#2: Vampire bats. Vampire bats (Desmodus rotundus) are livestock pests because they feed on animal blood by landing lightly on a large animal, opening wounds with sharp incisors, and—yes—drinking the blood that flows. They can and do transmit rabies, being a particularly long-lasting reservoir of rabies virus. Although only a small amount of blood is drunk at any one time, the bite site is vulnerable to an infection. If the animal gets rabies, it ultimately kills the animal, hurting farmers and their families financially. Infected bats are also a serious public health risk to other livestock and to humans. To reduce the spread of the disease, authorities usually try to poison as many vampire bats as possible in a given region in response to an outbreak of bat-transmitted rabies.
A common vampire bat (Desmodus rotundus) clinging to a cave wall ( (P/C Wikipedia)
As with the green crabs in California, there’s an unexpected consequence. The usual way of killing bats is to poison them with “vampiricide,” a paste applied to the backs of bats that friendly bats back in the colony lick off of their peers, but which kills them not long after a few good licks.
When bats were poisoned before rabies was detected in an area, the researchers found that the culling could slow the spread of rabies in that area. This might be because fewer bats means fewer opportunities for virus transmission.
But when culling bats from an area started in reaction to a rabies outbreak, it had little reduction in the numbers of livestock suffering rabies, and even worse, the colonies that were culled increased the spread of rabies. Culling also didn’t reduce the numbers of livestock dying from bat attacks—once bat-carried rabies starts up in an area, the level of disease was the same regardless of whether bats were killed. Surprise! It turns out that merely reducing populations of vampire bats actually does little to limit a rabies outbreak. [Blackwood]
Despite the widespread use of vampiricide for over 40 years, vampire bat rabies still persists throughout Latin America. It turns out that bat colony size is more related to density of the local cattle herds rather than the effectiveness of culling campaigns. Unless the cull kills all of the local bats, the effect is small.
If you test bats in a colony, the level of rabies exposure is not related to colony size. Interestingly, juvenile and sub-adult bats have a higher rate of rabies exposure than adult bats. Why would that be? Young bats tend to hang out in the colony with other young bat pups and mothers, sharing diseases like all young children in colonies. Since vampiricide kills primarily adult bats, it’s not a surprise that bats from periodically culled colonies actually show higher rabies exposure levels than those from colonies that were never culled.
Notably, the efficacy of culling for rabies control has had no effect or (much worse) positive effects on the occurrence of rabies in vampire bats. [Blackwood, et al.] One possible explanation is that culling via the vampiricide paste preferentially targets adult bats due to population age structure and behavioral factors, but that adults may be more likely than juveniles to have acquired immunity to rabies from past exposures.
3. Cats. Like bats, low-level culling of feral cats in open populations just doesn’t seem to work. Contrary to expectation, the relative numbers and activity of feral cats increased in the cull-sites, even though the numbers of cats captured during the culling period declined, just as you’d expect. However, numbers during the culling period, increases in the estimated numbers of cats living in the area ranged from 75% to 211% of the original, compared with pre- and post-cull estimates. This surprising result probably happened because of the influxes of new individuals after dominant resident cats were removed. Space abhors a vacuum, and that applies to cats in open spaces as well—removing feral cats from an area simply makes space for nearby neighborhood cats to move in. Cats are very mobile animals [footnote: ask anyone who’s had to bring an errant cat back home.] and easily move from place to place as vacancies arise. Unless the cull area is surrounded by a definitively cat-free area, taking away one cat just makes space for another to move in. [Lazenby, 2015]
Feral cats living in the Largo di Torre Argentina, Rome. (P/C Paolo Monti, 1969. From Wikipedia.)
