Background Information on
Amphibians
Five mass extinction events and consequent
biotic rebounds have been documented in the geologic record. The
event at the end of the Permian period eliminated 96 percent of
extant species (Eldredge, 1999). The causes of these historical
extinctions are difficult to determine. Current theories suggest
that the mass extinctions resulted from changes in global climate,
increases in volcanic activity (Eldredge, 1999), or collision between
the earth and some extraterrestrial object (Alvarez and others,
1980). There is strong evidence that a sixth extinction event is
ongoing (Wilson, 1993). In contrast to the earlier mass extinctions,
human activities, specifically direct habitat destruction, overexploitation
of natural resources, and introduction of non-native species are
believed to be the cause of this sixth event (Eldredge, 1999).
Amphibians evolved around 350 million years
ago during the Devonian period and have survived three mass extinction
events. The three main groups of amphibians are frogs, salamanders,
and caecilians. In North America, the greatest density of anuran
(frog) species is in the south Atlantic coastal plain and the Gulf
Coast, while the greatest density of salamanders is in the southern
Appalachian mountains, extending south to the Gulf Coast. In the
last few decades, the decline and in some cases loss of amphibian
populations has been identified as a global problem (Houlahan and
others, 2000). Many instances of amphibian decline have been documented,
and many causes of amphibian decline have been proposed (Blaustein
and Wake, 1990; Orchard, 1992; Koonz, 1992; Corn, 1994; Dodd, 1997;
Alford and Richards, 1999; Rouse and others, 1999). Instances of
amphibian population decline may be the result of natural fluctuations
(Pechmann and others, 1991), but some of the declines are likely
the result of changes in local conditions, such as the introduction
of a contaminant or the alteration of a habitat. Declining amphibian
populations have also been attributed to regional phenomena like
changes in climate, introduction of non-native competitive species,
or exposure to pathogens. Many herpetologists believe that a combination
of stresses is being placed on amphibian populations, and that the
combination of stresses puts some species more at risk than others.
Rigorous long-term census studies of amphibians worldwide are needed
to assess the magnitude and direction of changes in amphibian populations
(Blaustein and others, 1994).
Almost all amphibians spend a part of their
life-cycle in aquatic habitats, primarily for mating, oviposition,
and larval growth (Semlitsch and others, 1996). Amphibians have
many unique or unusual developmental characteristics. Some representatives
from this class of animals can breath through their skin, regenerate
lost body parts, change skin color, produce deadly poisonous secretions,
or during cold weather, enter a state of torpor during which their
heart stops and they cease to breath air only to return to normal
when warmer weather returns (Cunningham and others, 1912). Because
amphibians have highly permeable skins, they may be more susceptible
to damage from environmental toxins than other terrestrial and aquatic
invertebrates (Alford and Richards, 1999). There is a need for a
better understanding of habitat suitability and the stressors that
affect amphibian populations over a wide range of ecoregions. Specifically,
an evaluation scheme that incorporates hydrologic variables is needed
to: (1) evaluate natural habitat suitability for amphibians, (2)
determine the vulnerability of the habitat to stressors such as
changes in the hydrologic regime, increases in ultraviolet radiation,
modification of natural water chemistry, or introduction of chemical
contamination, and (3) determine if potential stressors to amphibian
populations are present at biologically relevant times.
References
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