Gray squirrels, introduced from North America in the 1900s, are now driving continental Europe’s native red squirrels to the brink of extinction. 

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Alien species are accelerating their march across the globe

Invasive species, from feral pigs to Japanese knotweed, can devastate ecosystems. They damage crops, clog rivers, and cost farmers and homeowners billions of dollars to control each year. People aren’t the only ones suffering: The invaders have been linked to the decline of some four in every 10 endangered or threatened species in the United States. Now, the first-ever look at just how quickly these species have spread reveals more bad news: Since 1800, the rate at which alien species have been reported around the world has skyrocketed, with almost 40% of them discovered since 1970.

“There are no signs of a slowdown [except for mammals and fish], and we have to expect more new invasions in the near future,” says study leader Hanno Seebens, an ecologist at the Senckenberg Biodiversity and Climate Research Centre in Frankfurt, Germany. He cautions that not all nonnative or “alien” species are a problem—only those that adversely affect the environment, which are known as “invasive.”

To figure out how to help scientists deal with the problem, Seebens first had to find out how big it was. To do so, he and a team of researchers from across Europe, Asia, and the United States combed through more than 500 years of records from scientific publications, books, and unpublished works taken from more than 280 countries and islands. The documents revealed the very first sightings of alien species in each region, from squirrels to mosquitoes. Altogether, the scientists found 16,926 records of alien species of plant, mammal, insect, bird, and fish, they report today in Nature Communications.

Then, the team analyzed the speed at which new incursions were taking place, broken down by major taxonomic groups. Since 1800, that speed has increased for all groups, with the absolute number of new species reaching 1.5 sightings per day in 1996. Part of this is inevitably because of better recordkeeping over time, says Mark Hoddle, an entomologist at the University of California (UC), Riverside, who was not involved in the work. But Hoddle, who directs the Center for Invasive Species Research at UC Riverside, adds that the key trends are not surprising.

The introduction of nonnative plants exploded in the 1800s thanks to the growth of globalized trade, and it has remained high ever since. Mammals and fish peaked around 1950. But other groups, including algae, mollusks, and insects rose steeply after 1950, thanks to climate change and the post–World War II wave of global trade. For those plants and animals that can easily stow away in the ballast of ships, there is a strong correlation between the spread of nonnative species and the market value of goods imported into each region.

One glimmer of hope is that biosecurity measures that limit the movement of species and pathogens between countries—which have been growing in use since the early 20th century—seem to be having an effect. Since the early 1950s, for example, the speed of fish and mammals spreading beyond their native habitats has decreased, going from more than 150 sightings per year to just 24 from 1996 to 2000.

In New Zealand, the study found a significant decline in the number of new alien plants since the 1990s, which coincides with the country’s 1993 Biosecurity Act and the Hazardous Substances and New Organisms Act 1996. These acts regulate imports using a white list of permitted species, and they require a risk assessment for all new species entering the country. As an island, says Hoddle, New Zealand has a distinct advantage. For countries without such geographically distinct borders, preventing the introduction of nonnative species is much harder. He says success requires science-based policy that accommodates increasing tourism and trade.

“The good news is that biosecurity and quarantine measures [have worked] for some more obvious taxa, so we know we can take actions that have positive outcomes,” says Margaret Stanley, an ecologist at the University of Auckland in New Zealand, who was not involved in the new analysis. The challenge now, she adds, is to set policies that prevent more inconspicuous nonnative species from becoming established.

Conservationists hope that better awareness of the threats of species, coupled with improved global biosecurity, will continue to slow the spread of nonnative species. Some researchers predict the rate of spread will reach a saturation point before tailing off. Unfortunately, Seebens’s data suggest that may be a long way off.