Two invasive species - the brown tree snake and the American bullfrog - cost the world more than $16 billion between 1986 and 2020, according to a study.

Researchers say the already-hefty price tag should be seen as a lower limit on the true cost of invasive reptiles and amphibians, especially in under-studied regions such as Africa and South America. The study results were published in the online journal Scientific Reports.

Invasive species are animals, plants or other living things that aren't native to the places where they live and damage their new environments. Humans spread many of the more than 340 invasive reptile and amphibian species - as stowaways in cargo or through the exotic pet trade, for instance.

Invasive reptiles and amphibians can damage crops, destroy infrastructure, spread disease and upset ecosystems. The damage is costly, but scientists still don't fully understand the extent of the economic impact wrought by invasive species.

For the study, biologist and study author Ismael Soto of the University of South Bohemia, and Ceske Budejovice in the Czech Republic, and his colleagues, estimated the global cost of invasive reptiles and amphibians using a database called InvaCost. The database collects the results of thousands of studies, reports and other documents produced by scientists, governments and non-governmental organizations.

FILE - A brown tree snake is held by a U.S. Department of Agriculture wildlife specialist at Andersen Air Force Base on the U.S. territory of Guam, Feb. 5, 2013.

The data revealed that invasive reptiles and amphibians have cost at least $17 billion worldwide between 1986 and 2020.

'But this cost mostly focused on two species - the brown tree snake [and] the American bullfrog,' Soto told VOA in an interview via Zoom. 'But there are almost 300 invasive species of reptiles [and] amphibians. So, this means that our cost is really underestimated.'

The two species have received a disproportionate amount of attention from researchers, said economist Shana McDermott of Trinity University, who was not involved in the study.

'When you talk about invasives, people immediately will probably say, 'Oh, the brown tree snake,' just because its impacts are so wide-ranging,' she said via Zoom. 'It's got ecosystem biodiversity impacts. It's got impacts to human health - it sends people to the hospital every year with bites. It takes down energy infrastructure. ... And so, of course, people are like, 'Oh God! That's an incredibly dangerous invasive! Let's understand it better.''

The research bias toward a few well-known species also skews the distribution of costs worldwide. For instance, 99.6% of the $10.4 billion in costs from reptile invasions were in Oceania and the Pacific Islands, largely reflecting damage dealt by the brown tree snake in Hawaii, Guam and Northern Mariana Islands. Likewise, most damage from amphibians was in Europe.

But that doesn't mean invasive reptiles and amphibians aren't problematic elsewhere. Soto said there are many invasive amphibians in Africa, but their costs probably haven't been quantified.

'There's not enough research in these countries [to] detect the economic costs,' he said.

Soto also noted that the current cost estimate only includes costs that are easily quantified. Destroyed crops or property are easier to count than reduced quality of life or indirect damage to human health and assigning dollar values to ecological damage is trickier still, McDermott said.

'We're still in this very early stage of trying to understand the economic costs, and trying to understand how invasive species impact ecosystems, how they impact people's quality of life,' she said, adding that she wants to include the price of biodiversity losses in future cost estimates.

Soto and McDermott agreed that future studies should not only quantify the costs of more species in more regions but also project how the costs will evolve with time, especially as climate change continues to facilitate the spread of more invasive species.

'There is a lot still left to be determined. ... I do think that quantifying it is the first step, though,' said McDermott. 'Unless you can put a dollar value on it, unfortunately, you don't get [policymakers'] attention for policy. So, this is an incredibly important topic. ... We really shouldn't be waiting on more studies to act.'