Incredible shrinking frogs: The price of deforestation?
By Matt Kaplan (Image: Luis J Villanueva-Rivera) Human disruption to habitats not only causes populations to get smaller, it also seems to cause the individuals of some species to literally shrink. Johanna Delgado-Acevedo and Carla Restrepo at the University of Puerto Rico collected specimens of two common species of Puerto Rican frogs from nine sites in the northern regions of the island. The sites were all subtropical, moist environments, but differed dramatically from one another in the amount of foliage present. Some were heavily forested, while others had barely any forest left at all. Collected frogs were X-rayed and had their bones measured. Remarkably, the team found that frogs collected in habitats with foliage coverage of 20% or less were physically 5 to 10% smaller than those collected in habitats with 70% or more foliage cover. They also found that the frogs collected in more disturbed habitats had bodies that were less symmetrical than those in pristine areas. “It has been reported before that amphibian body size decreases when the animals are exposed to large numbers of predators,” says Delgado-Acevedo, “but discovering this in response to human environmental disruption is really surprising.” The reduction could be the result of natural selection. With few resources available in deforested areas, smaller frogs that make more modest demands on the habitat may be the most successful. However, the disturbed habitats might also be affecting the frogs during their early development, by exposing them to stresses that they would normally not encounter. It is difficult to say if the size reduction is for better or for worse. Small size usually goes hand in hand with slower movement, greater heat loss, and faster dehydration, but it also makes hiding easier. “Being smaller has some advantages, but these species are not small because of natural causes, they are small in response to habitat disturbance, [which] we think is a bad thing,” says Delgado-Acevedo. “They have uncovered some interesting trends that need further investigation,” says Rachel Santymire, an endocrinologist who measures stress in endangered species at the Lincoln Park Zoo in Chicago, Illinois. “We think that body size combined with measurements of symmetrical traits could be used as a tool to evaluate the health of natural populations,” says Delgado-Acevedo. “What I really wonder is whether pollutants, pathogens, or competition are causing the morphological changes in the fragmented forest dwelling frogs,” says Santymire. Journal reference: Conservation Biology (DOI: 10.1111/j.1523-1739.2008.00930.x) Endangered species – Learn more about the conservation battle in our comprehensive special report. More on these topics: