Factors contributing to declines in amphibian population sizes and occurrences
Are amphibian population sizes and occurrence declining?
Researchers from around the globe have been asking this question since 1990. The short
answer is yes, sure they are. Have you seen many frogs in downtown Toronto lately? How
about Vancouver or Edmonton? They used to be there. There used to be lots of them before
we replaced woodlots with parking lots and wetlands with highrises. Of course that was a
long time ago. And that's not really what we mean when we muse about declines in amphibian
population sizes and occurrences is it?
Amphibian populations are declining in size and occurrence, but then so are many other
groups of organisms aside from humans and those species that have adapted to our ways such
as Cockroaches and Starlings. Are amphibians declining above and beyond the average rate of
decline of other species? That is a much more difficult question to answer. At this point the
answer is not clear.
We do know that amphibian habitat is declining. On the prairies approximately 70% of
historic wetlands have been drained, most turned into farmland. In southern Ontario, the
situation is even more dire: 90% of wetlands have been drained. Think about that for a
moment. Only one in ten ponds still remains. Imagine if we removed 90% of the apartment
buildings. Would anyone be surprised if the number of humans in the area declined?
And yet habitat loss alone cannot explain the situation. Amphibian declines have also
been reported in relatively pristine sites. For example, the Golden Toad of the Monteverde
forests of Costa Rica, (only discovered by science in the 1960s) is now extinct, despite the
fact that its habitat was protected by a vast nature reserve.
No Canadian species has yet gone extinct, however,
Northern Cricket Frog is no longer
believed to occur in Canada (extirpated). A number of species have declined dramatically,
particularly the Northern Leopard
Frog in western Canada. Even Leopard Frogs can be very abundant where they still occur,
they just aren't found in as many places as in the past. In total 17 of 45 species of
Canadian salamanders, toads and frogs are known to have undergone loss of populations.
Habitat loss is likely the primary cause of all species declines in Canada. That doesn't
mean other factors aren't important, only that habitat loss is a large and ongoing problem.
We may feel that because Canada is so large we don't need to worry as much, but Canada's
size only means we have a larger a responsibility to take care of what we have.
Why are amphibians in trouble?
In addition to habitat loss there are a large number of ways we are modifying the Earth,
consciously or not. All of these factors have some effect on amphibians. It is often
difficult or even impossible to sort out which factor or group of factors are the most
threatening to amphibians and many organisms.
Habitat Fragmentation
Even when good habitats remain, habitat fragmentation can cause declines. If remaining ponds
are isolated or the land between ponds is inhospitable, those few ponds may not support
amphibian populations for the long-term. Many populations rely on immigrants from other
populations to support them when local reproduction has been poor, to reduce inbreeding or
to help them recover from local catastrophes. When the connections among ponds are destroyed
these processes cannot continue and slowly these small populations die out and the ponds may
not be recolonized.
Traffic Mortality
On warm, rainy nights in the spring, many amphibians migrate from overwintering sites to
breeding ponds. If a road should separate these two habitats, slaughter can be the result.
Along one stretch of road less than 4 km long at Long Point, in southern Ontario, over 10 000
Leopard Frogs were killed in just one year. Even a moderate volume of cars can wipe out
entire populations.
Collecting
biology. In some parts of the world collecting alone appears to cause declines. In eastern
Ontario up to 45 000
Bullfrogs have been collected in a single year (this is now prohibited) and over a
million Leopard Frogs have been collected in Manitoba in some years. It is how much
collecting amphibian populations can sustain or what would be a sustainable harvest. In many
jurisdictions in Canada even the number of frogs each year isn't known.
Acid Rain
The dangers of acid rain have been known for years -- it acidifies lakes and ponds killing
many plants and animals. The eggs and larvae of amphibians are particularly sensitive and,
unfortunately, it is generally in the spring that ponds are the most acidic, a result of the
melting of "acid snow." In addition to the direct effects, acid rain also mobilizes other
toxins into ponds, poisoning eggs, larvae and juveniles.
Agricultural Chemicals
Some amphibians thrive in agricultural areas enjoying the benefits of irrigation and
artificial ponds for watering livestock. Nonetheless the increasing use of pesticides poses
a threat. In Québec, grossly deformed frogs have been found in areas where pesticide
use is especially heavy. Pesticides can also have lethal effects years after their use. For
example, it has been over 20 years since DDT was banned in Canada and yet it is commonly
found in the tissues of many amphibians in certain areas. Some governments are taking
action: Australia has recently banned one herbicide because it caused declines in local
amphibian populations. Ironically, irrigation and artificial ponds may actually attract
amphibians to breed in sites where their chance of success is low.
