Amphibians In Crisis

Amphibians have received much attention during the last two decades because of a now-general understanding that a larger proportion of amphibian species are at risk of extinction than those of any other taxon (SN Stuart et al., 2004). Why this should be has perplexed amphibian specialists. A large number of factors have been implicated, including most prominently habitat destruction and epidemics of infectious disease (Pechmann and Wake, 2006); global warming also has been invoked as a contributing factor (Pounds et al., 2006). What makes the amphibian case so compelling is the fact that amphibians are long-term survivors that have persisted through the last four mass extinctions.

Paradoxically, although amphibians have proven themselves to be survivors in the past, there are reasons for thinking that they might be vulnerable to current environmental challenges and, hence, serve as multipurpose sentinels of environmental health. The typical life cycle of a frog involves aquatic development of eggs and larvae and terrestrial activity as adults, thus exposing them to a wide range of environments. Frog larvae are typically herbivores, whereas adults are carnivores, thus exposing them to a wide diversity of food, predators, and parasites. Amphibians have moist skin, and cutaneous respiration is more important than respiration by lungs. The moist, well-vascularized skin places them in intimate contact with their environment. One might expect them to be vulnerable to changes in water or air quality resulting from diverse pollutants. Amphibians are thermal-conformers, thus making them sensitive to environmental temperature changes, which may be especially important for tropical montane (e.g., cloud forest) species that have experienced little temperature variation. Such species may have little acclimation ability in rapidly changing thermal regimes. In general, amphibians have small geographic ranges, but this is accentuated in most terrestrial species (the majority of salamanders; a large proportion of frog species also fit this category) that develop directly from terrestrial eggs that have no free-living larval stage. These small ranges make them especially vulnerable to habitat changes that might result from either direct or indirect human activities.

Living amphibians (Class Amphibia, Subclass Lissamphibia) include frogs (Order Anura, =5,600 currently recognized species), salamanders (Order Caudata, =570 species), and caecilians (Order Gymnophiona, =175 species) (AmphibiaWeb, 2007). Most information concerning declines and extinctions has come from studies of frogs, which are the most numerous and by far the most widely distributed of living amphibians. Salamanders facing extinctions are centered in Middle America. Caecilians are the least well known; little information on their status with respect to extinction threats exists (SN Stuart et al., 2004).

Amphibians are not distributed evenly around the world. Frogs and caecilians thrive in tropical regions (Fig. 2.1). Whereas caecilians do not occur outside the tropical zone, frogs extend northward even into the Arctic zone and southward to the southern tips of Africa and South America. Salamanders are mainly residents of the North Temperate zone, but one subclade (Bolitoglossini) of the largest family (Plethodontidae) of salamanders has radiated adaptively in the American tropics. The bolitoglossine salamanders comprise nearly 40% of living species of salamanders; =80% of bolitoglossines occur in Middle America, with only a few species ranging south of the equator.

The New World tropics have far more amphibians than anywhere else. Fig. 2.1 shows the number of species in relation to the size of countries [all data from AmphibiaWeb (2007)]. The Global Amphibian Assessment completed its first round of evaluating the status of all then-recognized species in 2004 (SN Stuart et al., 2004), finding 32.5% of the known species of amphibians to be ''globally threatened" by using the established top three

FIGURE 2.1 Global amphibian species diversity by country visualized using density-equalizing cartograms. Country size is distorted in proportion to the total number of amphibian species occurring in each country relative to its size. (Inset) Baseline world map. Brazil (789 species) and Colombia (642) have the largest number of species. China (335) has the largest number of species in the Old World. The Democratic Republic of the Congo (215) has the largest number from continental Africa. However, 239 species are recorded from Madagascar. Australia has 225 species, and Papua New Guinea has 289. In North America, Mexico has the largest number of species (357). There are 291 species in the United States. Prepared by M. Koo (see Acknowledgments).

FIGURE 2.1 Global amphibian species diversity by country visualized using density-equalizing cartograms. Country size is distorted in proportion to the total number of amphibian species occurring in each country relative to its size. (Inset) Baseline world map. Brazil (789 species) and Colombia (642) have the largest number of species. China (335) has the largest number of species in the Old World. The Democratic Republic of the Congo (215) has the largest number from continental Africa. However, 239 species are recorded from Madagascar. Australia has 225 species, and Papua New Guinea has 289. In North America, Mexico has the largest number of species (357). There are 291 species in the United States. Prepared by M. Koo (see Acknowledgments).

categories of threat of extinction (i.e., Vulnerable, Endangered, or Critically Endangered); 43% of species have declining populations (International Union for the Conservation of Nature, 2007). In general, greater numbers as well as proportions of species are at risk in tropical countries (e.g., Sri Lanka with 107 species, most at risk; nontropical New Zealand has an equivalent proportion, but has only 7 species) (Fig. 2.2). Updates from the Global Amphibian Assessment are ongoing and show that, although new species described since 2004 are mostly too poorly known to be assessed, >20% of analyzed species are in the top three categories of threat (Global Amphibian Assessment, 2007). Species from montane tropical regions, especially those associated with stream or streamside habitats, are most likely to be severely threatened.

We present a case study from our own work to explore the reasons underlying declines and extinctions of amphibians.

0 0

Post a comment