Culebra Stream- Las Cruces
“The rain forests of tropical America are not a uniform realm in which frogs are always conspicuous elements. Individual frogs can be rare even in places that are a home to a wealth of species.” Forsyth and Miyata in Tropical Nature (1984)
This sums up my experience in looking for frogs over the last two weeks. Over the last two weeks I have seen zero frogs or toads despite the lengthy list of species provided on Las Cruces’ website (see link in References), but I can take some comfort in the fact that I am not the
Las Alturas Stream
only one to have experienced this phenomenon. Forsyth and Miyata recall how colleagues searching for frogs in the Brazilian Amazon would only see one or two frogs for every hour spent searching while they (Forsyth and Miyata) would find dozens of frogs in the same time period. They go on to explain that this variation in abundance is mostly a mysterious phenomenon, but that the cycles of rain may play a role.
For the purposes of my group’s study on stream bioindicators, precipitation likely wasn’t a factor as it consistently rained after we had sampled for the day. This still leaves us answering the question where did all the frogs go? I propose three possible explanations; 1) tropical frogs do not congregate in streams, 2) the temperature/elevation gradient negatively impacted frog diversity and 3) our observations may be part of the global amphibian population decline.
First, frogs favor small, temporary pools over established pools for breeding (Forsyth and Miyata 1984). Forsyth and Miyata (1984) conclude that since frog eggs and tadpoles are such tempting targets to aquatic predators such as fish and water insects, frogs have developed a preference for small, temporary pools as a way to minimize the likelihood of predation and maximize the likelihood of nutrient availability. Since the streams surveyed in this study were all established streams, it can be suggested that we simply weren’t looking in the right places if we wanted to find frogs.
Second, the temperature/elevation gradient at a mid-elevation site like Las Cruces influences frog diversity patterns (Laurencio and Fitzgerald 2010). Researches looking at abiotic, non-living, factors effecting species diversity patterns of amphibians across Costa Rica found that at mid-elevation sites, including Las Cruces, elevation plays a significant role in determining frog diversity (Laurencio and Fitzgerald 2010). This suggests that elevation and by association temperature may have affected the likelihood of observing frogs at our survey sites in Las Cruces.
Finally, our lack of amphibian sightings may have been part of the greater global amphibian decline. In 2004 the International Union for Conservation of Nature (IUCN), publishers of the Red List of Threatened Species, launched the first Global Amphibian Assessment (GAA) to comprehensively determine the conservation status of amphibians worldwide (IUCN 2008). The findings of this assessment were updated in 2006 and 2008 (2008). Key findings from the 2008 GAA include that nearly one-third (32%) of the world’s amphibian species are known to be threatened or extinct and 42% of all species are declining in population (2008). The highest numbers of threatened species are in Latin America, where our study took place (2008). Habitat loss, the chytrid fungal disease and climate change are the only known causes of amphibian population declines, but IUCN notes that many species are declining for unknown reasons. Three factors may have contributed to my group’s lack of amphibian observations; unsuitable habitat sites were surveyed, temperature and elevation negatively affected diversity and our observations may have been part of the global amphibian population decline.
References
OTS Las Cruces heretofauna species list: http://www.ots.ac.cr/images/downloads/biological-stations/las-cruces/species-lists/herpetofauna_list-2011.pdf
Forsyth A, Miyata K. 1984. Tropical nature: Life and death in the rain forests of Central and South America. First Touchstone Edition. New York: Simon & Schuster. 172-174.
International Union for Conservation of Nature . 2008. Analysis of Data [Internet]. Washington D.C. (District of Columbia):International Union for Conservation of Nature; [2008, cited 2016 June 11] . Available from: http://www.iucnredlist.org/initiatives/amphibians/analysis
Laurencio D, Fitzgerald LA. 2010. “Environmental correlates of herpetofaunal diversity in Costa Rica”. Journal of Tropical Ecology [Internet]. [2010 Sep, cited 2016 June 11] 26(5):521-531. Available from: : http://www.jstor.org/stable/40925452