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, the
Blanchard's 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.
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.
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
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.
The dangers of acid rain have been known for years -- it acidifies lakes and ponds
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, DDT was banned in Canada in 1972 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 Mussels 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/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:
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?