The Unpredictable and Unexpected Impacts of Invasive Species
You may have noticed that, at ARC, we spend a lot of time on invasive species management. From removing invasive bullfrogs in southwestern wetlands to controlling fire ants and a multitude of plants in southeastern ecosystems, it seems like our invasives work is never-ending. In fact, it may seem pointless, and some have argued that we just need to get used to the idea that invasive species are now part of our ecosystems. So, why do we work so hard to control invasive species? The answer is simple: invasive species are often detrimental to imperiled native species.
While that answer may be simple, the many ways introduced species impact natives are not. What happens when a new species arrives in an ecosystem can be difficult to predict because natural systems are complex and dynamic. It turns out that some species don’t create too many issues for native species, and others have an oversized impact, sometimes causing big problems, sometimes in ways we wouldn’t have expected. When a species disrupts the ecosystem, it’s classified as invasive.
A recent case in Africa demonstrates this perfectly. An invasive ant species moved into a Kenyan savanna about 20 years ago, and as a result, lions now kill significantly fewer zebras there (Kamaru et al. 2024). This was unpredicted and unexpected.
After its introduction, the invasive big-headed ant, which is thought to be native to an island in the Indian Ocean, outcompeted and eradicated native acacia ants. Acacia ants and a savanna tree called the whistling thorn tree have a special relationship, a mutualism. The tree provides acacia ants with food and a place to live, and the tree is provided protection from browsing elephants by the ants, which deliver an irritating bite to the elephants.
With the acacia ants gone, the elephants were no longer deterred and were free to pluck the trees almost bare. This, in turn, had a surprising effect on lions. Whistling thorn trees were an important source of cover for lions, which used the trees to hide them as they snuck up on zebras. Because of this, the number of zebra kills was decreased by almost threefold in these areas (Kamaru et al. 2024).
Given all of the factors at play in intricate ecosystems, though, perhaps it’s not surprising that ecosystem health might be more important than the characteristics of the introduced species itself. In some places, high-quality habitat seems to act as a buffer for vulnerable native species when invaders are added to these systems.
For example, in a long-term study in Oregon, researchers investigated the effects of invasive bullfrogs from the Eastern US on native amphibians, including rough-skinned newts and northern red-legged frogs. Bullfrogs are voracious eaters and often impact native species by eating them directly and outcompeting them for food. However, scientists found that when habitat conditions were favorable, several species of native frogs and salamanders were able to maintain stable populations even though they were sharing the landscape with bullfrogs (Rowe et al. 2019). Unfortunately, though, in many other places in the Western and Southwestern US, bullfrogs are decimating populations of imperiled native amphibians and reptiles, including the Threatened Chiricahua leopard frog.
The unpredictable nature of the potential cascading effects invasives can cause illustrates just how damaging they can be. In fact, invasive species are one of the leading contributors to the worldwide biodiversity crisis.
So, what do we do, given that the results are often unpredictable when species are introduced? It’s challenging to manage the unexpected consequences of invasive species in ecosystems with numerous players enmeshed in complicated webs of interactions. Plus, in some cases, the impacts aren’t fully understood until decades after an invasion has taken hold.
The solutions are sometimes as complex as the ecosystems we’re working to protect, but there’s hope. The obvious first step is prevention. We know that when introductions are allowed to occur, they can be costly and devastating. Effective public education, laws, and regulations work when implemented well and enforced at large scales.
Also, early detection is critical so that control measures can be carried out before an invasion gets out of hand. At ARC, we regularly survey for introduced species in PARCAs, or Priority Amphibian and Reptile Conservation Areas, across the US. Plus, we’re conducting long-term population monitoring of sensitive native species in these locations to watch for trends indicating that we need to step in and remove invasives. Learn more about how we do that here.
Most important, however, is the protection and restoration of habitats for imperiled species. When habitats are healthy, populations of native species tend to be healthy, too. In all of our PARCAs, invasive species removal and control are at the center of the many actions we’re taking to return these landscapes to the conditions imperiled amphibians and reptiles need to flourish once more.
References
Kamaru, D.N., Palmer, T.M., Riginos, C., Ford, A.T., Belnap, J., Chira, R.M., Githaiga, J. M., Gituku, B.C., Hays, B.R., Kavwele, C.M., Kibungei, A.K., Lamb, C.T., Maiyo, N.J., Milligan, P.D., Mutisya, S., Ng’weno, C.C., Ogutu, M, Pietrek, A.G., Wildt, B.T., & Goheen, J.R. (2024). Disruption of an ant-plant mutualism shapes interactions between lions and their primary prey. Science, 383: 433-438. https://doi.org/adg1464
Rowe, J.C., Duarte, A., Pearl, C.A., McCreary, B., Galvan, S.K., Peterson, J.T., & Adams, M.J. (2019). Disentangling effects of invasive species and habitat while accounting for observer error in a long-term amphibian study. Ecosphere, 10: e02674. https://doi.org/10.1002/ecs2.
