The two invasive species have cost the world $16bn 

Two invasive species, one reptile and one amphibian, have cost the world more than $16 billion in damage, scientists have found.

The brown tree snake and American bullfrog caused $10.3 billion and $6 billion in damage, respectively, between 1986 and 2020.

This makes them by far the most costly of 21 invasive species analyzed by scientists in the journal Scientific Reports, with costs totalling $17 billion over the same time period.

The United States Geological Survey (USGS) describes invasive species as plants, animals or organisms that have been introduced to places outside of their native areas. These then spread and can harm the environment, human health and the economy.

On the American island of Guam in the North Pacific Ocean, for example, the brown tree snake causes island-wide power cuts and costly damage by slithering over electrical wires, according to the BBC.

Over a 20-year period, the snakes caused more than 1,600 power cuts, a study on the website ScienceDirect found. This cost the island’s economy at least $4.5m a year. The brown tree snake has also driven local bird species to extinction.

Brown tree snakes were first spotted on Guam in the 1950s and probably got there by stowing away on cargo ships coming from New Guinea, one of their native areas, the USGS says.

The Scientific Reports study found that islands in the Pacific Ocean were most at risk from the economic impact of invasive species – at 63% of total costs – followed by Europe (35%) and North America (2%).

In Europe, there have been programmes to control or eradicate the American bullfrog across countries including France, Germany, the United Kingdom, and the Netherlands, according to one of the studies cited in the Scientific Reports analysis.

It would cost Germany €4.4 billion ($4.47 billion) to control or eradicate the American bullfrog if the species spread country-wide, the European Environment Agency (EEA) warns in its report, The impacts of invasive alien species in Europe.

The bullfrog, which is native to North America, can grow to 20cm in length and almost half a kilogram in weight. It is a prolific predator that can “colonise a whole range of habitats and feed on many species”, the EEA says.

This makes it a “major conservation concern” for native species with the potential for spreading disease.

Invasive species are one of the top global causes of species becoming extinct and of declining biodiversity – the falling variety of life in an area – according to conservation organization, the International Union for Conservation of Nature (IUCN).

For example, rats are linked with “catastrophic declines” of island bird populations, the organization says. In some parts of the world, their fleas also transmit the bacteria that causes the plague, described by the World Health Organisation as an infectious disease that can cause severe illness and death.

In North America, an invasive insect species, the Emerald Ash Borer beetle, is “decimating some of the most prominent ash tree species”, the IUCN says, and could potentially destroy more than eight billion ash trees.

Invasive insect species alone cost the global economy at least $70 billion a year, it adds.

Climate change is also heightening the threat posed by invasive species, the IUCN notes. As greenhouse gas emissions warm the Earth’s atmosphere, the more extreme weather events this causes – including floods and droughts – makes habitats more vulnerable to invasions.

For example, melting ice caps in the Arctic make shipping routes between Asia and Europe faster, increasing the likelihood of unwanted invasive species onboard surviving the trip.

To combat this additional threat, governments should make sure their climate change policies take the impact of invasive species into account, the IUCN suggests.

For example, where forests are used to naturally soak up carbon dioxide or control erosion, only native tree species should be used.

Risk assessments of invasive species also need to consider how climate change might create threats from new alien species in the future.

The IUCN also suggests prioritizing the world’s ecosystems according to the level of threat they face from climate change and invasive species. Control measures can then be put in place on the main pathways that introduce or spread harmful organisms, with “early warning and rapid eradication” systems supporting this.

“International cooperation is key,” the IUCN concludes.