Habitat loss is generally considered to be a negative process, causing population isolation, edge effects, and extinctions. However, a new study suggests that habitat degradation can sometimes have a positive effect. Last week, a paper in PNAS showed that habitat loss may actually suppress the spread of a deadly disease in some amphibians.

Chytridiomycosis, a disease caused by the fungus Batrachochytrium dendrobatidis (Bd), is one of the greatest challenges facing amphibians today. The fungus causes skin loss, ulcers, convulsions, and often death. Bd has affected more than 30 percent of all amphibian species in the world, including some in North America, South America, and Australia, and has caused the extinctions of about 200 frog species.

To examine the relationship between this disease and habitat loss, researchers surveyed several populations of the golden lesser treefrog in areas of Brazil that had undergone varying levels of habitat disturbance. The greater the extent of habitat loss in each sampling area, the fewer Bd infections they found. Furthermore, the infections in disturbed areas were far less intense than those in pristine areas. None of the 19 other bioclimatic variables they looked at, including various measures of temperature and precipitation, were significantly correlated with infections.

The same relationships were seen among rain frogs in Costa Rica and Stony Creek frogs in eastern Australia: populations in disturbed areas had far fewer infections. In all analyses, the researchers controlled for the amphibian density by using capture rates as proxies for population density.

The authors suggest two possible mechanisms that may drive this relationship. First, pristine areas with little or no habitat loss tend to have high species richness, allowing for higher disease transmission rates between species; infections may not spread as quickly or as effectively in areas of habitat degradation. Another possible mechanism is that habitat loss may lead to microclimatic conditions that aren't favorable to Bd. For example, deforestation may create temperature variability that is greater than the range at which the fungus can survive.

The authors warn that these findings apply to wide-ranging, generalist amphibians that can tolerate some habitat degradation. It’s more than likely that the interaction between habitat loss and disease may affect specialist species in different ways, and it is likely that endemics may be at particularly high risk. In any case, the finding that disturbed areas can act as safe harbors from disease, even for just some species, is a surprising one.

PNAS, 2011. DOI: pnas.1014497108 (About DOIs).

Listing image by Image: Guilherme Becker