Pregnant, nursing and lactating mongooses fully replenish populations within two months of removal—likely bad news for rabies transmission
Local removal of invasive mongooses that have been eating their way through native Caribbean species has little long-term effect as surrounding populations quickly move back in.
“The recolonization happened super quickly,” said Caroline Sauvé, a wildlife veterinarian and researcher at the Maurice Lamontagne Institute in Quebec.
Since removing mongooses from islands has been nearly impossible, some wildlife managers have focused on more local efforts. But rather than giving native species temporary relief from these small predators, culling might actually cause an increase in rabies as animals move in to newly empty habitat.
First introduced to the Caribbean in the late 19th century to help control rodents and snakes in sugar cane fields, the small Indian mongoose (Urva auropunctata) has since become an invasive species with a destructive ecological footprint. As generalist carnivores, they eat native birds, amphibians, reptiles, birds and mammals. Their appetites have led to declines in native reptiles and ground-nesting birds.
Mongooses also carry diseases that can infect humans as well as domestic animals—most infamously, rabies. In Puerto Rico, Grenada and Cuba, small Indian mongooses are the primary source of rabies infections, which have a 100% fatality rate once symptoms begin.
As early as 1902, land managers have tried to remove mongooses from the islands through culling or poisonous baits. But the latter can cause unintentional ecological effects when nontarget species eat the poison or when scavengers consume poisoned mongoose carcasses. Neither approach has been very successful at limiting mongoose numbers long-term on all but the tiniest of islands.
For her doctoral thesis at the University of Montreal, Sauvé researched the mongooses of St. Kitts and Puerto Rico: their ecology, population densities, movements, home range, contacts with one another and rabies epidemiology. With this information, she created a mathematical model to understand rabies transmission and control strategies in the region.
Despite her knowledge of the minutiae of a mongoose’s daily habits, she didn’t know how ongoing removal efforts affected mongoose behavior and demographics.
In a new study published in The Journal of Wildlife Management, Sauvé monitored mongooses on St. Kitts Island before and after an intensive, localized removal program to determine how their behavior—and therefore, disease transmission—might change after a cull.
The team euthanized 91 mongooses over six days, which was an estimated 88% of the population of the 1 square kilometer study area. “It’s difficult because they’re my study species, and so you get attached,” she said. Each carcass was donated to St. Kitts’ Ross University School of Veterinary Medicine for research, which Sauvé said has resulted in several other published studies.
Monitoring of the area subsequently revealed that mongooses began to immigrate as early as the first days after culling. By seven weeks post-removal, the population density had completely recovered. “We would have expected that at least for a few weeks, it would have stayed somewhat lower than what it used to be, but that wasn’t the case.”
Why do mongoose populations bounce back so quickly?
The researchers also found a stark change in demographics as mongooses repopulated the study site. The sex ratio of new recruits skewed heavily female, and the mongooses recolonizing the study site were mostly either pregnant or nursing. “This really accelerated recolonization,” Sauvé said. Before experimental removals, about 1-3% of the population were juveniles. At seven weeks after culling, that number had risen to 14%. By six months after removal—which is also the age that mongooses reach sexual maturity—the proportions were back to baseline levels.
Sauvé also monitored the behavior of 19 mongooses via VHF radiotelemetry collars for seven months before the cull and for seven weeks afterward. The collared mongooses spared in the cull increased their daily presence in the study area for five to 30 days after removal before going back to business as usual.
Can mongoose removal affect disease transmission?
St. Kitts is rabies-free, but Sauvé is worried that removing mongooses from parts of islands with the disease might lead to a phenomenon called the perturbation effect.
The perturbation effect has been demonstrated in European badgers (Meles meles), which are reservoirs for bovine tuberculosis. Disease transmission to cattle increased, rather than decreased, after culling. “Culling disrupts social bonds and hierarchy,” Sauvé said. “The remaining animals increase their home ranges, establish new social relationships—meaning more fights, interactions, and contacts than they used to [have]—especially in territorial species.” These changes in behavior can result in an increase in disease transmission compared to more stable populations.
A similar thing might be happening in the Caribbean after mongoose removal. Sauvé said that the increase in activity and strong attraction to the territory left open by culling may increase opportunities for rabies transmission. “There’s a chance there’s some perturbation effect here,” Sauvé said, but she emphasized there need to be studies conducted on islands where rabies is present in mongoose populations.
Island-wide eradication efforts are nearly impossible. Her effort included five people working full-time for six days and covered an area that was a little less than one square kilometer. “In theory, if you culled mongooses across an island, of course they wouldn’t survive. But in practice, that’s impossible for any large island,” she said. Even poisoned bait dispersals from planes can get caught in thick tree canopies before reaching the ground, where they can get eaten by mongooses. “I think this helps explain why it’s so hard to eradicate mongooses,” Sauvé said.
Since controlling mongoose populations on large islands seems near impossible, Sauvé said that wildlife managers might now focus instead on rabies control. Oral rabies vaccinations are a potential solution for mitigating the disease in small Indian mongoose populations, but Sauvé said you need to know a lot about the species before starting a wildlife vaccination campaign. “Everything we do for mongoose ecology will hopefully inform a future oral rabies vaccination program,” she said. “There’s still a lot we don’t understand about them.”
This article features research that was published in a TWS peer-reviewed journal. Individual online access to all TWS journal articles is a benefit of membership. Join TWS now to read the latest in wildlife research.
