Invasive Species
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Invasive Species

In a few words…
Invasive, exogenous, allochtonous… There are many ways to name these species, which are at the root of a surprising phenomenon caused by human activity. A species is called “invasive” when it is introduced, accidentally or deliberately, in a new environment outside its natural distribution area but in which it can still breed. A famous example is the introduction of 24 common rabbits in Australia by an English farmer, in 1859. In 1926, the size of their population was estimated at ten billion, a true calamity for this country.

Overview of the situation
The idea that animal and plant species can pose serious threats to biodiversity may seem strange, yet the problem is very real: in ecology just like anywhere else, everything is a question of balance.

The Australian rabbits provide an educational example. They are common in Europe, where they fall prey to numerous predators such as cats, dogs, falcons, ermines and foxes. The situation is very different in Australia where rabbits are almost completely free from natural predators. There, they multiplied without the “biological control” normally exerted by predators. Today, due to their sheer numbers, rabbits threaten many plants unique to Australia, and they are in direct competition with several threatened or vulnerable species.

Numerous measures have been implemented to counter this veritable “biological invasion”: vast fences were built, a rabbit disease (myxomatosis) was introduced, poison was used, lairs were destroyed with explosives and bulldozers or sometimes fumigated, and rabbits were hunted intensively. In spite of certain successes, particularly with myxomatosis in the 50’s, the current population of common rabbits in Australia remains significant with over 300 million individuals.

In a sample of 57 countries, over 542 invasive species were found – including vascular plants, fish, mammals, birds and amphibians – whose impact on biodiversity is known. In each country, an average of 50 exotic invasive species were found, and it is believed that this may actually be an underestimation of the real threat.

We estimate that among the 11,000 exotic species present in Europe, approximately one in ten has an ecological impact, and that a slightly higher proportion is at the root of financial losses. The current structure of commercial exchanges on a global scale suggests that the European situation is most likely reproduced everywhere else and that the scale of the invasive species problem is increasing all over the world.

The control or eradication of invasive species can decrease the extinction risk for many species affected by this phenomenon. Success has been achieved in this regard: eleven species of birds (since 1988), five species of mammals (since 1996) and one species of amphibians (since 1980) have seen their risk for extinction decrease thanks to such measures. The Red List Index, however, still shows that almost three times as many birds, twice as many mammals and over 200 times more amphibians have seen their state of conservation deteriorate, essentially under the increased threat of invasive animals, plants and microorganisms.

Generally speaking, on average, birds, mammals and amphibians have become increasingly threatened because of invasive species. In spite of the fact that other groups of species have not yet been thoroughly evaluated, we do know that invasive species are the second most important root cause behind the disappearance of freshwater mussels and that they are key factors in the extinction of endemic species (species that are unique to a specific location) in general.

Islands are particularly vulnerable to exogenous species. Over 950 catalogued islands are experiencing similar issues all over the world. The invasive species that threaten these insular environments are, in the overwhelming majority of cases, rats, goats, cats, pigs, foxes and mice.

A few classic cases of invasive species
– Rabbits, foxes and cats in Australia
– Nile perches in Victoria Lake
– Zebra mussels in the Great Lakes
– Stonefish in the Caribbean Sea
– Water hyacinths (aquatic plant) all over the world

Did you know?
Approximately 85 invasive species have been identified in the St. Lawrence River. Among them, 60% are plants, 20% are fish and another 20% are invertebrates. The Great Lakes are now home to nearly 165 exogenous species (species that come from elsewhere).

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