DeBruyn & Ring 1999

DeBruyn AMH, Ring RA. 1999. Comparative ecology of two species of Hydroporus (Coleoptera: Dytiscidae) in a high arctic oasis. The Canadian Entomologist 131: 405-420.

These authors examined the beetles living in two ponds in a polar oasis on the east coast of Ellesmere Island, during the summers of 1992 and 1993. The two ponds are different from each other in a number of important respects, including sediment and vegetations characteristics, shoreline structure, temperature profile, and duration. Pond A is smaller than Pond B, has a greater diversity of aquatic plants and benthic sediments, and never dries completely, while Pond B dries up in August and has benthic sediment composed mainly of bare rocks with occasional patches of silt and sand. These features may explain why both species of Hydroporus were found in Pond A but only one species was found in Pond B.

While high arctic conditions are generally considered extreme for organisms, because abiotic conditions approach the physical limits for life (e.g. Downes, 1964), aquatic habitats are considered relatively benign, because the large mass of water and winter ice cover buffer temperature changes compared to adjacent terrestrial habitats. However, other authors such as Danks (2007) have noted that smaller ponds freeze completely in winter and may respond to winter temperature variations in much the same way as terrestrial habitats. Interestingly, Danks (1987) does consider aquatic habitats to be more favourable to organisms, and invokes this effect to explain the higher ratios of species richness of aquatic versus terrestrial insects at high latitudes.

Polar oases are described more fully in a book edited by Svoboda and Freedman (1994); briefly, these are locations of high biodiversity and mild conditions. Alexandra Fiord’s lowland (78°53” N) is such an oasis because of the gentle slope near sea level and the surrounding landscape providing good exposure to summer sunlight while restricting exposure to chilling winds. The two ponds examined in this study had exceptionally high temperature profiles in summer, with some microhabitats rising to 37.5°C on one particularly sunny day in July of 1993.

The discussion of how these species of beetles are able to persist at such a high latitude site is somewhat confusing. Both species overwinter as adults, one in microhabitats that dry to at least some extent in winter while the other in microhabitats that remain wet and consequently probably freeze solid. These authors state that the dry-winter species must resist both desiccation and low temperatures, but do not consider that desiccation is a strategy employed by freeze-resistant insects, especially in terrestrial habitats. This is very confusing considering that the second author has published extensively about cold adaptations in insects.

Low temperatures have been invoked to explain the frequent pattern of longer life cycles among Arctic insects compared temperate conspecifics and congeners (e.g. Danks, 1981). These authors do not dismiss this possibility, but point out that the longer development time of one species may explain its absence from the temporary pond, rather than temperature per se. Other potentially explanatory variables, such as various aspects of water chemistry, are dismissed as unlikely, given the other places these species have been found.