Hormone-mimicking Chemicals
A number of chemical contaminants are hard to detect and study because they rarely outright
kill their victims. Instead, they mimic natural hormones we (and amphibians) have in our
own bodies. In some fish and reptiles these contaminants interfere with reproduction and
can even cause sterility. Toxicologists continue to try and understand this complex subject,
but meanwhile we continue to emit more and new kinds of these contaminants into our
environment.
Introductions of Exotic Species
Humans are constantly introducing animals and plants from their native habitats into new
areas. Sometimes this is done intentionally, for example stocking a lake with game fish,
and sometimes accidentally, such as the release of Zebra Mussells into the Great Lakes.
Many amphibians can only survive in fishless ponds. When these are stocked with fish the
amphibians are eventually eliminated. Ironically, amphibians themselves are sometimes the
introduced species. Bullfrogs escaped or released from "farms" where they were raised for
frogs' legs have resulted in their introduction to many areas, such as British Columbia.
Bullfrogs are voracious predators of smaller frogs and have been implicated in declines in
the western US. Introduced species may also out-compete and eventually displace native
species. However, the most insidious effect of introducing exotic species comes with the
diseases they may harbour.
Disease
The tragic story of how European diseases killed thousands of native Canadians is well known.
Humans are more susceptible to diseases they and their ancestors have never been exposed to
before. The same is true of plants and animals. There is some evidence that amphibian
populations in Australia and Central America may have declined because of diseases
introduced by releasing non-native fish. Releasing non-native or captive reared amphibians
or reptiles could also spread disease. Never release a pet into the wild.
Learn about testing amphibians for Chytridiomycosis
From December 2002 Froglog
(newsletter of IUCN/SSC Amphibian Specialist Group
(formerly IUCN Declining Amphibian Populations Task Force (DAPTF)):
"The DAPTF has received two sets of notes concerning methods currently available for
testing amphibians for the presence of chytrid fungus. These have been provided by Alex Hyatt
(Australia) and Pisces Molecular (USA).Both can be accessed from the front page of our
website: www.open.ac.uk/daptf
Ultra-violet Radiation
The UV Index has become a familiar part of the summer weather forecasts. Decreased
stratospheric ozone permits more UV radiation to reach the Earth, causing an increase in
skin cancers and cataracts. Frogs don't have the benefit of sunblock. And their eggs may
be even more unprotected. Many frogs lay their eggs at the surface of the water so the sun
can warm them and speed their development. Recent research from Oregon has shown that
current levels of UV radiation can reduce the hatching success of the eggs in some
species of frogs. Research continues to try and determine which species are the most
susceptible to increasing UV.
Global Climate Change
Amphibians like warmth; global warming should be good for them, right? Unfortunately nature
is never that simple. Some species may benefit, however global climate change includes more
than just warming. It is likely to be accompanied by drought in some areas and flooding in
others, more severe storms, rising sea level and less predictable temperatures. Amphibians
are very dependent on the weather, particularly for the survival of their eggs and larvae.
Dramatic changes in the climate could eliminate some species from large areas. For example,
increased drought across the prairies could prevent many species from transforming before
ponds dry up each year.
Why should I care?
Why should we care about any species other than ourselves? There are a lot of reasons why
people do care. Frogs and Salamanders are fascinating. Studying them can teach us about
the variety of ways animals live and how natural systems function. Just as learning about
other cultures can teach us about our own, learning about other species can give us insight
into what it means to be Human. Amphibians are as strange and different from us as Aliens
or Dinosaurs, but unlike such creatures, amphibians live in our backyards. We can learn
directly from them.
Many species of amphibians provide a direct benefit to humans. They eat enormous numbers
of insects and are significant predators of both agricultural pests and biting insects. They
provide food for fish, birds and mammals. Some of the large species provide food for humans.
There is even a pain killer derived from the toxins of the Poison Frog of South America.
Amphibians may be especially important as indicators of environmental health. Because
most species have both aquatic and terrestrial life stages they can signify changes in both
types of environments. In addition, because amphibians breathe through their skin, they may
be more susceptible to pollution than other species and provide early warning of serious
problems. We would be foolish not to heed these warnings.
Most important of all though, we have a moral and ethical responsibility to all species.
We have become one of the most powerful species on this planet, capable of shaping the land
and altering the very atmosphere. If we judge Human rulers on how they treat the poorest in
our societies, should we not judge ourselves on how we treat other species -- not just the
dramatic, beautiful or the useful ones -- but even those which are tiny, ugly or far removed
from our daily lives?
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