My cat caught a starling this week. By the time I intervened, the poor bird’s leg was broken, the kitchen floor was strewn with feathers, and I had to make one of those awful decisions. Was I to leave the bird to the satisfaction of my cat; do all I could to keep her alive; or wring her neck and put her, as they say, out of her misery? I wrung her neck. The toes of her good leg clenched around my finger and she was dead within seconds.

Reading this story, many ecologists will condemn my cat-husbandry – and understandably so. What business do I have, after all, letting an obligate carnivore scamper around on my roof where who-knows-who might end up in her jaws?

For most of these ecologists, however, the evil of this particular incident will be mitigated by the fact that my cat’s victim was a starling, an exotic bird; indeed, in the hectic phrasing of the Western Australian Department of Agriculture, a member of “one of the world’s worst invasive alien species”.

Invasive status is of course not intrinsic to a species. In 2007, the “invasive” starling joined the bottlenose dolphin and the otter in an official list of declining and threatened species on Britain’s Biodiversity Action Plan.

Most ecologists understand the term “invasive” not as a heading under which to categorise a species permanently, but as a way of describing an ecological phenomenon, the process and effect of a species migrating into a new bioregion.

The term “invasive species” has additional legitimacy among conservation biologists because it refers not only to a species’ impact on humans but on ecosystems and on non-human species. Some of these will become extinct or will be pressured into behavioural and perhaps genetic change as a result of the invasive species\’ arrival.

Nonetheless, there is still a profound anthropocentrism lodged in the idea of the invasive species. Australia’s Department of Sustainability, Environment, Water, Population and Communities defines the invasive species as one “occurring, as a result of human activities, beyond its accepted normal distribution and which threatens valued environmental, agricultural or other social resources by the damage it causes.”

The idea that a species becomes invasive because of its impact on “valued” elements of an ecology is all too obviously anthropocentric (it is humans that value the threatened “environmental, agricultural or other social resources”). Indeed, many ecologists attempt to avert the human-interested bias from their talk of species value by investing all constituents of a “pre-invasion” ecology with value.

But just as anthropocentric is the idea that a species introduced to a new environment “as a result of human activities” should belong to a significantly different category from a species introduced to a new environment as a result of non-human activities, such as continent drift, wind or water currents, or via bird droppings.

The history of life on earth is punctuated with such migrations. The reason why every continent on earth is inhabited is because organisms did not conform to their “accepted normal distribution”.

The vegetation on coral atolls gives us a good example of how species migration produces distinctive life forms. Vegetation arrives at the atoll because birds or ocean currents import seeds, not because the plants and the atoll came into existence in the same instant.

Inevitably, newly migrated species will shift an ecology in which they establish themselves. In some cases these shifts will be insignificant, and in other cases they will entail some species’ extinction. Often, subsequent to an original migration, a migrant species will produce variant offspring. The distinct climate and ecology of the new host environment will advantage some individuals over others so that the variant might flourish and a new variety – perhaps, eventually, a new species – will arise that is peculiar to its bioregion, at least until it, in turn, migrates.

This process of ecological change is intrinsic to the production of biodiversity. Natural selection, the mechanism that allows some individuals to give rise to new varieties, and in turn new species, operates when environmental pressures produce a differential death rate. Some individuals, and varieties, are extinguished, and others prosper.

Species change and species extinction – the casualties attributed by conservation biologists to “invasive species” and that are widely supposed to threaten biodiversity – are also the agents by which biodiversity is produced.

Is there a difference between the seed deposited by the bird in its droppings and a seed introduced into a new environment via, say, human droppings? Is there a difference between a seed that catches in the fur of a migratory bat and a seed that catches in a human traveller’s sock? Clearly the seed itself is the same, regardless of the method by which it is transmitted from place to place. But I want to suggest that the agents of transmission – bird, bat, human – are also the same, or of the same category: they are all animals, they are each, equally, part of nature. (In fact, once we recognise that humans are not separate from nature, the idea of nature itself becomes redundant, or tautological.)

There is no denying that our species’ numerousness and our own migratory activities have meant that human-enabled species migration is currently far more prevalent than species migration enabled by other vectors such as birds, bats, wind, water.

There is no denying that humans are collectively exerting pressures on the world’s ecologies – indeed, on the world ecology – that are accelerating both species extinction and species adaptation beyond the rates known within geological memory.

In their multitude, humans, as Dipesh Chakrabarty has observed, currently constitute a force of nature, something equivalent to a geological force. They are not, however, outside nature.

The species that manage to prosper in part because of human agency are no less natural than the species that are made extinct because of human agency.