SAT Reading - Khan Diagnostic Quiz level 2 - reading 9

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
₅ 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
₁₀ 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
₁₅ 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.
₂₀ 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
₂₅ 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
₃₀ 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
₃₅ 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
₄₀ result from either direct or indirect human activities.
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
₄₅ 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
₅₀ extinction exists.
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
₅₅ 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
₆₀ 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
₆₅ streamside habitats, are most likely to be severely

threatened.