A disappearing act

What combination of threats faces this tropical amphibian?

What combination of threats faces this tropical amphibian?

I’ve spent the last few days trying to get a sense of how frogs are faring around  the world, and what I found was generally depressing- according the to the first Global Amphibian Assessment which was completed in 2004, at least 43% of amphibian species are in decline worldwide (Ben-Ari 2005). This trend seems to have gained momentum over the last few decades, and scientists are investigating a wide variety of possible causes, including habitat loss, global climate change, environmental pollution, and disease. One of the things that amazed me was how quickly populations could be lost in some cases- with the golden toad and harlequin frog in Costa Rica, for example, populations that had seemed healthy in 1987 were all but gone a year later (Pounds and Crump 1994). To put that in perspective in terms of my life, it was as if the town of 2000 that I grew up in was reduced to a single person in the space of 12 months- that’s horrifying. Why should we pay attention to this? Well, frogs and other amphibians are sensitive to changes in their environment, so their demise could very well be pointing to changes that are negatively impacting all of us; and the loss of frogs and tadpoles changes ecosystem processes, for example by allowing more algae to grow or more mosquitos to survive (Norris 2007).

Is habitat loss the key? It’s definitely a major player. Habitat loss seems to be central to frog declines worldwide (Lips et al. 2005, Puglis and Boone 2012). When Davidson et al. (2001) looked at the decline of the California red-legged frog which had disappeared from more than 70% of its CA range, they found that habitat loss was one of the most important factors, especially with regard to urban development. And it’s not just the issue of more roads and parking lots and houses- other habitat alterations are creating consequences you might not expect. 14 sites in Brazil were surveyed before and after the reservoir for the Serra de Mesa dam was flooded (Brandao and Araujo 2008)- after three years, 8 of 19 frog species along the edge of the reservoir had disappeared from the area and only 1 species remained on the islands in the reservoir. Now, we might think that expanding the amount of water available to frogs would help them, but hydroelectric flooding tends to create deep bodies of water with less shallow habitat for frogs and tadpoles, waves that can disturb eggs, and a line of dead vegetation along the water’s edge, all of which create problems for frogs.

Climate change is also altering frog ecology. Rising global temperatures are leading to earlier mating among frogs. When researchers compared data from Ithaca, NY for 1900-1999, they found that 4 of 6 target species initiated breeding at least 10 days earlier (Gibbs and Breisch 2001). Researchers in Poland also found that body size was increasing among some species, they believe as a result of warmer temperatures and greater insect populations (Tryjanowski et al. 2006). While this might seem like a positive development in terms of competition for resources, since females of one species preferred smaller males, it may impact breeding success in the future. One of the big concerns with climate change is also any alterations in moisture patterns, since frogs are so sensitive to this. Pounds and Crump (1994) felt that one of the factors leading to loss of the golden toad, among other species, in Costa Rica was decreased moisture as a result of stronger El Nino patterns.

And elements within ecosystems may be contributing to frog declines. According to Buck et al. (2012) 30-60% of shallow ground water and 60-95% of streams in the US contain pollution from at least one pesticide. Some pesticides, such as carbaryl (which is used on apples and cherries, among other things), were shown to delay metamorphosis in frog species. Pounds and Crump (1994) also suggested that climate disturbance may be interacting with atmospheric pollution to create toxic mist and clouds. One of the big areas of concern right now is disease caused by the (possibly) introduced fungus Batrachochytrium dendrobatidis. Bd has been implicated in continent-wide (as in Australia, and Central and South America!) amphibian declines (Norris 2007), and there are big questions about its role, for example why frogs in the northeastern US carry it with little impact (Gahl et al. 2011), but Panamanian populations have been heavily impacted (Lips et al. 2008).

So there are many factors and many questions regarding current amphibian declines. I’ve provided a brief overview here, but scientists are attempting to tease out the details and hopefully find ways to support frog populations and stop what seems to be a free-fall in some areas. In my next post, I’ll provide information on very recent research as well as possible responses from the scientific community.

Works cited:

Ben-Ari, E. 2005. A new piece in the puzzle of global amphibian declines. BioScience 55: 96.

Brandao, R.A. and A.F.B. Araujo. 2008. Changes in anuran species richness and abundance resulting from hydroelectric dam flooding in central Brazil. Biotropica 40: 263-266.

Buck, J.C., Scheessele, E.A., Relyea, R.A. and A.R. Blaustein. 2012. The effects of multiple stressors on wetland communities: pesticides, pathogens and competing amphibians. Freshwater Biology 57: 61-73.

Davidson, C., Shaffer, H.B., and M.R. Jennings. 2001. Declines of the California red-legged frog: climate, UV-B, habitat, and pesticide hypotheses. Ecological Applications 11: 464-479.

Galh, M.K., Longcore, J.E. and J.E. Houlahan. 2011. Varying responses of northeastern North American amphibians to the chyrtrid pathogen Batrachochytrium dendrobatidis. Conservation Biology 26: 135-141.

Gibbs, J.P. and A.R. Breisch. 2001. Climate warming and calling phenology of frogs near Ithaca, New York, 1900-1999. Conservation Biology 15: 1175-1178.

Lips, K.R., Burrowes, P.A., Mendelson, III, J.R. and G. Parra-Olea. 2005. Amphibian population declines in Latin America: a synthesis. Biotropica 37: 222-226.

Lips, K.R., Diffendorfer, J., Mendelson, III, J.R. and M.W. Sears. 2008. Riding the wave: reconciling the roles of disease and climate change in amphibian declines. PLoS Biology 6: e72.

Norris, S. 2007. Ghosts in our midst: coming to terms with amphibian extinctions. BioScience 57: 311-316.

Pounds, J.A. and M.L. Crump. 1994, Amphibians declines and climate disturbance: the case of the golden toad and the harlequin frog. Conservation Biology 8: 72-85.

Puglis, H.J. and M.D. Boone. 2012. Effects of terrestrial buffer zones on amphibians on golf courses. PLoS ONE 7: e39590.

Tryjanowski, P., Sparks, T., Rybacki, M. and L. Berger. 2006. Is body size of the water frog Rana esculenta complex responding to climate change? Naturwissenschaften 93: 110-113.


Posted February 10, 2013 by Mirka Zapletal in Amphibians

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