Invasive Species

It was written extensively about the ways we affect our surroundings. The media is full of headlines screaming about global warming, rainforest decimation, oil spills and ozone depletion, but there is one major risk to the world's biodiversity that we are the major cause of and that is not being spoken about so often: invasive species.

Invasive species are defined as:

species introduced deliberately or unintentionally outside their natural habitats where they have the ability to establish themselves, invade, outcompete natives and take over the new environments. They are widespread in the world and are found in all categories of living organisms and all types of ecosystems. However, plants, mammals and insects comprise the most common types of invasive alien species in terrestrial environments.

Invasion of novel species is not a new concept. It is probabbly a sort of human disturbance that has changed the course of human history in the strongest manner: The balance of power today would probably be completely different, had the New World natives been resistant to some of the strains of disease that the Old World colonist brought with them. Colonization of the unexplored lands has brought the dramatic increase in the number and impact of alien species on the pristine niches, when the colonists brought the native species of plants and animals for their use in the New World. Since then, the things are just going downhill and invasive species is now part of the infamous top 3 damaging effects that we extend over our environment, joining pollution and habitat destruction.

Islands are the most vulnerable habitat in regards to invasive species effect. Due to the lack of space, animal and plant species on islands are made out of smaller number of smaller populations. Extremely strong speciation, due to the isolation, makes this habitat prone to huge disturbances due to novel species mainly introduced by humans. Some of the most striking examples to this phenomenon are invasion of the Brown Tree Snake (Boiga irregularis) to Guam and historical invasion of rabbits to Australia.

Plants are much more common invasive species than animals, due to their widespread use as food, building material and in horticulture. Botanical gardens are often hotspots for novel species that spread from there and invade hosting countries' habitats. An extreme example is invasion of Acacia Saligna in several countries with Mediterranean-type climate, such as South Africa or Israel.

In addition to the damage that alien species can inflict to the local biodiversity, they can pose great threat to the human health and economy. In addition to the mentioned infective diseases brought by European colonists to the New World, one of the more recent examples is a spread of the dengue fever in the US, through the far-east native Asian Tiger Mosquito (Aedes albopictus) brought into the country inside of imported tires.

The economical impact of invasive species is hard to measure in the majority of the cases due to several reasons:

• the delayed effect - sometimes it can take years for an introduced species to become alien and start affecting the local biodiversity
  
  
• the multi-level impact - due to their effect on often key players in the food webs, it is impossible to estimate the total effect on the whole of the ecosystem. Sometimes it is just one link in the chain that is affected, sometimes it is the whole chain that collapses.
  
  
• the lack of ability to put an exact price tag on biodiversity loss

However there are some well documented cases that can just give an example of the possible costs of invasion of alien species. Zebra Mussels (Dreissena polymorpha), native to the Caspian Sea, have been noticed in the Great Lakes area in the late 80-s. They have probably arrived with the ballast water released by the commercial cargo ships coming through the Hudson River. Due to their ability to multiply almost exponentially, they have quickly started to affect the local mollusk species. In addition to the biodiversity impact, the relatively small Zebra Mussel, has found its way into the drains and water pipes of lake shore cities in US and Canada and started causing immense damage. In 1989 there was even a case of a complete three day long water supply shortage in a city of Monroe, MI. Totally US Fish and Wildlife Service estimates the cost of the damage caused by Zebra Mussels in the next 10 years to about $5 billion in the US and Canada alone.

There are quite a few ways of dealing with this problem and the best one is obviously prevention. Appropriate legal measures should be taken in order to prevent any introduction, intentional and unintentional, into the country. Laws protecting biodiversity should be introduced. A good example of a country that takes its invasive species seriously is New Zealand that has introduced its Biosecurity Act which provides the border officials with extensive authorities in goal of prevention of introductions. This is understandable taken that New Zealand is specialy vulnerable to introduction of alien species being an isolated island.

All in all, if we do not take this problem seriously in any country we occupy, they may pretty soon look very bland due to the extensive loss of biodiversity brought upon by the introduction of alien plants and animals