Rich biodiversity depends on the ability of species to coexist. And yet, ecologists are still grappling with the requisite mechanisms. Recent findings published in Nature suggest that predators might not be as helpful a driver of species coexistence as previously thought.
“These results should serve as a warning that introducing new predators to a system can increase diversity of their prey but can also decrease diversity,” says Pacifica Sommers, an ecologist at University of Colorado in Boulder who was not involved in the study.
Many ecologists think that predators can boost biodiversity by hunting just enough of one prey species to make room in the ecosystem for another competing prey species to coexist. But predators can also harm biodiversity by hunting prey species to extinction.
Looking to test how predators affect the coexistence of prey species, study coauthor and Princeton University ecologist Robert Pringle carried out an experiment on 16 small islands in the Bahamas that are widely populated with semiterrestrial brown anole lizards (Anolis sagrei). On eight of the islands the team introduced between five and seven curly tailed lizards (Leiocephalus carinatus)—a ground-dwelling predator of brown anoles and the arboreal green anoles (Anolis smaragdinus). On four islands, they also added 10 to 11 of the green anoles, a competitor to A. sagrei. On another four islands, they introduced just the green anoles. The remaining four islands were left untouched to serve as a control.
In 2011 and in each subsequent year up to 2016, the team conducted a baseline population count for the brown anole; they assessed the population sizes, habitat use, diet, and trophic position of all three lizard species. Over the five-year experiment, the curly tailed lizard population grew by more than fivefold to an average of 30 individuals across all experimental islands. On islands where only green anoles were introduced, the arboreal lizard’s population grew on average by eightfold (up to 161 individuals per island by 2016).
But on islands where both the curly tailed predators and green anoles were introduced, the latter mostly fared poorly—two populations went extinct, another remained stable, and one grew by 50 individuals, or threefold. The results show that predators hinder rather than help the prey species to coexist. “We found that the presence of top predators destabilizes coexistence of prey species,” says Pringle.
Observations suggested that the green anole populations did not dwindle because they served as food for the predatory curly tailed lizards. Rather, they died out because the curly tailed predator managed to outcompete them in their niche; the land-dwelling brown anoles retreated to the trees in the presence of the curly tailed predator. A molecular analysis of the lizards’ diets backed up the team’s observations, suggesting in part that the consumption of green anoles was rare and that brown anoles moved away from a diet of ground dwelling insects.
“In the absence [of] the predator, the two prey occupy different ecological niches,” Pringle explains. “When the predator is present, both prey species are forced into a hiding place where they compete much more intensely to the point that they are not able to coexist.”
Pringle says the brown anoles likely got the upper hand because they started out with more individuals per island—between 100 and 300—compared to green anoles with around 11 individuals. “The identity of the winner and loser may depend on initial conditions,” he says. Pringle speculates that the population of green anoles that did manage to flourish was helped along because brown anole numbers on that island fell to very low levels due to predation, and this eased the competitive pressure.
Sommers says the study “is a nice illustration of the fact that predators do not have only one type of effect on ecosystems.” She urges conservationists to consider these results before introducing predators with the aim of boosting biodiversity. Pringle says that in future studies he plans to investigate further what drives one prey species to get the upper hand over another.