RESEARCH AND DISCOVERY

Amphibians: Off-the-charts unpredictable

Closer look at amphibians reveals layers of variability at play

David M. Green with the skeleton of a tree frog double-stained to show the bone (red) and the cartilage (blue).
CLAUDIO CALLIGARIS

By Daniel Spitzberg | Maybe if someone had watched that top-hat-wearing frog a few minutes longer, they might have caught him singing "Hello my baby, hello my honey, hello my ragtime gaaal…" and made it on Broadway.

Just like in the Warner Brothers' cartoon, real amphibians are not so easy to observe. An ongoing study entering its twentieth year is only beginning to draw back the curtain on how they live, move and reproduce.

"Reports of amphibian declines are raising concern worldwide. They succumb easily to environmental stress, and people blame a variety of causes," says Dr David M. Green, professor of biology and director of the Redpath Museum. "Now I have sufficient amounts of data that I can look for correlations and ask all sorts of questions. If you look at a population of frogs, can you detect if it's declining or not? Even over 20 years, you only begin to get hints."

Dr Green and his field assistants have been tracking the Fowler's toad (Bufo fowleri) at Long Point, Ontario, every spring and summer since 1988. Through this work, the Fowler's toad—a threatened species in Canada—is one of the most thoroughly studied amphibians.

This is the longest running study of its kind and Green expresses concern over the paucity of long-term views of amphibian populations. "Most PhD students will see only three or four field seasons, and amphibian populations can do almost anything in that time," says Green.

"Looking only at the first four years of our data, when the population was doubling annually, you would think, 'Great! Toads for everyone'. But then, the population crashed and remained at very low levels before coming back four years later."

Green describes how weather patterns have a deep impact on reproduction and survival. One harsh winter storm in 1986 opened up many new breeding sites. A particularly cold winter in 1994 was devastating to toad survival but then a mild winter in 1996 allowed larvae and juveniles to mature and adults to age another year. Because older toads lay more eggs, a huge cohort of offspring surged in the population for years to come.

"We see multiple scales of variation and stochasticity (random behaviour) at play," says Green. "This is not the standard view. Theoretically, there should be one variable that is fairly constant, like the birth rate or the growth rate. But amphibians have larvae, which grow faster or slower depending on weather conditions, lake levels and a host of other factors. Our years of data show they are totally off-the-charts unpredictable compared to birds and mammals."

The researchers head to Long Point on Lake Erie in the early spring. Every evening, they go out with headlamps and scoop up as many toads as possible. They clip the toe of every frog to gauge its age and collect a DNA sample. GPS tracking reveals that while 70 percent stay within a few metres of where they were first spotted, two to three percent will roam as far and wide as field researchers look, all across the study site's 10-kilometre shoreline. In other words, toads can really hop around.

Great dispersal distances signify that the toads in Long Point are all members of a large, single population, one of only three in Canada, explains Green. This implies more vulnerable conditions for toads and runs counter to those who say they live in "metapopulations," where the population gains security from segmented resources and refuges.

A long-term study, like the work with Fowler's toads, maps dispersal patterns on a fine scale, but Green says it takes a study on a large scale to see if the geographic ranges of amphibians are declining or expanding.

In a recently completed study, Green, along with a former master's student and a former postdoc, tracked wood frogs (Rana sylvatica) hopping across all of northern Canada after the glaciers from the last ice age receded. According to the study, amphibians are ideal species for examining impacts on animals from climate change both today and during the Pleistocene epoch. Using genetic samples from more than 500 frogs in 116 locations collected by a former PhD student, the researchers traced the wood frog re-colonization back to two high-altitude refuges, thereby showing how declining amphibian ranges may not imperil the population for good.

"We want to know how well they are doing for conservation purposes," says Green. "The geographic distribution of different wood frog lineages recreates the post-glacial history of primary colonizing vertebrates in North America."