Speed limits: Thresholds curb warming's acceleration of animal movement

A recent study from UNL biologists has found that rising temperatures may exert less influence over the movement of cold-blooded animals after crossing certain thresholds that vary among species.

Biologists have long considered temperature a pace car of ecosystems, pushing and slowing the speed of organisms as it rises and falls.

By reducing the energy needed to activate biochemical processes, rising temperatures essentially act as a catalyst that elevates the metabolism of animals and ultimately fuels their movement. But a recent study from University of Nebraska-Lincoln researchers has found that this ecological pacesetter may begin to redline at certain temperatures.

Led by doctoral students Jean Philippe Gibert and Marie-Claire Chelini, the study suggests that the movement of cold-blooded animals becomes less responsive to warming after crossing temperature thresholds that vary among species.

“The current paradigm predicts that as temperature increases, the frequency (of movement) should increase at a constant rate,” Gibert said. “We actually found that that’s not what happens at all.

“That’s both good news and bad news for animals. It’s good news in the sense that, if temperatures keep increasing, they’re not going to be as affected as they could be. But at the same time, it means that they’re still going to be moving faster (than they are now), and that has all sorts of potential consequences for the way they interact with each other.”

The authors came to their conclusions after adapting mathematical models to account for both the biomechanics of animal movement and the known relationships among metabolism, body size and temperature.

They then plugged in previously collected data on the movement of 18 species that included alligators, ants, crickets, crabs, flies, lizards, snakes and turtles. Though much of that data quantified the speed of locomotion, some described the rates of other movements – the rattling of a tail or the rubbing of wings, for example – used for communication and courtship.

Because movement naturally affects the frequency of predator-prey encounters, the study also examined the density, growth and attack rates of those populations. Gibert, Chelini and their colleagues discovered that rising temperatures generally destabilized populations far more severely when factoring in the species’ respective thresholds. Yet the team actually detected the opposite effect in ecosystems that support only small numbers of prey, with seasonal fluctuations also altering the dynamics.

“We’re finding that even the patterns we thought to be universal … are not actually universal,” Chelini said. “Understanding (these interactions) is going to be much more complicated than we originally thought.”

Gibert and Chelini said tackling this daunting complexity is a necessary step toward grasping the comprehensive effects of global warming. The Intergovernmental Panel on Climate Change currently projects that the average global temperature will rise at least several degrees Fahrenheit by the year 2100.

“When we think about the consequences of climate change, we think about extinctions of particular species and the effects they will have on a very broad scale, such as whole ecosystems,” Chelini said. “But because animal movement has so much importance for things taking place within a species – communicating, mating, interacting with one predator but perhaps not another – I think this really highlights the importance of looking at how temperature affects all these intermediate scales, as well.”

The team’s study was published in the journal Global Change Biology and featured as a research highlight in Nature Climate Change. Gibert and Chelini co-authored the study with John DeLong, UNL assistant professor of biological sciences, and Malcolm Rosenthal, a postdoctoral researcher at the University of Toronto Scarborough. The researchers received support from the National Science Foundation.