Distribution and Conservation StatusSee also: Decline in amphibian populations
Frogs are found on all the continents except Antarctica but they are not present on certain islands, especially those far away from continental land masses. Many species are isolated in restricted ranges by changes of climate or inhospitable territory such as stretches of sea, mountain ridges, deserts, forest clearance, road construction or other man-made barriers. There is usually a greater diversity of frogs in tropical areas than in temperate regions such as Europe. Some frogs inhabit arid areas such as deserts and rely on specific adaptations to survive. Members of the Australian genus Cyclorana bury themselves underground where they create a water-impervious cocoon in which to aestivate during dry periods. Once it rains, they emerge, find a temporary pool and breed. Egg and tadpole development is very fast in comparison to that of most other frogs so that breeding can be completed before the pond dries up. Some frog species are adapted to a cold environment. The Wood Frog (Rana sylvatica), whose habitat extends into the Arctic Circle, buries itself in the ground during winter. Although much of its body freezes during this time, it maintains a high concentration of glucose in its vital organs which protects them from damage.
In 2006, of 4,035 species of amphibians that depend on water during some life cycle stage, 1,356 (33.6%) were considered to be threatened. This is likely to be an underestimate because it excludes 1,427 species for which there was insufficient evidence to assess their status. Frog populations have declined dramatically since the 1950s. More than one third of frog species are considered to be threatened with extinction, and more than 120 species are believed to have become extinct since the 1980s. Among these species are the gastric-brooding frogs of Australia and the golden toad of Costa Rica. The latter is of particular concern to scientists because it inhabited the pristine Monteverde Cloud Forest Reserve and suffered a population crash in 1987, along with about twenty other frog species found in the area. This could not be linked directly to human activities such as deforestation and was outside the range of normal fluctuations in population size. Elsewhere, habitat loss is a significant cause of frog population decline, as are pollutants, climate change, increased UVB radiation and the introduction of non-indigenous predators and competitors. A Canadian study conducted in 2006 suggested that heavy traffic in their environment was a larger threat to frog populations than was habitat loss. Emerging infectious diseases including chytridiomycosis and ranavirus are also devastating populations.
Many environmental scientists believe that amphibians, including frogs, are good biological indicators of broader ecosystem health because of their intermediate position in food chains, their permeable skins and typically biphasic life (aquatic larvae and terrestrial adults). It appears that species with both aquatic eggs and larvae are most affected by the decline, while those with direct development are the most resistant.
There has been an increase in frog mutations and genetic defects since the 1990s. These often include missing limbs or extra legs. Various causes have been identified or hypothesized including an increase in ultraviolet radiation affecting the spawn on the surface of ponds, chemical contamination from pesticides and fertilizers, and parasites such as the trematode Ribeiroia ondatrae. Probably all these are involved in a complex way as stressors, environmental factors contributing to rates of disease and vulnerability to attack by parasites. Malformations impair mobility and the individuals may not survive to adulthood. An increase in the number of frogs eaten by birds may actually increase the likelihood of parasitism of other frogs because the trematode's complex life cycle includes the ramshorn snail and several intermediate hosts such as birds.
In a few cases, captive breeding programs have been established and these have largely been successful. In 2007, it was reported that the application of certain probiotic bacteria could protect amphibians from chytridiomycosis. One current project, The Panama Amphibian Rescue and Conservation Project, has subsequently been developed in order to rescue species at risk of this disease in eastern Panama and to develop field applications for probiotic therapy. The World Association of Zoos and Aquariums named 2008 as the "Year of the Frog" in order to draw attention to the conservation issues faced by them.
The cane toad (Bufo marinus) is a very adaptable species native to South and Central America. In the 1930s it was introduced into Puerto Rico, and later various other islands in the Pacific and Caribbean region, as a biological pest control agent. In 1935, three thousand toads were liberated in the sugar cane fields of Queensland, Australia in an attempt to control cane beetles such as Dermolepida albohirtum, the larvae of which damage and kill the canes. Initial results in many of these countries were positive but it later became apparent that the toads upset the ecological balance in their new environments. They bred freely, competed with native species of frog, ate bees and other harmless native invertebrates, had few predators in their adopted habitats, and poisoned pets, carnivorous birds and mammals. In many of these countries they are themselves now regarded both as pests and invasive species, and scientists are looking for a biological method to control them.
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