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
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, the
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 other
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
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.
People collect amphibians for food, medicines, bait, pets and even for teaching
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 unclear how much
collecting amphibian populations can sustain or what would be a sustainable
harvest. In many jurisdictions in Canada even the number of frogs collected each
year isn't known.
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.
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.
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.
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 Declining Amphibian Populations
"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:
If you require a paper copy, contact John Wilkinson, DAPTF Coordinator."
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
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