The opening up of trade between China and the USA came at a cost. It introduced to the USA the Asian long-horned beetle, which were brought in through soft-wood packaging of crates. These beetles have already caused cities like Chicago to cut down some of their centuries-old street trees in a desperate attempt to halt their spread. The real risk is that the beetles may decimate the maples and other trees that give the north-east of the USA their beautiful autumn (fall) colours —an important source of tourist revenue.

Other "mistakes" lead to invasions. Some commercial opportunists tried to farm alien mussels in the fresh-water lakes in Poland. Not only did the mussels prove to be non-commercial, but they also proved to be vociferously invasive. These mussels now make up a staggering 98% of the biomass (weight of all living organisms) in these lakes. They have destroyed the lakes as both vibrant ecological systems and as a source of food.

The Argentine ant is seen as a particular threat to the biological diversity of the Fynbos (the smallest of the world’s six plant kingdoms). Brought into Southern Africa with the grain for feeding horses during the South African War, these ants displace indigenous species that have evolved to be totally dependent upon various plant species. Unlike the indigenous ants, the Argentine ants do not bury the seeds of indigenous plant species, thereby putting these plant species — along with the indigenous ants — in danger of extinction.

This is an impact that we are unlikely to reverse. We can withstand only a very limited number of such disasters, before collapsing our natural systems.

If this level of destruction can occur in one of the most prosperous and technologically advanced countries in the world, then the hard reality is that the rest of the world is extremely vulnerable to invasions of various kinds.

As the world gets "smaller’ it will surely get sicker too. We are all in this together. If the rich do not assist the poor in defeating their invasions, they may well be next in line.

With rights come responsibilities. We must educate those who trade in (or own) exotic pet species, so that they do so with an understanding of the implications of their actions. But we must also take strong action against those whose greed brings profits at the expense of society, of the environment and of future generations.

Strong laws are not going to impact negatively on responsible nurseries. If anything, these laws (coupled with a generous education drive) will mean that customers will choose to buy at responsible nurseries.

The remote bird sanctuary of Marion Island was invaded by rats from anchoring ships. Someone then decided to introduce cats to catch the rats. The problem was that the millions of birds that breed on the island had not evolved defences against cats, and the nesting birds were "sitting ducks". Needless to say, the cats decided that eating defenceless birds was easier than catching unco-operative rats, and the cats flourished.

The story has a partially happy ending, though. Biological control was used in the form of introducing cat flu. Those cats that survived were shot. The cats are now gone. The rats persist, and will probably be impossible to eradicate.

Sometimes brave action is needed. When an invasive fouling mussel was found in a marina in Australia (brought in by ships), the authorities poisoned the entire marina, killing all living organisms in the bay. The indigenous species returned, but the invasive species is no longer there — at least for now!

It’s not all doom and gloom. Water lettuce is one of the many waterweeds from South America that have ravaged African, and other, waterways. Great carpets of this weed have caused water bodies to virtually cease to exist as functioning ecological systems, because of eutrophication, thermal changes, light deprivation and sheer (unpalatable) biomass. But biological control agents — introduced herbivorus species from the invasive weed’s natural habitat — have been used to great effect.

The careful use of biological control agents can be an essential component in the control of invading alien plants.

This could have been the fate of more than half of Southern Africa’s arable land and natural veld — an unproductive monoculture of invading Prickly Pear from Central America — were it not for the successful use of biological control. This accomplishment by the Plant Protection Research Institute saved the country billions of rands in direct costs, and brought incalculable savings in terms of indirect costs such as unemployment, poverty, threats to rural development, and so on.

In a foolhardy, if romantically inspired, step many years ago, someone decided that all the bird species mentioned in Shakespeare’s plays should fly over the USA. He released these species in New York. Most died, fortunately. But several species have become pests of major proportions — like these European starlings (which are also a plague in South Africa). They decimate indigenous bird and other animal populations, plunder crops, and cause disease. Decisively "not to be"!

A lesson here is that an appreciation of fine literature does not necessarily lead to wisdom.

Some of our best efforts to control invasive species have been in attempts to protect our most valuable agricultural crops. Even so, the risks to the crops remain very high as the movement of species around the world increases. (The crops are usually alien introductions themselves, and turn some indigenous species into pests — such as the quelea in South Africa - see photo.)