Exotic pets — companion animals that are not native to the area where they are kept — can become invasive pests when they escape or are released by pet owners, breeders, collectors and wholesalers
It’s no secret that the global market for exotic pets is large and growing, particularly recently, because so many of us stayed home for the past year to comply with the coronavirus pandemic quarantine restrictions.
In the last decade alone, billions of individuals comprising thousands of animal species were traded annually, fuelling a multibillion-dollar global industry (i.e.; ref and ref). This rapidly increasing demand for pets has been driven by the easy access provided by the internet and, increasingly, by bored and lonely housebound people who are looking for ‘something different’ — nontraditional or ‘exotic’ pets — basically, animals that lack a long history of domestication (ref).
But what happens to these exotic non-traditional pets when they get too big, too expensive, or too ornery or too boring to keep in a home? Often, they are intentionally released by their keepers or, sometimes, they escape. And this offers the potential for establishment of nonnative species. Free-roaming non-native species present powerful implications for the conservation of native species, whose populations are often dwindling, as well as for the establishment of invasive species in places where they can cause a lot of harm.
A surprising number of invasive animal pest species start out as exotic non-traditional pets. For example, a 2017 study (ref) showed that most (53%) invasive vertebrate species were introduced around the world through the exotic pet trade (957 out of 1,822 species). A number of other studies (i. e.; ref, ref, and ref) provided extensive evidence that pet owners intentionally release some portion of their animals frequently, and this results in biological invasions.
But what about the exotic non-traditional pets themselves? It’s obvious that the pet trade is responsible for the global movement of invasive animals, but it’s unclear if the most commercially successful exotic pets are invasive species or if exotic pets become invasive as the result of being kept as pets. If we are to deal successfully with invasive species, it is important to better understand the many overlapping threats — environmental, economic, social and human health — posed by exotic non-traditional pets by identifying their potential invasion risk. To do this, we must distinguish between these two very similar and tightly interwoven questions:
1) Is being a pet increasing the probability of becoming invasive? (Pets become pests)
2) Or is being invasive increasing the probability of becoming a pet? (Pests become pets)
The first question has been answered, mostly, because of the documented predilection of pet keepers for releasing unwanted exotic non-traditional pets.
“The second question [do pests become pets] is much more difficult to answer because most invasive vertebrates are invasive because of the pet trade (they escaped captivity or were released by their owners),” ecologist and myrmecologist (ant scientist) Jérôme Gippet, a postdoc in conservation biology at the University of Lausanne, said in email. “And that’s why we used ants to solve this problem.”
According to Dr Gippet, ants are an ideal model system to test the ‘do pests become pets’ question because:
- Exotic ants can be invasive. So far, there are 255 species of ants that are invasive (i.e., established outside of their native range). But this is not the result of the pet trade but because ants are transported worldwide with traded commodities (especially potted plants)
- The trade in ant colonies as pets/ornamentals is becoming increasingly easy, popular and global since the early 2000’s, when internet animal sales were more fully developed. Thus, the pet trade in exotic ants is too recent to be responsible for any new ant invasions.
If invasive exotic ant species are overrepresented among those that are traded as pets, one interpretation is that invasiveness is linked to the probability of being traded. But this issue is complicated.
“While working on this project and trying to understand this very strong overrepresentation of invasive species among traded pet species, we realized that it was sort of a ‘chicken or the egg’ problem,” Dr Gippet said in email.
Do pets become pests?
In his newly published study, Dr Gippet collaborated with Cleo Bertelsmeier, a professor at the University of Lausanne who specializes in biological invasions and climate change. To investigate whether the pet trade specifically favors species that are invasive, Dr Gippet and Professor Bertelsmeier conducted a literature review of 14 recently published lists from eight publications of non-native vertebrate species and their abundance in the pet trade. The surveyed animals included mammals, birds, reptiles, amphibians, and fish (Figure 1).
Dr Gippet and Professor Bertelsmeier then conducted a meta-analysis on these data and found that invasive species represent 12.6% of the exotic non-traditional pet trade. They also found that invasive species are, on average, 7.4 times more frequent in the pet trade than in the global pool of vertebrate species (mammals, 4.2–7.2; birds, 2.5–7.4; reptiles, 4.0–12.7; amphibians, 8.0–9.0; and fish, 7.2–13.1).
To test the study’s main question — whether the exotic pet trade actually prefers species that are invasive — Dr Gippet and Professor Bertelsmeier followed up this meta-analysis with a statistical analysis of the diversity of ant species offered for sale online between July and December 2017 by the exotic non-traditional pet trade.
Do pests become pets?
As I previously mentioned, we know that 255 of the 15,377 described ant species already became invasive since the 1800s (Figure 2C), but these ant invasions did not stem from the exotic non-traditional pet trade.
So far, exotic pet ants have not established any invasive populations because there is a time lag of one or more decades between the initial introduction and the spread of an invasive species (i.e.; ref), and this is usually when we first notice them. However, if invasive ant species are already known to be overrepresented amongst ants currently offered as pets, a reasonable person may conclude that being invasive is linked to greater commercial success.
