Worse than pollution?!

On a dark night last week a group of animal rights activists in Donegal made their own special contribution to the International Year of Biodiversity. They cut their way into a fur farm and released 5,000 mink . This, within their circles, was considered a clever thing to do. A spokesperson for the Alliance for Animal Rights said: "I commend whoever risked their freedom to do this." The Coalition to Abolish the Fur Trade announced that "we fully support what has happened".

Had these people tipped a tanker load of bleach into the headwaters of the river Finn, they would have done less damage. The effects would be horrible for a while, but the ecosystem could then begin to recover. The mink, by contrast, will remain at large for years, perhaps millennia. Like many introduced species, American mink can slash their way through the ecosystem, as they have no native predators, and their prey species haven't evolved to avoid them. Is there anything the animal lovers in Donegal could have done that would have harmed more animals?

But there's a second question raised by this act of preternatural imbecility: what were the mink doing there anyway? In other respects the Irish Republic appears to be a civilised country, in this case it looks barbaric. While the United Kingdom banned fur-farming in 2000, Irish governments have resisted prohibition, to protect a tiny but wildly destructive industry. The republic's five remaining fur farms are the sole source of continuing releases of mink, either through raids or accidents. They are also places of astonishing cruelty, in which intelligent carnivores are confined to cages the size of a few shoeboxes. The Irish government is considering phasing out fur farming in 2012. Until then, its citizens will continue to pay more to eradicate mink than they make from breeding them.

But Ireland is a small player. Two-thirds of the world's mink farming and 70% of its fox farming takes place in other EU countries. Denmark alone produces 40% of the global supply of mink pelts. Feral American mink on the continent are even more damaging than they are here, as they drive out the endangered European mink . The EU's 6,000 fur farms are an affront to the values it proclaims.

This month governments meet at Nagoya, in Japan, to review the Convention on Biological Diversity . It has, so far, been a dismal failure. Perhaps the starkest botch has been their inability or unwillingness to control the spread of invasive species. The stories I am about to tell read like a gothic novel.

Consider, for example, the walking catfish , which is now colonising China, Thailand and the US, after escaping from fish farms and ornamental ponds. It can move across land at night, reaching water no other fish species has colonised. It slips into fish farms and quietly works through the stock. It can burrow into the mud when times are hard and lie without food for months, before exploding back into the ecosystem when conditions improve. It eats almost anything that moves.

Its terrestrial equivalent is the cane toad, widely introduced in the tropics to control crop pests. It's omnivorous and just about indestructible: one specimen was seen happily consuming a lit cigarette butt. Nothing which tries to eat it survives: it's as dangerous to predators as it is to prey. Unlike other amphibians, it can breed in salty water: it's as if it had waddled out of the pages of Karel Capek's novel War With the Newts .

The world's most important seabird colony – Gough Island in the South Atlantic – is now being threatened by an unlikely predator: the common house mouse. After escaping from whaling boats 150 years ago, it quickly evolved to triple in size, and switched from herbivory to eating flesh. The seabirds there have no defences against predation, so the mouse simply walks into their nests and starts eating the chicks alive . Among their prey are albatross fledglings, which weigh some 300 times as much as the mice. A biologist who has witnessed this carnage observed that "it is like a tabby cat attacking a hippopotamus".

On Christmas Island the yellow crazy ant does something similar: it eats alive any animal it finds in its path. It is also wiping out the rainforest, by farming the scale insects that feed on tree-sap. Similar horror stories are unfolding almost everywhere. The species we introduce, unlike the pollution we produce, don't stop when we do. A single careless act (think of the introduction of the rabbit or the lantana plant to Australia) can transform the ecology of a continent.

According to a government report, invasive species cost Britain several billion pounds a year. The global damage they cause, it says, amounts to almost 5% of the world economy. A single introduced species – a speargrass called Imperata – keeps 2 million square kilometres in the tropics out of agricultural production, equivalent to the arable area of the US, while ensuring that the native ecosystem can't regenerate.

In most cases there's a brief period in which an invasive species can be stopped. So you would expect governments to mobilise as soon as the threat appears. But in many parts of the world the policy appears to consist of staring dumbly at the problem while something can be done, then panicking when it's too late. When museum weed (Caulerpa taxifolia) escaped into the Mediterranean from the Oceanographic Museum in Monaco, the authorities responded by bickering over whose fault it was. In 1984, when the invasion was first documented, the weed occupied one square metre of seabed. It could have been eradicated in half an hour. Now it has spread across 13,000 hectares and appears to be uncontrollable.

Australia, the continent that has been hit hardest by introductions, still seems incapable of regulating the trade in dangerous species. As the Guardian's new Biodiversity100 campaign shows, 90 potentially invasive plant species are being sold in nurseries there, while 210 species of aquarium fish can be imported without a licence. The UK has some good policies at home. It spent £10,000 in 2006, for example, on a strategy (successful so far) for excluding the South American water primrose, whose control now costs France several million euros a year. But in its overseas territories – of which Gough island is one – it reacts slowly, if at all.

The mink, the walking catfish, the cane toad, the mutant house mouse, these are potent symbols of humanity's strangely lopsided power. We can sow chaos with a keystroke in an investment bank, one signal to a Predator drone, a seed dislodged from the sole of a boot, a fish tank emptied into a canal. But when asked to repair the mess we've made, we proclaim our impotence. Our challenge this century is to meet our capacity for harm with an equal power for good. We are not, so far, doing very well.

• A fully referenced version of this article can be found at www.monbiot.com