When cats are trapped and removed from an area, new cats quickly move in to fill the vacated territory and start the breeding process all over again. This phenomenon in cat groups was discovered by British biologist Roger Tabor and is referred to as the “vacuum effect” (Tabor, 1983). However, if a group of cats is “neutered and returned to its area it will continue to hold the location and keep other cats out by its presence.” [Tabor, 1995]
A perfect example of the vacuum effect is illustrated by a recent study conducted by Lazenby et al. (2015) in the forests of Tasmania, Australia. “Low-level culling of feral cats” actually caused an increase in the number of cats in the area, despite the initial illusion that there was a decrease in population. At the end of the study, researchers noted a significant increase in feral cat numbers with an average of 75% at one site and 211% at the other site.
Contrary to expectation, the relative abundance and activity of feral cats increased in the cull-sites, even though the numbers of cats captured per unit effort during the culling period declined. Increases in minimum numbers of cats known to be alive ranged from 75% to 211% during the culling period, compared with pre- and post-cull estimates, and probably occurred due to influxes of new individuals after dominant resident cats were removed. Our results showed that low-level ad hoc culling of feral cats can have unwanted and unexpected outcomes, and confirmed the importance of monitoring if such management actions are implemented.
4. Lionfish. Red lionfish (Pterois volitans and P. miles) have rapidly colonized the tropical Western Atlantic from Florida to North Carolina, and into the Caribbean from Key West to Utila on the east coast of Honduras. With long, trailing red, brown, and white fins, lionfish are popular in-home saltwater aquariums, and were possibly released into the Atlantic when aquarists drained their tanks, possibly getting rid of the fish which are beautiful, but have a very painful sting. (Effects of being stung are extreme pain and nausea, with possible side orders of convulsions, dizziness, fever and numbness. You can see why home aquarium enthusiasts might like the look, but hate the sting.) The first lionfish was reported in South Florida waters in 1985 with many additional sightings occurring until they were documented as well-established throughout the Caribbean by the early 2000s.
Red lionfish (Pterois volitans) as seen on a shallow reef in the western Caribbean. (P/C Daniel Russell)
The big problem with red lionfish is that they’re voracious eaters, consuming any little fish, larvae, and eggs they find on a coral reef. Their arrival on shallow patch reef systems has been associated with declines in native fish of up to 79% and of prey fish declines of up to 65%. [Anradi-Brown, et al., 2017] It’s a top predator competing for food and space with overfished native fish such as snapper and grouper. In affected areas, the lionfish continues increasing its range. Basically, lionfish have no known predators in the Atlantic and Caribbean, while reproducing all year long. A single mature female lionfish releases roughly two million eggs a year and can flood a tropical reef system rapidly. (Footnote: In an attempt to motivate lionfish controls, there are no limits on the number of lionfish humans can take—it’s always open season. I can tell you from personal experience that lionfish fillets are mighty fine eating.) (Footnote: Another attempt to control lionfish have been various attempts by scuba diving outfits to teach the local shark population to eat lionfish, which they don’t recognize as prey. Unfortunately, the local sharks aren’t as keen as humans about lionfish, possibly seeing them as prickly prey.)
Attempts to control the lionfish population have always met with limited success. While they’re easy prey, being very colorful and slow-moving, they also range down to 786 feet (240 meters) in depth where humans just don’t go. Even if you remove 100% of lionfish down to 100 feet (33 meters), you’ve only scratched the top of local population—there’s still another 686 feet of lionfish down below.
Culling of lionfish has successfully lowered the number of animals in an area temporarily and has also led to the side-effect of moving the older, wiser, and more prolific survivors into deeper water to avoid predation, with increased vigilance on those left behind, naturally enough. So, the possibility of culling 100% of lionfish is slim to none. There are always more in the deep that will happily repopulate the free space in the shallower waters.
Cats, bats, lionfish, and crabs are all very different animals, yet culling didn’t work very well to manage their numbers. And yet, somehow, we keep driving different species into extinction, often when we didn’t mean to do so. The million-dollar question is this: How well does culling work in general?
In these four examples, culling to completely remove a species often fails. It turns out that culling in a wildlife population can paradoxically sometimes lead to an INCREASE in the level of the disease. [Prentice, 2019]
Culling efforts usually are not large enough in scope to avoid leaving remnant individuals around to restock the culled area. That is, local extinctions are usually temporary, with animals moving back into the niche after some time.
