Phenotypic responses to climate change: Reduced body size on a 60-year time scale in arctic insects

Background/Question/Methods

The impact of climate change in the arctic is likely to be significant because northern ecosystems are inherently fragile and predicted increases in temperature and precipitation are high. Responses to climate change are seen at multiple levels from ecosystem dynamics to individual organisms. Reduced body size is one predicted response to increased temperatures, although body size decrease has primarily been documented in endotherms; studies on ectotherms lag far behind. Insects, flies (Diptera) in particular, are dominant animals in northern terrestrial ecosystems, and thus understanding their responses to climate change is critical in order to predict broader impacts. Our objective was to assess whether Diptera body size has decreased over a half-century time scale at a subarctic site (Churchill, Manitoba) and to what extent the trophic role of the species influences the response to warming. Contemporary specimens of selected species collected in 2010 were compared to museum specimens collected by the 1947-1962 Northern Insect Survey. Hind tibia length was used as a proxy for overall body size. Because of sexual size dimorphism, each sex of each species was analysed separately to quantify the decrease in mean body size at the two time periods.

Results/Conclusions

We documented a significant decrease in body size in multiple (but not all) species of Diptera at Churchill from 1948–1952 to 2010. In the widespread species Scathophaga furcata, which has saprophagous larvae and predaceous adults, the decrease was greater at Churchill (5% in males, 8% in females) than at two other north boreal and subarctic sites, correlated with a greater increase in mean annual temperature (4.29°C) at Churchill over that time period. Other saprophagous species at Churchill mirrored the pattern seen in S. furcata, but no body size decrease was documented in a phytophagous species. Our results show that small ectotherms respond to climate change through decreased body size just as larger endotherms do, but the response is taxon-specific and may be influenced by the trophic group of the species. Saprophagous flies may be directly impacted by environmental factors, whereas phytophagous species may be buffered from changes by idiosyncratic responses in their host plants. Given the ecological diversity of Diptera, and their dominance in arctic ecosystems, changes in body size may have cascade effects in trophic networks, but the body size decrease may not conform to a general pattern, even within a single insect order.