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