This is an important thing to know: when a population reduction effort is too low, a ‘perturbation effect’ happens—this means that culling just leads to increased movement of individuals within a region. Likewise, if culling isn’t pursued for long enough, the dynamics of a population can lead to an overcompensation effect. In effect, this means that culling has to find the ‘Goldilocks zone’, where, for a restricted combination of culling intensity, coverage and duration, the species can be reduced in total number without driving them to extinction—it has to be just right. Culling has to remove the right number of individuals to improve the circumstances (such as, to reduce spread of a disease, or to remove aggressive feeders from the ecosystem).
A mistaken assumption that eradication is complete when all it actually is NOT complete--that can have disastrous consequences: the culled species can frequently bounce back from a partial reduction in numbers and go on to expand its range, making the eradication campaign redundant. [Rout et al., 2013]
This is a strange idea—that partial culling is sometimes a better strategy than removing all of the individuals. The concept of intentionally “leaving some behind” in culling activities may be a difficult paradigm shift for invasive species managers and stakeholders. However, time and time again, studies of culling activity suggest that removal efforts are often incomplete in any case, and the total time, money, and effort spent on suppression would be better if you don’t try to kill them all but leave a small population behind. [Taylor and Hastings 2004] [Green, 2021]
REFERENCES
Andradi-Brown, Dominic A., et al. "Depth-dependent effects of culling—do mesophotic lionfish populations undermine current management?" Royal Society Open Science 4.5 (2017): 170027.
Blackwood, Julie C., et al. "Resolving the roles of immunity, pathogenesis, and immigration for rabies persistence in vampire bats." Proceedings of the National Academy of Sciences 110.51 (2013): 20837-20842.
Benavides, Julio A., William Valderrama, and Daniel G. Streicker. "Spatial expansions and travelling waves of rabies in vampire bats." Proceedings of the Royal Society B: Biological Sciences 283.1832 (2016): 20160328.
Ens, Nicholas J., et al. "The Green Wave: reviewing the environmental impacts of the invasive European green crab (Carcinus maenas) and potential management approaches." Environmental Reviews 30.2 (2022): 306-322.
Green, S. J., & Grosholz, E. D. (2021). Functional eradication as a framework for invasive species control. Frontiers in Ecology and the Environment, 19(2), 98-107.
Grosholz, E., Ashton, G., Bradley, M., Brown, C., Ceballos-Osuna, L., Chang, A., ... & Tepolt, C. (2021). Stage-specific overcompensation, the hydra effect, and the failure to eradicate an invasive predator. Proceedings of the National Academy of Sciences, 118(12), e2003955118.
Lazenby, B. T., Mooney, N. J., & Dickman, C. R. (2015). Effects of low-level culling of feral cats in open populations: a case study from the forests of southern Tasmania. Wildlife Research, 41(5), 407-420.
Prentice, J. C., Fox, N. J., Hutchings, M. R., White, P. C., Davidson, R. S., & Marion, G. (2019). When to kill a cull: factors affecting the success of culling wildlife for disease control. Journal of the Royal Society Interface, 16(152), 20180901.
Rout, T.M. et al. “When to Declare Successful Eradication of an Invasive Predator?” Animal Conservation (2013): 125–32. Web. 14 Nov. 2014.
Tabor, Roger K. The Wild Life of the Domestic Cat. London: Arrow Books, 1983.
Taylor, C. M., Hastings A. 2004. Finding optimal control strategies for invasive species: a density-structured model for Spartina alterniflora. J Appl Ecol 41: 1049–57.
Viana, Mafalda, Julio A. Benavides, Alice Broos, Darcy Ibañez Loayza, Ruby Niño, Jordan Bone, Ana da Silva Filipe et al. "Effects of culling vampire bats on the spatial spread and spillover of rabies virus." Science Advances 9, no. 10 (2023)