Why do tropical species appear more sensitive to small environmental changes? New paper out by the Record Lab

Read, Q.D., B. Baiser, J. Grady, P.L. Zarnetske, S. Record, J. Belmaker, and L. Beaudrot. 2018. Tropical bird species have narrower body-size niches. Biology Letters 14 (1), 20170453.
Why do species in the tropics appear to be more sensitive to small changes in their environment? This is a question that has puzzled ecologists for generations. Two famous papers from fifty years ago explored this topic. One suggested that species closer to the equator tend to have more specialized diet requirements, and the other observed that environments near the equator have very stable climates, so that species that live there cannot tolerate big swings in temperature. Those ideas led us to predict that bird species outside the tropics would vary more in their body size than birds in the tropics. We tackled that question by compiling bird specimen data from museums worldwide and comparing pairs of sister species. In each pair, one of which lives in the tropics and the other closer to the poles, we compared how much the body size varied. We found that tropical birds indeed have less variable body sizes, supporting our prediction and suggesting that tropical birds will be more susceptible to ongoing human-caused environmental changes. 
This research is supported by a grant from the National Science Foundation. Quentin Read and John Grady are postdoctoral fellows co-mentored by Dr. Record (Bryn Mawr) and Dr. Zarnetske (Michigan State University).