In the mid-1990s, children in Minnesota were horrified to find the frogs they were catching had missing legs, horror-movie features, extra legs, and missing eyes. After that, these misshapen critters started showing up seemingly everywhere across the U.S., less so in Canada.
This past November the U.S. Fish and Wildlife Service released the results of the 10-year study of amphibians, 48,081 individuals representing 32 species of frogs and toads at 462 wetland sites. Its purpose was to address “the broad spatial distribution and temporal dynamics of amphibian abnormalities that have hindered efforts to understand the extent of the problem.”
Amphibians are the most imperiled class of vertebrates on earth. Amphibian Ark, a coalition of conservationists, estimates that 165 species have gone extinct in recent years. Malformations and mortality rates seem to be on the increase. Their permeable skin and shell-less eggs make them extremely vulnerable to the environment, and because they never venture far from home, they’re close to perfect bio-indicators of local conditions.
Various reasons for their decline have been suggested, but people have fastened on climate change, ultraviolet radiation seeping through the thinning ozone layer and damaging the molecular structure of just about everything.
The FWS study actually turned up surprisingly few abnormalities: only two per cent when the results were averaged over the U.S.; only 22 cases of extra limbs .6 per cent of all the abnormalities recorded; and one-third of the 675 collection events yielded no abnormal amphibians at all.
Two per cent or fewer abnormalities is considered within a normal range. Malformations can result from predation and parasites. Even dragon flies nibble on frog larvae.
However the hot spots—the Mississippi Valley, the Central Valley in California and south-central and eastern Alaska—revealed serious abnormalities, shortened or odd-shaped limbs and missing eyes, at rates close to 40 per cent in some years, other years nothing. (Bravo for this long-range survey: a smaller sampling might have come to wildly different conclusions.)
But there’s a codicil: all the sites studied were in U.S., refuges where amphibians are provided protection from man-caused environmental stressors—except of course ultraviolet radiation and climate change. So the study doesn’t tell us why exactly amphibians are doing so poorly in less pristine environments.
One of the chief causes of malformations and high levels of death in amphibians, although it doesn’t explain all the cases, is the parasite, Ribeiroia ondatrae, or flukes. This flat worm, after reproducing asexually in snails, dispels 40 to 1,000 larvae every night per snail. Once released, they head straight for a frog, toad or salamander where they target their limbs, forming cysts there, obstructing their growth, sometimes causing their deaths or simply rendering them lame and more susceptible to predators.
The infected amphibians are eaten by birds (especially herons), which then become the second home for the flukes, and a place where they sexually reproduce. Their eggs are expelled in the bird’s feces, which if they land in water, they hatch and seek out snails, burrowing into their shells and setting up home again. R. ondatrae thrive in algae- rich farm ponds, so the increase in amphibian mortality may be related to an increase in nutrients.
The Canadian Journal of Zoology reports that infected amphibians have been found along the migratory flyways in the U.S., as you might expect given that the dispersal agents are birds, but curiously enough, the flyways in Canada haven’t turned up infected amphibian populations—a mystery for a future gap analysis. (This is not to suggest that R. ondatrae aren’t found in Canada, they are.)
A recent study in Illinois connected the decline in amphibians to the invasive European buckthorn, now evident in two thirds of the U.S. This rampaging ornamental plant releases a compound that is toxic to amphibian embryos, killing off populations of Western chorus frogs and the African clawed frog in particular, but it is also suspected in the high mortality rates of other amphibians.
A Canadian study has drawn a line between pesticide use and amphibian mortality. In Minnesota, a field study has linked UVB penetration in a pond to a depth of 10 cm. and the percentage of malformed frogs there, but light penetration in water is affected by shading from trees and aquatic plants, and by the amount and quality of dissolved organic carbon. So UVB doesn’t rank very high as a cause of amphibian mortality. However it could be acting on natural and manmade compounds that then make them toxic to frogs. The potential combination of biological, physical and chemical substances is limitless.
It’s useful to know that amphibians are doing more or less O.K. in pristine environments like U.S. refuges. By inference, we’re left with contaminated environments, squashing them under our tires, interactions with invasive species, chemicals, nutrient runoff, any or all combinations of these factors plus outright habitat destruction as the real stressors. We can deal with these issues at the local level, unlike climate change.
Too often climate change induces a paralyzing stupor; it makes us feel bad and block us from taking on the challenges we should be fighting. For frogs everything is local. These critters don’t have enough room, and we too often mess with what room they do have. We can stop that.
Localized Hotspots drive Continental Geography of Abnormal Amphibians in U.S. Wildlife Refuges, Mari K. Reeves, Plos One
Ribeiroia ondatrae causes limb abnormalities in a Canadian amphibian community, C.D. Roberts, T.E. Dickinson, Department of Biology, Thompson Rivers University, Kamloops, BC V2C 0C8, Canada. Canadian Journal of Zoology
USGS, Science for a Changing World. Malformed Frogs in Minnesota: An update
Frog Deformities: North Temperate Lakes: Long Term Ecological Research from Lakeland Times, John Bates
legless frog photo credit: Scientific American, USFWS/Fred Pinkey