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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