Thermal tolerance explains unusually rapid decadal range shifts and extinctions of Andean dung beetles

Background/Question/Methods

Tropical ectotherms are predicted to be among the most sensitive organisms on earth to climate change because of their narrow physiological tolerance, but almost no studies have examined this response. To assess the decadal response of montane biodiversity to climate change, I sampled dung beetles using standardized baited pitfall traps in 1999 and in 2009 along a steep altitudinal gradient from 1050-3450 masl in the Andes in southeastern Peru. To help understand the mechanisms underlying this response, I also measured the critical thermal limits (CT_max) of 30 dung beetle species.  

Results/Conclusions

After ten years, total species richness declined by 20% and abundance declined by 45%. Eleven species were lost while one species, which previously occurred only in the lowlands, was gained. Species’ mean altitudinal range increased by 72 m. This rate is faster than average range shifts observed for other taxa worldwide, but is consistent with warming trends in the region, suggesting that dung beetles are quickly tracking temperatures as they change. Because 16 species shifted upslope and only 3 species moved down, there was a net loss of species richness at the lowest sites and a small net gain of species at the highest sites. Out of species that persisted, 15% expanded their upper range limit and 20% contracted their lower range limit, while none showed upper limit contractions or lower limit expansions. Species with low warming tolerance (difference between CT_max and mean ambient temperature) showed greater upslope range shifts. These results suggest that climate change is rapidly affecting sensitive taxa such as tropical ectotherms due to physiological constraints, causing extinctions, population declines, and range shifts. Biotic attrition due to lack of species replacement appears to be strong at low elevations. In addition to biodiversity loss, range shifts are resulting in novel communities with unknown ecological consequences.