FILE - Volunteers remove Himalayan balsam, a non-native invasive species that spreads along river banks and can cause erosion problems, during a clean-up on the Hogsmill River, in Kingston-upon-Thames, southwest London, Britain, June 26, 2018.
FILE - Volunteers remove Himalayan balsam, a non-native invasive species that spreads along river banks and can cause erosion problems, during a clean-up on the Hogsmill River, in Kingston-upon-Thames, southwest London, Britain, June 26, 2018.

Asian kudzu vines smothering the southern United States. Pacific lionfish devouring Caribbean sealife. South American cane toads killing their way across Australia.  
As bad as invasive species are today, a study says they will get worse.    
Researchers predict that non-native—or alien—species introductions will increase globally by around 36 percent during the first half of the 21st century.
The researchers call for better monitoring and regulations to contain the spread of alien species.
The movement of plants and animals around the planet soared over the last century as human trade and travel opened new global pathways.
Not all alien species are problematic, but invasive alien species—like kudzu—wreak environmental or economic havoc in their new homes.  
“Together with climate change and land use change, invasive alien species are posing one of the greatest threats to biodiversity,” said Hanno Seebens, ecologist at the Senckenberg Biodiversity and Climate Research Centre and lead author on the study.  
From devouring crops to clogging water pipes, invasive species cost the United States alone around $120 billion each year. Some species are intentionally introduced to new regions by humans. Others arrive accidentally as stowaways in goods shipped by planes, trucks and ships or as hitchhikers on luggage. 

“A species can only arrive in a new region when you connect different [regions],”  Seebens explained. “When we extended our trade networks, we connected more and more [regions], which allowed more and more species to come.”  
However, the number of possible species could taper off in the future. “We may just run out of species to be transported, because at some point, all species may have been transported already.”
To forecast alien species introductions for each continent between 2005 and 2050, the researchers used past records of alien species introductions and estimates of the number of possible species that could be introduced.
Alien species introductions will increase on every continent, they predict.

FILE - An Asian hornet chases a bee near a beehive in Loue, northwestern France, Sept. 14, 2019.

The largest increase is expected in Europe, with a 64 percent rise in alien species introductions totaling more than 2,500 species. In Australia and New Zealand, they predict just a 16 percent increase or about 1,286 new alien species.
“We know that a certain proportion of alien species will be problematic, so the more of them that there are, the higher the likelihood that we'll have problems,” said Cascade Sorte, associate professor of ecology and evolutionary biology at the University of California Irvine, who did not contribute to the research.  
“In some ways it's shocking to think that, with all of the impacts that we’ve already seen [from alien species], there's even a possibility that things can get worse,” she added.
On all continents, the study predicted the greatest increase in introductions for insects and their relatives. One reason is that these tiny animals can easily escape detection and be accidentally transported to new regions.
The researchers predict just small increases in alien mammal introductions, ranging from just 0 to 16 percent—or up to 12 species. According to Seebens that is because the number of possible mammal species is limited.
“Mammals are often very big. And so they are not easily transported to other areas. And the small ones, mice or rats for example, have already been transported all over the world,” said Seebens.
“We could, if we chose to as a global society, do something about [alien species],” said Bethany Bradley, professor of spatial ecology and biogeography at the University of Massachusetts, who was not involved in the study. “It requires more inspections; it requires more regulation. It's definitely costly.”
“But on the other side of things,” she added, “there are estimates of hundreds of billions of dollars being spent every year on managing invasive species. I think we have the solutions to the problem that this paper lays out; we just haven't implemented them.”
One reason for fewer alien species introductions in Australasia is the strict border regulations that Australia and New Zealand have in place, says Seebens. The isolation of those countries also means there’s less opportunity for species to cross borders.
The study predicts alien species introductions in a “business as usual” future, providing a baseline for comparing different future scenarios. For example, increased trade and transport or accelerated climate change would likely boost alien species introductions. On the other hand, adoption of new regulations could slow introductions.
“Our recommendations would be to have stricter regulations and stricter border controls,” said Seebens. People should also be aware of alien plants in their gardens and avoid releasing non-native pets into the environment, he added.