Unfortunately, we like pets that are likely invasive species.

In addition to being home to men with questionable decision-making skills, Florida also seems to have some issues with strange animal behavior, whether it’s frozen iguanas falling from trees or alligators fighting pythons in the Everglades. However, when it comes to those animals, Floridians can really blame non-natives. Neither pythons nor green iguanas made the Sunshine State their home until we brought them there as pets.

In fact, there are many troublesome invasive species that have spread through the pet trade, from predatory fish that can crawl between bodies of water to a crayfish that is cloned to reproduce. Those high-profile cases lead to some obvious questions, like whether pets are actually more likely to be invasive, and if so, why?

Two Swiss researchers, Jérôme Gippeta and Cleo Bertelsmeier, have now tried to answer these questions. And his conclusion is that yes, our pets are more likely to be a problem.

We have bad taste

To answer the question of whether pets are really problematic, the researchers generated some basic statistics for different groups of animals (mammals, birds, reptiles, amphibians, and fish). These included estimates of the total number of species, as well as the number that are classified as invasive and the number that are part of the pet trade.

If pets weren’t more or less likely to be invasive, you’d expect to see invaders occupy similar fractions of both the pet trade and the total number of species in that group. But that’s not what we see in no of the groups. Invasive mammalian species were present at a rate five times higher in the pet trade than in the wild worldwide. There was a similar result in birds; for amphibians, invasive species were eight times more common in the pet trade and about 10 times more common in fish.

In general, invasive species were 7.4 times more likely to be kept as pets than would be expected based on their frequency among vertebrate populations.

But cause and effect can be difficult to unravel. Do we choose species that are most likely to be invasive as pets? Or did pets end up with more opportunities to invade new environments because we transport them around the world?

Analyzing ants

Gippeta and Bertelsmeier had a good way to answer this question: using ants. Ants were quite rare as pets before the last few decades. (I had an ant farm as a child, but they barely managed to invade the habitat we had for them before they died, so it’s unclear if that species could contribute to the statistics.) As a result, ants have not had as many opportunities to invade new terrain through the pet trade. If the ants invaded any habitat, they did it the old-fashioned way: being invasive.

But it turns out that ants are not exceptional in terms of the trend the researchers saw in other species. Ants that were sold as pets were about 6.6 times more likely to be invasive than would be expected based on the frequency of the invasive ant species. Of the 19 most invasive ant species, 13 of them were offered for sale as pets. Among all the species being sold, invasive species were also offered for sale most frequently.

Are there any characteristics of invasive species that make them more likely to appear in the pet trade? Gippeta and Bertelsmeier analyzed five different properties common to invasive ants. Two of them – who had multiple queens in a nest and could nest in a variety of sites – were no more likely to have the species used by the pet trade. But two others were. One had a great variety in its original habitat and the second occupied a variety of habitats within that range. Both would apparently help the ants survive the questionable care they will likely receive once they reach their buyers.

There was a strange characteristic: invasive ant species tend to have smaller body sizes. But unsurprisingly, people tend to prefer species that they can see easily, so pets are more likely to have large body sizes, which means there is a reverse correlation here.

For many ecosystems, confirming that this is a problem is a bit like closing the barn door after the horses have reached escape velocity and headed for Pluto. But there are many places that are trying to control invasive species, often island nations or habitats like New Zealand and Hawaii. By identifying potential threats before they become problems, this information can help those locations prevent new threats from establishing themselves.

PNAS, 2021. DOI: 10.1073 / pnas.2016337118 (About DOIs).

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