At some point during the 1950s, a New Guinean cargo ship on its way to the tropical island of Guam inadvertently granted passage to a stowaway – a brown tree snake, Boiga irregularis. On Guam’s soil, the snake faced no competition nor predatory threat. Unchecked, its population exploded, doubling every year to a high of more than two million individuals during the 1980s. The snakes voraciously munched on the local wildlife, driving nine of the island’s 11 endemic bird species, five of six endemic lizards and two of three endemic bats to extinction. So outsized is the snake’s population today that the serpents routinely cause power outages by interfering with the island’s transmission lines. The US government spends millions of dollars each year repairing damages and inspecting outbound ships to prevent brown tree snake escapees causing similar extinctions on other Pacific islands.
Guam exemplifies the disruption that invasive species can bring to ecosystems and societies, but the issue is so widespread and diverse that it can be difficult to sum up globally. A new study by researchers at Université Paris-Saclay in Gif-sur-Yvette, France, has attempted to do so by quantifying the cost of invasions worldwide, reaching a figure of US$1.28 trillion between 1970 and 2017, at a mean of $26.8 billion per year. Costs can arise through direct damage, for example in crop-yield reductions, or due to management programmes, such as population control.
‘Costs are understood by industries, governments and the public alike. Many groups don’t care about exotic bird extinctions, but they do care about losing billions of dollars annually on damage reparation and management schemes,’ says ecologist Franck Courchamp.
Invasive invertebrates come out as the costliest category with an annual bill of $8.7 billion since 1970. Species of Aedes mosquito are the most expensive of all because they transmit a wide range of diseases, including yellow fever, dengue, Zika and chikungunya. Mostly native to tropical regions, they have been introduced to Europe and other regions with the movement of people and goods. Huge healthcare bills have followed their geographical expansion. Mostly native to tropical regions, they have been introduced to Europe and other regions with the movement of people and goods through the trade of used rubber tyres and bamboo. Their spread has also introduced wildlife diseases to new ecosystems: in the Hawaiian archipelago, avian malaria has caused the extinction of bird species, which have no naturally-evolved immunity.
Invasive rats come in second. Introduced to more than 80 per cent of the planet’s island groups, rats can reduce crop yields, eat stored grain and cereals, predate on birds and reptiles, destroy native vegetation and infrastructure, and carry disease. ‘There are also many species where the costs are not even studied yet,’ says Courchamp. ‘The coffee borer beetle, for example, is one of the most damaging pests to coffee crops worldwide, but we have no costs in our databases.’
The bill is rising exponentially, reaching more than $160 billion for the year 2017 – more than 20 times higher than the total funds available in 2017 for the World Health Organization and the UN combined. Intensifying global trade will more than likely ensure that the bill will continue to rise, as will the fact, says Courchamp, that ‘climate change is allowing invasive species to better establish, due to milder winters and a greater range of climatically suitable territories.’
By quantifying the costs of invasives, Courchamp and his co-authors aim to raise awareness of the issue. ‘We hope that it will help to build the case for a stronger international framework, better biosecurity and more assistance for low-income countries. The economic angle might be a bit cynical, but we have to play with the tools we have.’