Characterizing the Link Between Climate and Thermal Limits in Beetles
By Amanda Biederman
Amid concerns over a rapidly changing climate, the abilities of different insects to survive at warmer temperatures has become a major question of interest. Kimberly Sheldon, an assistant professor at the University of Tennessee, Knoxville, is employing a comprehensive approach to this problem.
Sheldon is studying the effects of climate change on temperate and tropical beetles across North and South America. She said it is important for researchers to consider not only increases in mean annual temperature but also increases in temperature variation across different environments.
Sheldon said her experiments were designed to test Daniel Janzen’s 1967 “seasonality hypothesis,” which postulates that an organism’s thermal physiology is driven by adaptation to thermal fluctuation in the environment. Further, the organism’s physiology should drive its capacity for distribution along a thermal gradient.
She said she first became interested in the link between thermal physiology and distributions when reading current perspectives on the topic as a graduate student.
“Around the time I was starting graduate school, I started reading papers by (Janzen’s) colleagues, and they started mentioning the holes and gaps in the work,” Sheldon said. “So then I thought that maybe there were some holes I could fill in.”
In a study published in 2014, Sheldon’s group analyzed four tribes of dung beetles (Canthonini, Dichotomini, Phanaeini) and one genus of carrion beetle (Nicrophorus). The beetles have different morphologies and life history strategies, but they are all distributed across North and South America.
Sheldon’s group collected specimens from the four beetle groups at four geographical sites in the United States, Argentina, Costa Rica, and Ecuador. She hypothesized that beetles from the tropical, thermally stable regions (i.e. Ecuador and Costa Rica) would exhibit a narrower thermal tolerance breadth and distributional range than those from the temperate, thermally variable regions (i.e. United States, Argentina).
To characterize thermal tolerance breadth, Sheldon analyzed beetle behavior during temperature change. The body temperature at which an organism loses its righting ability during either acute cooling or warming is described as its critical thermal minimum (CTmin) or critical thermal maximum (CTmax), respectively. As predicted, Sheldon found that individuals from temperate regions had a broader thermal tolerance than those from tropical regions.
Next, Sheldon examined the link between thermal tolerance and elevational distribution. Her group reported differences in habitat range among species. All four beetle groups are distributed along an elevational gradient, and individuals at higher elevations and latitudes tend to be exposed to cooler temperatures.
Sheldon observed a positive relationship between thermal tolerance breadth and distribution, meaning that beetles from more seasonal, temperate regions exhibited not only a broader thermal tolerance but also a greater capacity to exploit different regions within their environment. This trait may be critical for survival in the future due to the effects of climate change.
In a follow-up study, Sheldon said she plans to investigate differences in phenotypic plasticity among populations. She said she will expose the beetles to different temperatures and determine whether they can adjust their critical thermal limits in order to cope with environmental change. She said she wants to determine whether beetles are capable of shifting their thermal limits and whether there are differences in populations from different latitudes.
Sheldon said it is important for biologists to consider the full ecological picture when studying the effects of climate change on organisms. She said the link between temperature variation and range breadth may be due not only to thermal physiology but also other factors such as competition among groups.
“I think some of the broader ideas are that the organism that can compete really well in its current range may have a hard time competing outside of temperatures that it has experienced,” Sheldon said. “So, some of it could be, for example, temperature-mediated competition that incorporates both elements.”
Amanda Biederman is a graduate student at Ohio University, where she studies the thermal physiology of Antarctic fishes. She also works as a science writer for the Nanoscale and Quantum Phenomena Institute and writes science news articles for her blog, cennamology.com.