Whether it be voracious cane toads or pesky squirrels, there are creatures that invade a place they don’t belong and wreak havoc.
They compete with local animals, devour resources and define themselves as pests.
But is there anywhere in the world free of invasive species?
This question piqued the interest of Piero Genovesi, who chairs the International Union for Conservation of Nature (IUCN) Invasive Species Specialist Group.
«It’s not an easy question,» says Genovesi in conversation with the BBC, because the invaded lands far outnumber the still virgin places.
Where humans go, invasive species follow, Genovesi says, and «there’s literally no island in the world that hasn’t had human contact in the last century.»
Travelers and «taxis» of species
We have been «taxis» of species around the world for thousands of years, and detrimental entry of species continues to occur today: from the egg-hungry Argentine tegu or overo lizard that settled in Florida, USA, to the sudden arrival of exotic crazy ants in Texas.
Invasive species have been implicated in more than half of recent extinctions and cause more than $120 billion in damage annually in the United States alone.
Some of these stowaways are unwitting passengers: rats, cockroaches, and other pests that we ourselves cannot contain.
Others are released intentionally, either as a food source, as pets, or in an attempt to control another species that we want to eliminate.
Although they are everywhere, Genovesi thought that there must be places free of invasive species.
So he asked the more than 1,000 members of a global roster of experts on this question.
extreme free zones
Only the most remote and extreme ecosystems have managed to avoid invaders.
Thermophilic bacteria, the kind that thrive in overheated environments, probably do well without the company of alien species.
Those places include the hot springs of Yellowstone and Iceland, the edges of seafloor hydrothermal vents, and some areas of volcanic soil.
Very arid areas, such as the Arabian desert, also have few or no non-native species.
The pelagic zone of the open ocean – the layer of water between the surface and the sea floor – is also generally free of alien species, as is the deep sea.
Caves also tend to escape invasion, though the fungus that causes deadly white-nose syndrome in bats is showing up more and more in those habitats, especially in the United States.
Historically, the polar areas have been spared from the presence of invasive species.
But the situation is beginning to change with the increasing number of tourists, scientists and adventurers visiting these regions, whose extreme weather has been moderated by climate change.
Researchers in Svalbard, an archipelago located in the Arctic Circle, found more than 1,000 seeds of 53 species of exotic plants attached to the skin of visitors who arrived in a single summer, for example, and dozens of non-native species, from moths to flies, have been seen buzzing near Antarctic research stations.
Where there are humans there are invasive species, but remote areas of the rainforest are an exception to this rule.
Few if any alien species live deep in the Amazon or in Borneo, although there are people who do live there.
But that’s because those isolated tribes don’t come into contact with potential invaders.
Typically, even the most remote places – Gough Island in the South Atlantic, Palmyra Atoll in the Pacific – harbor living relics of the time humans have spent there.
Rats alone have reached at least 90% of the world’s islands thanks to our movements.
However, is it possible that some invaded ecosystem could join this list of virgin places again? New Zealand tries.
The world’s last major islands to be inhabited by humans are largely free of endemic mammals, making them vulnerable to the effects of warm-blooded creatures.
But today New Zealand’s wildlife is evenly divided between native and exotic species.
Of the country’s roughly 800 islands, less than 1% escaped the arrival of more than 30 alien mammals, including rats, weasels, mice, goats, pigs and Australian possums.
Around 1960 New Zealanders first thought about eradication and saw that it could destroy rodent infestations.
«Conservationists here became very aware that the ownership of wildlife and native forests involved killing rather than just protecting what we have,» explains Mick Clout, a conservation biologist at the University of Auckland, and former president of the IUCN Invasive Species Specialist Group.
Thanks to these efforts, around 150 New Zealand islands are now free of invasive mammals.
More than 1,000 removals of invasive species have taken place on islands around the world, and the benefits to the local environment are often evident.
On California’s Anacapa Island, for example, invasive rats were rampaging through the eggs and chicks of the Californian black-winged murrelet.
In 2002, the organization Island Conservation, which has carried out eradication operations on 52 islands in the last 20 years, got rid of the rats.
The birds almost immediately recovered and their eggs had almost three times the hatching success.
But while eradication often works, experts agree that the most effective way to control invasive species is to stop them from getting there in the first place.
The further we travel however, the more difficult it becomes to stop the flow.
Wherever we go, on foot, by boat, train, plane, car, bus or bicycle, we take other species with us.
Although there are still uninvaded lands, it is almost impossible to conserve all ecosystems.
«I don’t think we can stop the problem or get rid of the invasive species entirely,» Genovesi says.
«But we can mitigate their impact and stop the pattern of invasions that is really alarming right now.»