Cutting down the amphibian tree of life: how humans are changing amphibian habitats

A salamander crawling along the forest floor, its speckled brown body blending perfectly with the leaves. A tiny grey frog clinging with its webbed feet to a tree. Inhabiting every continent except Antarctica, amphibians play a major role in many forest ecosystems. They are key species for many reasons. First, amphibians are present in most forests in incredible numbers, and a lot of other animals depend on them for food. Second, amphibians are indicators of forest health. They have permeable, wet skin that allows them to “breathe” through their pores, but this also makes amphibians sensitive to changes in water, temperature and even the presence of harmful pollutants present within ecosystems. Finally, amphibians are an extremely diverse group of animals that contribute to ecosystem functioning in unique ways that we don’t yet fully understand.

Humans are drastically changing the world’s forests through deforestation for agriculture, logging, and urbanization. Some forest animals can adapt to changing circumstances by moving to a new habitat if their old one is lost. However, amphibians, with their small size, relative immobility, and permeable skin, are often unable to disperse to new habitats. Instead, amphibians are forced to cope with the ensuing environmental changes. As humans continue to alter forest habitats, the number of species threatened with extinction is in turn expected to rise. 

Previous studies have clearly established a link between forest destruction and a reduction of  amphibian populations. However, a recent study published in Ecology Letters provides new insight into how evolutionarily distinct amphibian species  are more likely to be affected by changing forests than species with more common evolutionary backgrounds.

Understanding the amphibian evolutionary tree is critical to understanding the importance of this study. All amphibians can be traced back in their evolutionary history to about 300 million years ago, to the first animal that scientists classified as an amphibian. Since then, through evolution, the original ancestor diverged into many different groups of amphibians to make up the many amphibian species living today. These species are all connected through their one ancestor, but some species have a particularly unusual evolutionary history – meaning their evolutionary links are distinct from most amphibians. The diverse ancestor-links are what the researchers refer to as “evolutionary diversity”.

In  this study, scientists at Simon Fraser University and Yale University examine how evolutionary diversity relates to persistence in changing forest habitats. To compare differences in evolutionary diversity between amphibian species, the scientists made use of a global evolutionary tree for amphibians developed by colleagues at Yale University. They used this data to give each amphibian a score called “evolutionary distinctiveness”, indicating how one species is different from another. The researchers also compiled data on amphibian occurrences in six different habitat types from sites across the globe, with habitats ranging from no human impact to extensive human impacts on the ecosystem. These habitats included intact forest, secondary forest, selectively logged forest, plantation site, cropland, and pasture land.

Using this data, the scientists created a model that revealed how often they could expect to find a species in each habitat based on its evolutionary distinctiveness score. They found that the intact forest habitat – the one with the smallest human impact – is the only place where amphibian species that are the most distinct on the evolutionary tree are likely to be found. These results indicate that almost any human disturbance in a forest can create an inhabitable environment for evolutionarily unique species.

Many evolutionarily distinct species are at risk of becoming extinct around the world. The more humans cut down forests, the fewer of these distinct species will exist. Intact forests are home to unique amphibian species and important for evolutionary diversity, but there are less and less forests around. Without amphibians, ecosystems and food webs could lose a crucial link that supports many other species. Only certain amphibian species will be able to survive in disturbed forest habitats, and human actions may ultimately change the progression of the amphibian evolutionary tree. If these intact forests disappear altogether, so will the unique genetic diversity of amphibians species – the consequence of which we may not be able to grasp for many years to come.