variables had generally a similar explanatory capacity in explaining the distribution of species than non-related ones. Our results indicate that desiccation resistance has not been an important physiological constraint for the colonisation of arid environments by this clade and suggest that other non-physiological factors are more important in shaping their distributions along aridity gradients. The studied beetle lineage might conserve a high basal desiccation resistance from relatively recent terrestrial ancestors, which could have provided a physiological advantage for the colonisation of arid areas. Further research could shed light on whether these unexpected results are common to other groups of aquatic insects living in arid areas or are particular to this group of beetles.