Climate and ice in the last glacial maximum explain patterns of isolation by distance inferred for alpine grasshoppers

Abstract

Cold‐adapted species are likely to have had more widespread ranges and greater population connectivity during the last glacial period than is the case today. This contrasts with the trend in many species for range and population size to increase during interglacials. We examined the pattern of genetic and morphological variation within an endemic, wingless, alpine grasshopper Sigaus australis (Orthoptera: Acrididae) in the Southern Alps of New Zealand, testing for isolation by distance using geometric morphometric and mitochondrial ND2 sequences to document variation. Presence/absence data were analysed to estimate the environmental envelope (niche) of Sigaus australis and the resulting model used to infer the extent of available habitat for the species during the last glacial maximum. Estimates of past range size were modified using models of montane ice extent during the LGM. Clinal patterns of pronotum shape variation and signatures of isolation by distance support the hypothesis of a formerly more connected species. A north/south division was observed in pronotum shape, but the phenotypic variation was not diagnostic, as one would expect within a single species. Although the current habitat area occupied by Sigaus australis is much smaller than estimates for the LGM from our climate model, we show that realised area differed less due to the extension of valley glaciers. However, the current distribution of S. australis is more fragmented than in the past. This and other flightless alpine species currently restricted to fragmented high elevation habitat demonstrate genetic lag but are subject to loss of diversity as anthropogenic climate warming proceeds.