In addition to being home to men with questionable decision-making skills, Florida also seems to have some issues with bizarre animal behavior, be it icy 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 let the sunshine be their home until we brought them there as pets.
In fact, there are many problematic invasive species that have spread through the pet trade, from predatory fish that can drag themselves between bodies of water to crayfish that clone themselves to reproduce. Those high-profile cases lead to some obvious questions, such as whether pets are really 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 their conclusion is that yes, our pets are more likely to have problems.
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 include estimates of the total number of species, as well as the number of species classified as invasive and the number included in the pet trade.
If pets were not more or less likely to be invasive, you would expect the invasive species to occupy similar fractions of both the pet trade and the total number of species in that group. But that’s not what we see each of the groups. Invasive mammal species were five times more common in the pet trade than in the wild around the world. In birds there was a similar result; for amphibians, invasive species were eight times more common in the pet trade and about 10 times more common in fish.
Overall, invasive species were 7.4 times more likely to be kept as pets than you might expect based on their frequency among vertebrate populations.
But cause and effect can be difficult to disentangle. Do we choose species that are more invasive as pets? Or have pets been given more opportunities to invade new environments because we transport them around the world?
Gippeta and Bertelsmeier had a good way to answer this question – by using ants. Ants were quite uncommon as pets over the decades. (I did run an ant farm as a kid, but they barely managed to get into the habitat we had for them before they all died, so it’s not clear if that species could contribute to the statistics.) As a result, ants haven’t had that yet. just as many opportunities to penetrate new territory through the pet trade. When ants invaded habitats, they did so the old-fashioned way: by being invasive.
But it turns out that ants are completely unremarkable in terms of the trend the researchers saw in other species. The ants sold as pets were about 6.6 times more likely to be invasive than you might expect based on the frequency of invasive ant species. Of the 19 most invasive ant species, 13 were offered for sale as pets. Of all the species that were sold, more invasive species were also offered for sale.
Are there any characteristics of the invasive species that make them more likely to appear in the pet trade? Gippeta and Bertelsmeier looked at five different traits that invasive ants have in common. Two of them – with multiple queens in a nest and able to nest in different places – probably didn’t get the kind used by the pet trade anymore. But two others were. One had a wide range in their native habitat, and the second occupied a variety of habitats within that range. Both would seemingly help the ants survive the questionable care they are likely to receive once they reach their buyers.
There was a strange feature: invasive ant species usually have smaller body size. But unsurprisingly, people prefer species they can see easily, so pets are more likely to have larger body size, which means there’s an anti-correlation here.
For many ecosystems, confirming that this is a problem is a bit like closing the barn door after the horses reach the escape speed and heading for Pluto. But there are many sites trying to control invasive species – often island states or habitats like New Zealand and Hawaii. By identifying potential threats before they become problems, this information can help those places prevent new threats from being identified.
PNAS, 2021. DOI: 10.1073 / pnas.2016337118 (about DOIs).