Degraded habitat alters frog microbiomes

Human-caused changes in the environment are linked to differences in the microbiome – the community of bacteria and other microbes that normally inhabit the skin – of a threatened species of frog, according to a new study.

Since the skin microbiome is essentially a major component of a frog’s immune system, the findings suggest that land use change could increase amphibians’ vulnerability to disease. In turn, this could be a clue to why some populations of frogs are more susceptible than others to a chytrid fungus, Batrachochytrium dendrobatidis, that causes a fatal skin infection and has resulted in declines and even extinctions of amphibian species worldwide.

Researchers collected specimens of Blanchard’s cricket frog (Acris blanchardi) from 11 sites in Ohio and Michigan. The inch-long frog was once widespread throughout the central United States, but since the 1970s has declined sharply in the northern portion of its range.

The researchers used a cotton swab to collect a sample of each animal’s skin microbiome and analyzed the bacteria characteristic of each one. Frogs collected from ponds surrounded by natural forest or prairie have a different microbiome than frogs from ponds near houses, farmland, athletic fields, parking lots, and golf courses, the researchers report in the journal Biological Conservation.

Other influences on the composition of the microbiome include latitude, pond size, and water conductivity – a measure that reflects the amount of chemical runoff entering a pond.

The researchers also bathed the frogs in a chemical solution that caused them to release natural peptide secretions (NPS), antimicrobial molecules that play a role in in amphibian immune defenses. The amount of NPS produced by the frogs differs across sites and is also linked to pond size and water conductivity, they found.

The researchers tested the activity of these skin secretions against the chytrid fungus grown in laboratory dishes. Surprisingly, higher NPS concentrations resulted in slightly greater growth of the fungus.

Earlier this year, a different group of researchers found that differences in frog microbiomes are linked to differences in the vulnerability to B. dendrobatidis infection.

Many researchers believe that chytrid infection is one factor contributing to the decline of Blanchard’s cricket frog, although the populations of frogs in this study were not infected with the fungus.

And the new study stops short of demonstrating a causal link between habitat change and disease resistance. (The researchers only collected data on the frogs’ microbiome composition, not on microbiome function.)

Still, the finding that altering the habitat around a frog changes the ecosystem on a frog is a striking one. Sometimes the changes our species touches off are small – microscopic even – but dramatic nevertheless. – Sarah DeWeerdt | December 22, 2015

Source: Krynak K.L. et al. “Landscape and water characteristics correlate with immune defense traits across Blanchard’s cricket frog (Acris blanchardi) populations.” Biological Conservation DOI: 10.1016/j.biocon.2015.11.019

Header image: A herpetologist holds a Blanchard’s cricket frog (Acris blanchardi). Credit: The Wandering Herpetologist via Flickr.

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