Lowland tropical stream insects are more vulnerable to climate warming than their temperate relatives

Alisha A. Shah, Brian A. Gill, Andrea C. Encalada , Alexander S. Flecker , W. Chris Funk, Juan M. Guayasamin, Boris C. Kondratieff, N. LeRoy Poff, Steven A. Thomas, Kelly R. Zamudio & Cameron K. Ghalambor

For decades, scientists have pondered how temperature shapes the physiology, ecology, distribution, and evolution of organisms. It has long been thought that high latitude regions, which experience large seasonal fluctuations in temperature, should favor the evolution of organisms with the ability to cope with a broad range of temperatures. On the other hand, tropical latitudes with stable climates should give rise to organisms that can only survive within a narrow range of temperatures. Few studies are able to test these hypotheses because of the difficulty of measuring thermal tolerance in closely related animals across large geographic gradients. Moreover, no studies have addressed such questions in aquatic systems where temperatures do not vary as dramatically as they do in air. In this study, we experimentally determined thermal breadths (maximum and minimum temperatures within which survival is possible) of aquatic insects from tropical streams in the Andes Mountains of Ecuador and temperate streams in the Rocky Mountains of Colorado, across a range of elevations. As predicted, we found that tropical insects indeed have narrower thermal breadths than their temperate relatives. In addition, we found that within each region, insects that experience greater temperature fluctuations in their native streams tend to have wider thermal breadths than insects that experience stable stream temperatures. Thus, local temperature variation in a given stream is the best determinant of thermal tolerance for a particular population of aquatic insects, regardless of whether they are from tropical or temperate latitudes. In our study system, low elevation tropical insects have the narrowest thermal breadths, making them the most vulnerable to extreme changes in their native stream temperatures. Low elevation temperate insects have the broadest thermal breadths and are the least thermally sensitive. Aquatic insects are important members of stream communities and contribute greatly to water quality in headwater streams, which are the ultimate source of clean water for humans. This study is a first step in predicting how aquatic insects will respond to increases in stream temperatures due to human manipulation of stream environments and global climate change.

Photo provided by authors.

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