Amphibiens en diminuation
Sont les amphibiens en diminuation?
Les cherecheurs autour du monde se posent cette question depuis 1990. La réponse
courte est " oui, ils sont en diminuation. Avez-vous vu plusieurs grenouilles en centre-
ville de Toronto ces derniers temps? Peut-être à Vancouver ou bien Edmonton?
Elles y étaient une fois. Elles y étaient en masse avant que nous avons
remplacé les forêts paysannes avec des parcs de stationnement et les milieux
humides avec des immeubles de grande hauteur. Certes, que ça fait longtemps. Et,
après tout, ce n'est pas ce qu'on veut dire quand on parle de la diminuation des
amphibiens, vrai?
Sont les amphibiens en diminuation?
Ceci n'est pas la question. Les amphibiens sont en diminuation, mais plusieurs groupes
d'organismes à l'exclusion des humaines sont aussi en diminuation et ses
espèces ont adaptés à nos manières comme les coquerelles
et les étourneaux. Est-ce que les amphibiens diminuent à taux mayen plus
vite que les autres espèces? Il est beaucoup plus difficile de répondre
à cette question. En ce moment, la réponse n'est pas claire.
Nous ne savons pas si l'habitat des amphibiens est en déclination. Environ
70% des milieux humides historiques sur les Prairies ont été
asséchés et la plupart sont devenus des terres agricoles. Au sud de
l'Ontario la situation est encore plus grave : 90% des milieux humides ont
été asséchés. Pensez-y un moment.Seulement un étang
en dix reste. Imaginez si nous détruisions 90% des immeubles d'habitation. Sera
personne surprise si le nombre des humaines dans la région diminuait?
Mais encore la perte d'habitat seule ne peut pas expliquer la situation. Une
diminuation d'amphibiens à été notée même aux sites
sains. Par exemple, le crapaud doré des forêts Monteverde de Costa Rica
(seulement découvert par les scientifiques dans les années 1960) est
maintenant disparu, malgré le fait que son habitat était
protégé d'une grande réserve naturelle.
Aucune espèce canadienne n'est disparue encore, par contre il est plus cru
que la rainette criquet du nord habite au Canada . Un grand nombre des espèces
ont considérablement diminuées, La grenouille léopard du Canada de
l'ouest en particulier. Les grenouilles léopards peuvent être nombreux
où elles existent encore, mais elles ne se trouvent pas où nous les avons
trouvées dans le passé. En total, 17 sur 45 espèces de salamandres,
de crapauds et de grenouilles canadiens sont connus d'avoir une perte de population. La
perte de l'habitat est probablement la cause principale de la déclination de
toutes les espèces au Canada. Ceci ne veut pas dire que les autres facteurs ne
sont pas importants, seulement que la perte de l'habitat est un grand et continuel
problème. Nous sommes de l'opinion que parce que le Canada est si grand que nous
n'avons pas bessoin de nous inquiéter autant, mais la taille du Canada veut
seulement dire que nous avons une plus grande responsibilité de soigner ce que nous
avons.
Pourquoi les amphibiens sont-ils en déclination ?
En plus de la perte de l'habitat, il y a un grand nombre de facons que nous changeons
la terre, si nous les changeons en le rendant compte ou non. Tous ces facteurs ont des
conséquences sur les amphibiens. Il est souvent difficile ou même impossible
de savoir quel groupe de facteurs est le plus menaçant aux amphibiens.
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
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.
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 Declining Amphibian Populations Task
Force):
"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
If you require a paper copy, contact John Wilkinson, DAPTF Coordinator."
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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