Amphibians In Losing Race With Environmental Change
Pastor Dale Morgan
Amphibians In Losing Race With Environmental Change*
In the face of current environmental challenges, amphibians appear to be
losing the battle - of 5,743 known species of amphibians on Earth, 43
percent are in decline, 32 percent are threatened and 168 species are
believed extinct. The impacts of changes are far more pervasive on
amphibians than many other vertebrates, such as birds or mammals.
Corvallis OR (SPX) May 03, 2007
Even though they had the ability to evolve and survive for hundreds of
millions of years - since before the time of the dinosaurs and through
many climatic regimes - the massive, worldwide decline of amphibians can
best be understood by their inability to keep pace with the current rate
of global change, a new study suggests.
The basic constraints of evolution and the inability of species to adapt
quickly enough can explain most of the causes that are leading one
species after another of amphibians into decline or outright extinction,
say researchers from Oregon State University, in a study published today
in the journal BioScience.
"We know that there are various causes for amphibian population
declines, including UV-B light exposure, habitat loss, pesticide
pollution, infections and other issues," said Andrew Blaustein, a
professor of zoology at OSU and one of the world's leading experts on
amphibian decline.
"But looked at in a different way, it's not just that there are threats
and pressures amphibians have to deal with," Blaustein said. "There have
always been threats, and these have been some of the most adaptive and
successful vertebrate animals on Earth. They were around before the
dinosaurs, have lived in periods with very different climates, and
continued to thrive while many other species went extinct. But right
now, they just can't keep up."
It has been estimated that the rate of plant and animal extinction is
greater now than any known in the last 100,000 years, the researchers
note in their report. Amphibians are of particular interest because
their physiology and complex life cycle often exposes them to a wider
range of environmental changes than other species must face - they have
permeable skin, live on both land and water, their eggs have no shells.
In the face of these challenges, amphibians appear to be losing the
battle - of 5,743 known species of amphibians on Earth, 43 percent are
in decline, 32 percent are threatened and 168 species are believed
extinct. The impacts of changes are far more pervasive on amphibians
than many other vertebrates, such as birds or mammals.
"Historically, amphibians were adept at evolving to deal with new
conditions," Blaustein said. "What they are doing is showing us just how
rapid and unprecedented are the environmental changes under way. Many
other species will also be unable to evolve fast enough to deal with
these changes. Because of their unique characteristics, the amphibians
are just the first to go."
In their analysis, the OSU scientists point out that evolution is not a
precise or perfect process - it takes time, is constrained by historic
changes and compromises, and does not always allow a species to adapt in
a way that meets rapidly changing conditions. Through genetic variation
and natural selection pressures, some species or populations will be
able to adapt - while others fail and go extinct.
The systems developed over millions of years to give amphibians survival
advantages have now turned against them, scientists say. Examples include:
+ Many amphibians lay their eggs in shallow, open water in direct
sunlight to provide a more oxygenated environment, increase growth rate
of larvae and reduce predation. But the increased levels of UV-B
radiation in today's sunlight, due to erosion of the Earth's ozone
layer, is causing mutations, impaired immune systems and slower growth
rates. Through evolution, amphibians were able to adapt to changing UV-B
levels in the past, but the current change has occurred too rapidly.
+ In the past, water was reasonably pure and clean. But increased
"eutrophication" of freshwater ponds due to use of modern fertilizers
and waste from grazing animals has led to higher rates of parasite
infections, and chemical contamination of aquatic systems is also more
common.
+ Many animal species lay their eggs communally or congregate socially,
often to avoid predation or improve resource use. But global warming has
caused higher levels of certain infectious diseases of some amphibians,
and it spreads more easily in closely connected communities.
"Although relatively rapid evolution may occur within some amphibian
populations when a novel threat arises, other threats may be too intense
and too new for amphibians to cope with them," the researchers wrote in
their report. "Behaviors and ecological attributes that have probably
persisted, and were probably beneficial, for millions of years . . .
under today's conditions may subject amphibians to a variety of damaging
agents."
Natural selection and species adaptation may, in time, allow amphibians
to react to and recover from the new environmental insults, Blaustein
said, if they don't go extinct first.
But evolution is an erratic, often slow and imperfect system, and the
complexities of amphibian life cycles makes them more immediately
vulnerable than many other species, the researchers said.