Not All Frogs are Created Equal When it Comes to Measuring Environmental Quality

Closely related frogs Pelophylax nigromaculatus (left) and Fejervarya limnocharis (right) respond in different ways to environmental changes (Photo by Y. Wu)

Don't jump to conclusions when it comes to using frogs as a measure of habitat quality, Integrative Biology scientists caution.

Prof. Jinzhong Fu and MSc student Vhon Oliver Garcia have found that two frog species - Pelophylax nigromaculatus (PN) and Fejervarya limnocharis (FL) - respond differently to changes in their environment.

"This was a unique research opportunity," says Fu. "Not only do PN and FL co-exist in the same habitat, but they are closely related as well."

Like other amphibians, frogs are highly sensitive to pollutants and environmental change, making them great indicators of environmental quality.  They are also valuable indicators of habitat fragmentation—the splitting of large natural areas into smaller, disconnected ones. This is because landscape features like roads and mountains are physical barriers that prevent frog populations from mixing genes with one another (also referred to as "gene flow").  Thus, by studying the genetics of frog species, it is possible to glean important information about how biologically fragmented their landscape has become. 

But there is risk to generalizing the response of one amphibian to others. Each species may react differently to the same physical changes, resulting in faulty conclusions about environmental impacts.

Enter PN and FL, two closely related frog species that co-exist in the same central China habitat. This scenario presented Garcia with the ability to compare differences in population-level gene flow of two similar species in response to the same habitat.

Scientists refer to this kind of study as "comparative landscape genetics". First, DNA from each population is compared to determine how closely related different subpopulations of the same species are. Second, geospatial data that captures landscape features like rivers, mountains, and physical distance is converted into numeric values. How much (or how little) these features act as genetic barriers for each species is then quantified.

Garcia's analysis revealed that FL populations were more sensitive to roads and type of land cover (cropland versus aquatic or forest habitat, etc.), whereas elevation was the most important barrier for PN.

This finding complements the observation that PN prefers more aquatic habitats, which are not found at higher elevations. Conversely, FL has a smaller body size and is less mobile than PN, translating to a greater sensitivity to the environmental stressors associated with land cover.

According to the researchers, this means there is no “one size fits all” when it comes to looking at important indicator species like frogs.

"We should pay attention to species-specific characters," advises Fu. "Even closely related species can have different responses to a landscape."

Fu intends to apply this analysis to urban environments in the future.

Contributors to this project include: Kui Liu (Capital Normal University); Catherine Ivy, Quin Shirk-Luckitt, Teresa Lewitzky, Patrick Burgess (University of Guelph); and Brad McRae (Nature Conservancy). This study was funded by a Natural Sciences and Engineering Research Council of Canada Discovery Grant.

Read the full article in Ecology and Evolution.

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