Calling them "exotics" may add a veneer of mystery to them, but invasive species are deadly by any name. While many invasive species begin as purposefully introduced ones, they often tend to take over new habitats and create unforeseen problems. These problems include a wide range of destructive ones from killing off other native species, to causing threats to health, agriculture and food security. It is estimated that the cost of dealing with invasive species and the wake of destruction that they leave behind is over US Dollars 1.5 trillion per year. This is equivalent of about 5 per cent of global GDP.  

The Invasive Species Compendium, an initiative of Agriculture Ministries in different parts of the world is a detailed database of invasive species threatening livelihoods and the environment which currently lists over 1500 species, though more are being added every day. As of now, 35 per cent of the database consists of aquatic and terrestrial plants, 30 per cent are pests that affect plants and the environment, 15 per cent are aquatic animals, 15 per cent are pathogens and 5 per cent are terrestrial vertebrates.

The list includes well known examples such as rabbits in Australia that were introduced in the 18th century by the First Fleet, to water hyacinths and lantana in Africa and Asia to the lesser known Asian longhorn beetle found in Kent. In fact, until humans learned how to build ships and became a sea-faring species, the problem of invasive species not significant. With global trade, transporting of plants and animals to new continents began.

Along with the intentionally transported, were those who went along, unknown to their human handlers. These included earthworms, now considered essential to agriculture and apparently considered by Charles Darwin as one the most influential creatures on the planet! More recent but equally unintentional immigrants are the Zebra Mussels that travelled from Russia to North America attached to ship hulls and now spread widely from Lake St. Clair to the Mississippi River and the Great Lakes. Not only have they managed to make the local mussels near extinct, they have also managed to change the real estate prices around the lakes as they have very sharp edges that are particularly good at cutting human feet!

In 2003, invasive species were declared by the Hawaian State Legislature as �€œthe single greatest threat to Hawaii�€™s economy and natural environment and to the health and lifestyle of Hawaii�€™s people.�€ With an idea to �€œstop the silent invasion�€, the Hawaii Invasive Species Council (HISC) was set up to provide policy direction, coordination, and planning for the control and eradication of harmful invasive species infestations. The HISC publishes a list of high profile invasive species in Hawaii, along with their description, the potential damage that they can cause and how they can be controlled. Interestingly this list also has potential invasives for Hawaii that could wreak chaos in the future, in case they do manage to get a foothold, such as the Brown Tree Snake that managed to do so in Guam shortly after World War II. So far, only dead Brown Tree Snakes have been found in Hawaii among goods that have been imported from Guam.

Another snake has been much in the news recently that has lead to a renewed debate on invasive species. A record breaking Burmese python measuring 17 feet in length was discovered in the wilds of Florida, not only managing to adapt to a completely new environment, but also containing 87 eggs. If all these had managed to produce viable offspring, the consequences might have been disastrous. How Burmese pythons managed to reach the Everglades of Florida remains a mystery, though circumstantial evidence points to careless pet owners.

So, while there has been some awareness and action about invasive species in recent years, it is expected that in the future, increasing travel and trade will inevitably increase the spread of invasive species. However, not all invasive species travel by human made transport. In the last year, millions of organisms have travelled across the Pacific Ocean from Japan to California after the tsunami in March 2011. The impact of these on their new ecosystems has yet to be seen and studied.  

Even more interesting, but potentially more dangerous as well, is the possibility of new species entering our ecosystem through our space probes and satellites. Whether these new exotics will change our environment only or the human species as well, is a question best left to our imaginations for the present.