Swimming through the seas, taking over forests, creeping into your garden - invasive species are everywhere, and they're becoming one of the world's major environmental concerns.
And research suggests that they may be costing big money in terms of agricultural damage as well - with damage totals set to grow because species invasions are closely linked to the beating heart of the global economy, namely international trade.
"As trade volumes continue to increase and more trade connections are made between countries, the pressures from invasive species will only intensify," Dean Paini, a senior research scientist at the Commonwealth Scientific and Industrial Research Organisation in Australia, and a group of colleagues write in a new study in the Proceedings of the National Academy of Sciences.
A big part of the concern surrounding invasives - which are non-native species that establish themselves in a new location, often at the expense of the organisms already living there - has to do with their effect on the native flora and fauna, especially if the locals are threatened already.
Burmese pythons in the Florida Everglades, for instance, are a prime example of how an invasive species can take over an entire ecosystem in just a short amount of time. These giant snakes - which were probably first released into the wild by pet owners in Florida - are eating their way through the small mammal population in the Everglades, and they've even been known to turn up with bigger prey, such as deer or alligators, in their stomachs.
But while looking out for the local wildlife is a high priority for many scientists and conservationists, there's an economic angle to the problem of invasive species: They can pose a terrible threat to agriculture by damaging crops.
The new study seeks to illuminate which countries are potentially most vulnerable to their presence - and which countries, at a time of burgeoning international trade, are most likely to send them out into the rest of the world.
The research finds that sub-Saharan Africa is the global region with the potential to be hit hardest, agriculturally speaking, by invasive species. On the other hand, the research suggests that China and the United States, both of which have huge trade volumes with other countries (and large numbers of pest species within their borders to begin with), are the biggest potential sources of invasives to the rest of the world.
When considering the potential effect of any pest species in a country, there are "two halves of a question" to consider, said Paini, the study's lead author: First, how likely is it to arrive in that country and, second, how likely is it to establish itself once it has arrived?
Paini and a group of colleagues from institutes in Australia, New Zealand and the United States, investigated those questions for 1300 invasive insect and fungal pest species in 124 countries around the world. They decided to use global trade connections, and the value of a country's annual importation, as a proxy for the likelihood that pest species might be carried across borders.
"Trade has been shown repeatedly to be correlated to the number of invasive species a country has," Paini said. And there are many specific examples, as well - invasive zebra mussels, for instance, which first started showing up in US lakes and waterways in the late 1980s, are thought to have been unintentionally carried over by European ships.
To evaluate a species' likelihood of establishment once it arrives in a country, they relied on existing data on the worldwide distribution of all 1300 species included in the analysis and the characteristics of locations where they're known to be able to survive. The researchers also collected data on crop production in each country so they could calculate the economic threat that would be posed if a species known to be a pest to any of the crops in that nation were to invade.
FACTORS AT PLAY
There was little correlation between geography and a country's vulnerability to imported pests. Factors include:
1 The types of crops grown
2 What pests are already present
3 Which places the country trades with and how often
4 What kinds of pests those other places house
Overall, the two countries with the potential to lose the most money to invasive species were the United States and China - not surprising, considering both are large agricultural producers.
But compared with other countries, the share of agriculture in their overall gross domestic product is fairly small. When the researchers considered the costs of invasion relative to GDP, they found that nations in sub-Saharan Africa, such as Malawi, Burundi and Guinea, were the most vulnerable.
Outside this region, though, the researchers noted that there was little correlation between geography and vulnerability, suggesting a complex array of factors are at play when it comes to whether - and to what extent - a country will be affected by invasive species. Those include the types of crops grown, what pests are already present, which places the country trades with, and how often and what kinds of pests those other places house.
"I spent a lot of time trying to find a pattern," Paini said. "Was it big countries, was it little countries, was it the number of invasive species that are already present in the country, was it whether the country was landlocked or not? The point is, nothing really popped out."
On the flip side, the research indicated that China and the United States ranked first and second as potential source countries - also not surprising because they are "characterised by large and diverse trade volumes and have been confirmed as network hubs in the international agro-food trade network," the researchers wrote.
Of course, there are a variety of other factors affecting the extent to which an invasive species could harm a country's economy once it's established, Paini said.
"It's things like how long is it established in the country before it actually grows to a large enough population that it does have an impact? How slowly or how quickly does it spread around the country?" he said, adding that management strategies in individual nations will play a large role as well.
The research does not suggest that its results will definitely happen - rather, it contends that some nations have the potential to be hit harder than others.
And Paini noted that the study is also not intended to make recommendations to any particular nation about which species they should most be on the lookout for or which nations they should avoid trading with.
The biggest takeaway, Paini said, is that some parts of the world may be particularly vulnerable - suggesting that international bodies may want to pay special attention to the risk of invasives in those places.
"It's everybody's responsibility to try to reduce or prevent invasive species from spreading, and just to keep in mind that those countries that are most vulnerable to those invasive species are those developing countries, particularly in sub-Saharan Africa," Paini said.
"There is a lot of work being done in that area already for developing countries, particularly in Africa - but I would just say that that work needs to continue."