Wherever you are, the aliens are coming.

But they don’t look like the little gray Martians from the movies (with the exception of the gray squirrel, which is currently making its way through Britain). The real “alien” threat comes from non-native species. 

A non-native species means any species that doesn’t originate from an area, but that has made its way to that place. It’s not quite as exciting as extraterrestrial aliens (sorry to let you down), but it’s actually a serious issue. These plants and animals are introduced from other places on Earth into places they don’t normally grow, and this can be extraordinarily harmful to the environment—and to humans.

That’s not to say that every non-native species is necessarily a threat; the non-native “aliens” that are classified as an invasive species are the ones causing problems.

In this case, invasive species “cause or [are] likely to cause environmental or economic harm or harm to human health,” according to the United States Fish and Wildlife Services.

Invasive species aren’t constrained by the same factors that limited their spread in their natural habitats; they may not have any natural predators whatsoever. About 20 to 30 percent of non-native species are eventually classified as invasive.

Some regions are more susceptible to non-native species than others and therefore more likely to suffer negative environmental and economic repercussions. In a recent survey, Durham University researchers attempted to determine the areas more prone to invasion.

Scientists studied non-native species on 186 islands and 423 mainland regions.

By looking at existing data, they were able to analyze the populations of eight groups of animals and plants.

The top three global hotspots with the highest numbers of established “alien” (that is, non-native) species were the Hawaiian Islands, New Zealand’s North Island, and the Lesser Sunda Islands of Indonesia. (Other problem areas included the coastal regions of the United Kingdom and the United States.)

Because of their geographic isolation, islands are more likely to have unique flora and fauna. In addition, human trade to and from islands frequently fosters the spread of non-native species. 

“We need to be much better at trying to prevent the introduction of species that can be harmful in the first place,” said Dr. Wayne Dawson of Durham University, one of the authors of the study. “Prevention is better than cure with invasive species.”

The simplest response, according to researchers, is to shore up defenses in coastal regions and to carefully analyze new species before intentionally introducing them to a new area. Otherwise, haphazard introductions can have dire consequences.

Kudzu is a notable example of an alien invader in the United States.

Native to Asia, kudzu is a perennial vine that reproduces through a process known as “asexual vegetative spread,” or “cloning.” In the 1880s, it was introduced in the American South as an ornamental plant for porches; in the 20th century, it was marketed as cattle fodder.

But in 1953, the United States Department of Agriculture officially declared kudzu a “weed,” indicating that it was detrimental to other plants. By some reports, it spreads over an additional 2,500 acres of land per year, drawing resources away from other plants.

Kudzu’s devastating environmental effects have led to the nickname “the plant that ate the South.”

The Burmese python is another interesting case.

Likely introduced to Florida by careless pet owners, Burmese pythons can easily grow to eight feet long within a single year. Florida’s subtropical climate is ideal for the reptiles, and they have spread through the Everglades, threatening endangered species.

The snakes can also pose a threat to Floridians. The Burmese python is one of the largest snakes in the world, with a maximum weight of almost 200 pounds. It reaches lengths of up to 23 feet, and its hatchlings are 19 to 31 inches—bigger than many native Florida snake species.

According to a paper published by the University of Florida, the pythons “have consumed prey as large as white-tailed deer and adult American alligators.” Despite an aggressive campaign to remove or kill the pythons, they’ve continued to spread.

To prevent additional reptile invasions, the state of Florida has passed legislation to require owners of “reptiles of concern” to microchip their pets and purchase a $100 annual permit.

Scientists believe that regulation plays a key factor in controlling the alien invaders.

However, they also warn that non-native species introductions are an expected byproduct of globalization.

“While species have been moved around the globe throughout history, more and more species will be introduced as the world becomes ever more connected, and the human population continues to grow,” said Dr. Dawson.

“Many of these incoming species are useful, and won’t establish and spread in their new homes, but some will, with varying levels of impact on resident species and ecosystems,” Dr. Dawson explains.

“The challenge for us is to understand what the consequences are of mixing up the world’s species in this way, to decide how to deal with this change, and what measures we can put in place to try and prevent further introductions to the most vulnerable regions.”

And to understand those factors, Dr. Dawson says, we should look at countries that have successfully curbed their invasions.

“This includes bolstering biosecurity measures at major points of entry to detect stowaway species, as is already done to an impressive degree in New Zealand.”

While islands and coastal regions are the primary targets for invasive species, everyone is affected. By one estimate, the United States is currently struggling with 50,000 invasive species, which cost taxpayers an astounding $120 billion per year.