Gondwanan mesotherms and cosmopolitan eurytherms: effects of temperature on the development and survival of Australian Chironomidae (Diptera) from tropical and temperate populations

In temperate regions of the northern hemisphere, where stream thermal regimes fluctuate seasonally and predictably, temperature has a role in niche segregation and maintenance of patterns of lotic diversity and distribution, as described by the ‘Thermal Equilibrium Hypothesis’. In Australia, the role of temperature in regulating patterns of diversity and distribution has been obscure, as seasonal variation in stream temperatures can be exceeded by stochastic fluctuation in flow. The thermal responses of five lotic Chironomidae (Diptera) species, contrasting in biogeographic (evolutionary) history, from warm tropical and cool temperate Australian populations, were investigated. All species, including postulated cool-stenotherms, showed broadly eurythermic developmental and morphological responses, and maintained both survivorship and oocyte production at elevated temperatures despite reductions in overall body size. There were subtle differences among species according to biogeographic affinity, with tolerances of Gondwanan species, which were narrower than those of cosmopolitan species, best characterised as ‘mesothermic’, but there was little divergence between populations. These results have implications for the understanding of diversity and distribution of Australian chironomids, and indicate that applicability of the Thermal Equilibrium Hypothesis to Australian lotic faunas may be limited.