新SAT阅读science level 2-4 Amphibians in Crisis 逐句精翻
2018-04-12 09:46阅读：
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.* 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
diseases; global warming also has been invoked as a contributing
factor. 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.
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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.
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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.
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Living amphibians (Class Amphibia, Subclass Lissamphibia) include
frogs (~5,600 currently recognized species), salamanders (~570
species), and caecilians (~175 species). 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
exists.
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The Global Amphibian Assessment completed its first round of
evaluating the status of all then-recognized species in 2004,
finding 32.5% of the known species of amphibians to be “globally
threatened” by using the established top three categories of threat
of extinction (i.e. Vulnerable, Endangered, or Critically
Endangered); 43% of species have declining populations. In general,
greater numbers as well as proportions of species are at risk in
tropical countries. 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. Species
from montane tropical regions, especially those associated with
stream or streamside habitats, are most likely to be severely
threatened.
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