How climate changes the timing of wood frog calls

Did you hear that? That's the sound of wood frogs calling.

Wood frogs have one of the most widespread ranges from the southeast U.S. to the Canadian subarctic and as far north as the Brooks Range in Arctic Alaska. It is the only amphibian found this far north and has a surprising adaptation to the cold--it freezes solid in the winter and thaws out to carry on with life in the spring.

Like many species of wildlife, amphibians time their breeding season to correspond to optimal conditions for success. Those habitat conditions change as the climate changes.

In Alaska, we've seen rapid changes in subarctic wood frog habitat. Annual air temperatures have increased by 2.4 ^oC and winter temperatures have warmed by 3.5 ^oC over the last century. Snow and frost cover has decreased 65% and the length of the growing season has increased 45%. Warmer air temperatures and less snow cover have changed the amount of spring runoff and reduced the amount of frog habitat.

Scientists are interested in a variety of questions related to the frog breeding season. Are the frogs able to adapt the timing of their breeding to compensate for the changing environmental conditions? If the breeding season advances, will the plants and animals frogs feed on adapt as well or will frogs go hungry? And what about migratory species like the sandhill crane, a common wood frog predator, will they migrate earlier in the season to arrive when wood frogs are present in shallow pools? The food web in Alaska is complex and it depends on plants and animals arriving at the correct time so they can take advantage of the short growing season. Many migratory species are cued by temperature in the overwintering habitat or drivers other than temperature, so they may not adapt as easily to changing environmental conditions as wood frogs appear to be.
The detection system we developed produced accurate and effective means of monitoring remote places. We also found a strong relationship between frog-calling activity with temperature and snowpack, suggesting that wood frogs will rapidly adjust breeding to changing spring conditions. As the climate changes, this monitoring could help us understand the impacts on wood frogs and the organisms that depend on them.

Monitoring the phenology of the wood frog breeding season using bioacoustic methods

Abstract

The breeding phenology of wood frogs (Lithobates sylvaticus) in subarctic ecosystems is closely tied to the weather conditions at the onset of spring. With climate change occurring at an accelerated rate at high latitudes, we expect changes in the timing of the onset of spring to impact the timing of wood frog breeding. Wood frogs in the subarctic emerge from hibernation just after the snow melts and ambient air temperatures increase, but local and regional drivers of breeding phenology are not well understood and are difficult to study due to the relative inaccessibility of the region. We deployed passive acoustic monitors at multiple sites from 2011 to 2017 to track the timing of wood frog breeding based on calling activity and determine how this activity relates to environmental conditions. We developed a semi-automated approach for detecting wood frog calls, including the manual removal of false positives, and show that these estimates of activity are closely related to the timing of snowmelt and ambient air temperature. Our results suggest that wood frog breeding phenology in subarctic and arctic ecosystems will likely change in response to warming conditions, and we conclude that passive acoustic monitoring paired with semi-automated detection of wood frog calling provides an accurate and efficient method for tracking changes in population-scale wood frog breeding in this challenging and rapidly changing ecosystem.

Larsen, A. S., J. H. Schmidt, H. Stapleton, H. Kristenson, D. Betchkal, and M. F. McKenna. 2021. Monitoring the phenology of the wood frog breeding season using bioacoustic methods. Ecological Indicators 131: 108142.