Sept. 11, 2001, a date now etched indelibly in our memories, provided an awfully pertinent lesson in human actions and human responses. Shock, fury, anger; all were reasonable, acceptable emotional responses to horrendous acts of terrorism.

By contrast, the attacks subsequently perpetrated on Muslim schools in Australia and on people mistaken for Muslims in the United States were extreme examples of what could be dubbed IER -- inappropriate emotional response.

Rarely are the triggers for IER so extreme as the events of Sept. 11. Nevertheless, the violent aftermath of that day serves as a reminder that, whatever our emotional responses to an event, they are not necessarily rational or reasonable.

Context confuses the issue. A given response may be the appropriate emotional response (AER) in some circumstances, yet an IER in others.

Take our common responses to certain animals. Just wander around any zoo observing the people, not the animals. You will see a wide range of emotions at play. Visitors may coo at the pandas, be bemused by the human-like antics of the apes, and display phobic horror at the snakes and insects. None of these responses are rational; all are emotional. None really relate to the species concerned, only to particular human imaginings.

Does it really matter how people feel about snakes? An emotional response is meaningful (or not) depending on whether it spurs an individual to action.

In various countries in Africa, Asia and South America, I have witnessed fear of a snake translate immediately into action -- the killing of the serpent, perhaps by machete or by crushing it under the wheel of a car. No consideration was given to whether the species was venomous or not; death was doled out anyway -- an IER taken to extremes.

Such a response is tough on reptiles, but it also impacts on rodent populations. These creatures thrive as their natural predators are exterminated, and may devastate crops or grain stores, as has been the case in Cambodia.

Take the Mustellidae: This fascinating group of species includes otters, martens, stoats and weasels. Stoats are among the most fleetfooted and aggressive of predators for their size.

These hunters play an important role in their ecosystems. Keen-eared and sharp-eyed, they home in on a diverse range of rodents and birds from those a fraction of their own size to others twice their weight and more.

Such predators inspire awe and respect. To watch a stoat pursuing its prey along an English hedgerow is every bit as exciting as watching a cheetah hunt gazelle across a Botswanan savanna. Awe, respect and inspiration may all be AERs under certain circumstances, such as in Europe where the stoat is the natural predator of birds and even rabbits -- but consider another scenario.

Imagine, if you can, a place that has never known land mammals. A country in which, for perhaps millions of years, insects, birds and plants have evolved without warmblooded hairy hunters. Call that land -- New Zealand. Or, closer to home, you could call it Miyake Island in the Japanese Izu island chain.

To both of these places, the stoat Mustela nivalis was introduced. In the case of New Zealand, it was assumed that the stoats would control the previously introduced rabbits that were already playing havoc with the ecosystem.

Not burdened by any human expectations, however, the introduced stoats made their own choices concerning suitable food. Rather than hunt large, predator-shy rabbits, they opted for the native birds that fed on or close to the ground and that nested equally accessibly in cavities in trees.

The stoat is not the only culprit in New Zealand, but it is one of the main offenders -- the result is that one may hike through stupendous landscape that 150 years ago would have rung with the calls and choruses of native birds, but which today is shockingly, eerily empty and silent.

On Sept. 11, 2001, I was hiking with David Melville, the ex-Director of WWF Hong Kong, through the beautiful forests of Mount Arthur in the north of New Zealand's South Island.

The silence was saddening. In about four hours, we noted just seven native bird species and counted only 14 individuals. When we encountered two male tomtits singing to each other, we both commented on how rare it is now in New Zealand to hear songs from adjacent territory-holders.

The stoat, so inspiring to watch in its native European habitat, is in New Zealand a marauding alien, dealing out death to natives. In New Zealand, it seems, the only good stoat is a dead one. The one shown above was killed crossing the road in an ancient beech forest in South Island, now devoid of nearly all its native creatures.

Human emotional responses are controllable, but taking responsibility for our feelings is a step many of us find difficult. Appropriate responses are, however, vital when facing the severest modern threat to the environment -- the introduction of alien species.

We need to understand the history of a species before we can respond correctly, but where a species is alien, its potential for damaging the local ecosystem is enormous.

Misguided members of the public may think the introduced raccoons that inhabit Kamakura or Hokkaido are cute, but such a response misses the point. I have heard a number of spurious arguments as to why raccoon should be protected (they are cute; it's not their fault; they are "wild" animals), but these overlook the crucial fact that raccoon are not a species native to Japan. We should instead consider the many species of native plants and animals that will suffer as the raccoon population spreads.

One ecologist I spoke to advocated doing nothing about the raccoon problem because it was "too difficult," but that simply sends the message that we have accepted this species -- and all its many consequences. Such tolerance leaves the door wide open for other aliens, whose arrival would steadily undermine the fantastic diversity that has evolved in isolation in the Japanese archipelago. It is appropriate to be pro-raccoon in North America, but simpering over its cuteness is definitely an IER over here.

Deliberately eradicating a species once it has become widespread is certainly not an easy task, but it can be done, with will, effort and persuasion of the public. Britain, for example, eradicated its alien populations of nutria (or coypu), a web-footed, water-dwelling rodent native to South America. New Zealand has done it with a whole panoply of alien predators.

Better still, though, is to crack down before a species has a chance to spread. Ultimately, aliens are a problem that people have caused -- and one that people must solve.

More on this contentious subject next time.