The lesson is clear: we need to improve our control mechanisms – in addition to appropriate regulations, education, enhanced customs – control, and working with neighbours and trading partners.The lesson is that any commercial use of a potentially invasive species must be subjected to a full environmental impact assessment, including a risk analysis, and the acceptance of insurance liability by those wishing to introduce the species as a resource. (The photo shows another species of mussel – the zebra mussel-a catastrophic invader) 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.Â 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.Sadly, many nurseries have been equally problematic as sources of invasions – often to satisfy someone’s desire to bring in some new, pretty plant with which to make more money. There may be a slightly more advanced level of understanding than in the pet trade, but the plant trade too suffers from negligible controls and virtually non-existent accountability.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 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.Any species introduction – even those that are meant to solve problems – must undergo an impact assessment, to avoid such expensive embarrassments.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!Desperate situations do sometimes warrant desperate measures.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.We need a suite of success stories upon which to build a practical strategy that can win political and public support. We should also remind the beneficiaries that “the cavalry” did respond, and that they should pay their dues.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.)Much wisdom led to the notion of the “precautionary principle” (to err on the side of caution), along with those of the “polluter pays” and “user pays”. But we still permit the importation of species by companies or individuals who have little genuine accountability.