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

Aim: Cold-adapted species are likely to have had widespread ranges with greater connectivity of populations during the last glacial cycle. We sought evidence of this in the level and distribution of variation within one alpine insect species.

Location: Southern Alps, New Zealand

Taxon: The endemic, wingless, alpine grasshopper Sigaus australis (Orthoptera: Acrididae)

Methods: The current fragmented alpine distribution of Sigaus australis was used to estimate its environmental envelope (niche) and this model was then used to infer the extent of available habitat for the species during the last glacial maximum. We subtracted the area covered by ice to infer available habitat and compared fragment size. Using our prediction of a more continuous species range we sought evidence of past gene flow in the form of isolation by distance using phenotypic and genetic variation. We used geometric morphometrics of pronotum shape and DNA sequencing (mitochondrial ND2) to document variation.

Results: The current distribution of Sigaus australis is of a similar size to that inferred during the last glacial maximum when valley glaciers would have covered much of the climatically suitable land. However, the current distribution of S. australis is more fragmented than inferred from the past distribution of suitable habitat. 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 this phenotypic difference was not diagnostic, as one would expect within a single species.  

Main conclusions: The distribution of ice during the last glacial maximum significantly alters inferred habitat availability revealing that models of past and future habitat need to include more than just climate variables to provide reliable inferences. Flightless alpine species that are currently restricted to fragmented high elevation locations are likely to harbour high levels of genetic diversity.

Methods
We collected Sigaus australis grasshoppers from 27 locations in South Island New Zealand, extending the documented range of this species. Location records were acquired for 14 species of New Zealand endemic grasshoppers were compiled with data from our own grasshopper collections to produce a binary table where the presence or absence of S. australis was recorded for each location. Analysis of shape variation used fourteen landmarks identified around the perimeter and the dorsal surface of the pronotum on each image of 503 grasshoppers. Landmarks 1, 2, 6, 8 and 12 relate to the main angles of the pronotum dorsal perimeter, 13 and 14 are at the intersection of sulci (surface grooves) with the dorsal midline, 3, 4, 5, 9, 10 and 11 on the lateral carinae. X and Y coordinates are then assigned to each landmark. Polymerase chain reaction designed to target the mitochondrial DNA gene NADH dehydrogenase 2 was used to produce the alignment data.