But why might invasiveness be linked to greater commercial success? Dr Gippet identified which traits are shared by invasive species. He found, for example, geographic origin is important for overall availability. For example, ant species endemic to the tropics in Africa were offered by fewer sellers (Figure 2A), whereas species originating in Europe and North Africa were offered by more sellers, mainly because the global ant trade is more developed in the Palearctic region. Nevertheless, there is a large diversity of interesting ant species in the Afrotropics that are mostly unknown by the pet trade at this time, but are likely to become invasives if presented with this opportunity by future pet ant keepers (Figure 2B).
It’s important to note that some ecological traits associated with greater invasiveness also increase the commercial success of particular ant species. Dr Gippet found that, first, ants that were generalists (Figure 3B, bottom panel) and second, ants with a large geographic range (Figure 3B middle panel) were associated with a greater risk of invasiveness. Both traits are associated with species with less specialized needs and are, therefore, easier to keep. Third, body size also was important for invasiveness: for ants, small body size was key (Figure 3B top panel), probably because it’s easier for small ants to escape and remain undetected.
But perhaps the reason that at least some ant species could become invasive is because exotic pet keepers like them — a lot.
“I think the main reason why some species become invasive is because humans transport them around the world all the time,” Dr Gippet explained in email. “[A]nd because human activities disturb habitats and damage native communities.”
Human-disturbed habitats are very similar, so a species that is adapted to live in one disturbed habitat can live in others, regardless of where in the world they may be located.
Overall, Dr Gippet’s statistical analysis revealed that invasive ant species appear in the pet trade 6.6 times more often than in the global species pool and are sold by 1.7 times more sellers than are noninvasive species.
Keeping pets should be an ecological as well as an ethical responsibility
“In general, this analysis shows that there is a clear relationship between the frequency of traded species and their distribution in the wild,” said ecologist Hanno Seebens, who was not involved in the study. Dr Seebens is a postdoctoral fellow at the Senckenberg Biodiversity and Climate Research Centre and is one of the coordinating and contributing authors of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services’ (IPBES) assessment on invasive alien species that will be published in 2023.
“Even without knowing the underlying causes, it is very interesting to see this,” Dr Seebens continued in email. “The results are not unexpected, as similar relationships have been shown already for plants and several studies indicated this for vertebrates, as the authors stated, but now it has also been shown for invertebrates, which is a notoriously under-studied taxonomic group.”
Although ant species are quite different from vertebrate species, the processes of introduction and the problems involved in the pet trade and animal husbandry are very similar: once a species is kept as a pet, there is a chance that the species may be intentionally released or escape. But ants could be particularly problematic in this regard.
“Maybe, it is even more difficult to keep ants under control due to their small size and single individuals may easily escape unnoticed,” Dr Seebens pointed out in email.
So what has this ant study told us about the ‘pets become pests’ versus the ‘pests become pets’ puzzle?
“It is impossible to disentangle these processes (ie. did the trade cause the wide distribution or did the traits of the species make them good invaders and good pets) in a correlative analysis like this one,” Dr Seebens elaborated in email. “And the author did not try to make this point, which is fine.”
Unfortunately, those who are the primary route for introducing invasive species are often poorly informed about the ethics of pet ownership.
“[W]hen you see the numbers of pets (not only cats and dogs, but also exotic pets) that are abandoned/released into the wild everywhere each year, it seems clear that the trade in domestic and non-domestic animals should be better regulated and people should be better informed about the ethical and ecological responsibility of buying an animal,” Dr Gippet pointed out in email.
Perhaps pet keepers should be limited to keeping only native species?
“It is very difficult to regulate pet keeping as a whole in restricting it to e.g. native species. I cannot imagine that this will be possible,” Dr Seebens responded in email. “But it would be much easier to ban the trade of certain problematic species, which has been implemented, for example, for threatened species. This could also be extended to the most problematic invasive species.”
If we are serious about limiting the spread of even more harmful invasive species, these recommendations, along with a number of other measures, must be carefully thought out and implemented regarding the possession of any and all pets.
“This does not necessarily mean prohibiting the trade in exotic pets, but I guess more pro-active regulations should be considered (e.g., white lists of authorized species rather than blacklists of banned ones),” Dr Gippet said in email.
“I think we should all […] consider that an exotic pet is a potential invader that could cause lots of damages to ecosystems and people if escaped or released,” Dr Gippet added in email. “Because pets can become pests, and the opposite seems also true.”
“It seems that, rather than a ‘chicken or the egg’ problem, we might face a ‘vicious circle’ problem.”
Jérôme Gippet and Cleo Bertelsmeier (2021). Invasiveness is linked to greater commercial success in the global pet trade, Proceedings of the National Academy of Sciences of the United States of America, published online on 22 March 2021 ahead of print | doi:10.1073/pnas.2016337118
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