(This exercise is based on Whitfield, S. M., K. E. Bell, T. Philippi, M. Sasa, F. Bolaños, G. Chaves, J. M. Savage, and M. A. Donnelly. 2007. Amphibian and reptile declines over 35 years at La Selva, Costa Rica. PNAS 104: 8352–8356.)
(Note: The reference above links directly to the article on the journal’s website. In order to access the full text of the article, you may need to be on your institution’s network [or logged in remotely], so that you can use your institution’s access privileges.)
In Chapter 1 of the textbook, we saw that morphological deformities have been found in many species of frogs and appear to be increasing in frequency. Several factors, including parasites, pesticides, and other chemicals, contribute to the deformities. Chapter 1 also describes how population sizes of frogs and other amphibians are declining, to extinction in some cases, in many parts of the world. These declines appear to be due to several factors, and these factors may differ across species and localities. While some of the causes for the declines are the same as those that appear to explain the deformities, some are not.
One of the factors hypothesized as a possible cause of some of the population declines is global climate change.
Figure 1 Amphibian and reptile density over 35 years at La Selva Biological Station, Costa Rica. Each point indicates mean density for all quadrats in a given year. The closed symbols and solid line indicate data from primary forest. The open symbols and dashed line indicate data from abandoned cacao plantations. (A) Trends for all terrestrial amphibians and reptiles. (B) Trends for frogs only. (C) Trends for lizards only.
Question 1
For more than 35 years, biologists have monitored the population densities of many species of amphibians and reptiles at La Selva Biological Station in the lowlands of Costa Rica. Figure 1 shows the densities of these organisms both at the La Selva Biological Station as well as in nearby abandoned cacao plantations. Based on these figures, what can be said about the densities of total amphibians and reptiles, just frogs, and just lizards at the station? What can be said about the densities of these organisms in the abandoned plantations? (Note that each y axis is on a logarithmic scale.)
Table 1 Population trends for leaf litter amphibians and reptiles at La Selva Biological Station (Click to enlarge).
Question 2
Table 1 shows population density trends for individual species. Identify two lizard species that have shown statistically significant population declines in forest habitat. Which frog species have not shown statistically significant declines in forest habitat? (Be sure to read the description of the data in the footnote to Table 1.)
Figure 2
Question 3
Figure 2 shows climate data taken at the La Selva Biological Station during the period 1970–2005. Based on this figure, what changes in the climate have occurred during this time period?
Question 4
Based on the data shown, can one, with confidence, state that the climate changes are directly responsible for the declines in frogs? Why or why not?
