The year is 1347. A rat boards a ship in Crimea – one of many, unnoticed by passengers. When the ship docks in Europe, plague violently erupts on a catastrophic scale, and death blooms across the continent. The stories of human-introduced invasive species, such as that of the Black Death, have been memorialised into popular culture for the sheer scale of destruction they wrought on humans and the environment alike. But what of contemporary human-introduced species?

An invasive species is defined by National Geographic as “any living organism not native to an area that causes economic or environmental harm, or is damaging to human health”. The element of environmental harm is increasingly forgotten in contemporary ecological and sustainability studies. Many human-introduced invasive species may be familiar to you, as they include household pets such as dogs and cats, as well as more alien species such as species of lionfish, Burmese pythons and the brown tree snake. However, these species can have disastrous consequences for the local environment and its biodiversity, which can domino-effect into more significant consequences for the global climate. 

Lionfish, whose original habitats are in the South Pacific and Indian Ocean, have been found in the Atlantic and Caribbean, where they wreak havoc on the local environment. They prey on native species of fish, which has knock-on effects for the rest of the food web, thus disrupting the entire ecosystem. According to conspiracy, these lionfish were unintentionally introduced into their new habitats following a hurricane in Florida, which caused an aquarium to shatter into the sea, washing its inhabitants into their new home.

For an example closer to home, consider the environmental impact of the average pet cat. Both domestic and stray cats are prolific hunters of native species like birds and small mammals, such as mice and rats. Domestic cats are especially damaging to local biodiversity, as they are not reliant on the prey they hunt because they receive their primary diet from their owners. This means that they can continue to hunt a species even when their numbers become scarce, as they are not dependent on them for sustenance. This allows domestic cats to hunt species to endangerment and even extinction. In Australia, over two-thirds of mammal species that have gone extinct are thanks to domestic and feral cats: the highest rate of mammal extinction in the world, as the beloved pets are also responsible for killing 1.5 billion native animals a year (at a conservative estimate).

Whilst it may sound exotic and farfetched, the Burmese python is another example of how pets can impact the environment – particularly if they are kept by irresponsible owners. After being released by ex-owners into the wild, Burmese pythons are able to cause chaos largely unchecked, due to having no natural predators in Florida. 

Maximising biodiversity and maintaining local food webs are crucial for the upkeep of functional ecosystems. Because food chains are so heavily intertwined, removing one piece of the puzzle can have disastrous effects for the other species in the food web (especially if it causes them to lose sources of prey). The biodiversity in these ecosystems provides stability, by providing a range of possible prey, thus removing the likelihood of far-reaching consequences if one element is lost. These links and networks in an ecosystem develop over millions of years of evolution, constantly adapting to improve. If biodiversity is lost, then these ecosystems are more liable to come crashing down.

All things considered, much of the damage from human-introduced invasive species is irreversible, but the next best thing would be to limit their predation, and remove as many individuals as possible. In some places, they are even being served as delicacies! After all… if you can’t beat ‘em, eat ‘em!