Trophic roles of black rats and seabird impacts on tropical islands: Mesopredator release or hyperpredation? (2015) Ringler et al., Biological Conservation, 185.

Guest post by Bart Peeters

THE CRUX

For centuries, rats have been portrayed as carriers of diseases and death; whereas our feline friends, worshipped by the ancient Egyptians, will definitely make your YouTube video go viral (a quick Google search of “cat video” shows 1 310 000 000 results). Both have been introduced, either accidentally or deliberately, to islands where endemic species have evolved and adapted to an environment without these generalist predators. So how do you know if eradicating one of them will make things better for the native wildlife?

Before taking radical conservation actions, it may be a good idea to understand how feral cats (the apex predator), rats (the mesopredator) and their common prey are affecting each other. Namely, if you kill all the cats, will there be more rats to prey on seabirds? On the other hand, will killing all the rats really reduce the predation by cats on seabirds?

HOW IT WORKS

The study was conducted on two islands off the coast of Madagascar: Juan de Nova and Europa. Both islands are important breeding grounds for seabirds, particularly sooty terns. Rats occur on both, but feral cats only on Juan de Nova.

By digging through cat scats and rat stomach contents, the authors analyzed and compared recent diets of these two predators during and outside the breeding season of seabirds. They also monitored seabird colonies for seabirds killed by cats and used video surveillance of sooty tern nests to assess visiting rates by both cats and rats. This way, they attempted to understand whether eradication actions, such as poisoning all the rats, could indeed be fruitful to the conservation of seabird species.

DID YOU KNOW: FOOD WEBS Plant and animal communities can be remarkably dynamic. Some animals are specialists, eating only one type of prey or plants, such as pandas eating exclusively bamboo. Others are generalists, eating nearly anything they find, or may switch to one resource when this gets more abundant during the season. Then there are herbivores, carnivores, omnivores, frugivores, scavengers…. If you imagine how all species in a community are connected, it does look a lot like a food web, or a hierarchical structure with plants in the bottom and apex predators on top. Some species have little impact on others, whereas a change in the presence or abundance of a keystone species transforms the ecosystem entirely (see “Did You Know: Trophic Cascades”).

WHAT THEY FOUND OUT

During the breeding season, cats shifted from eating rats and mice to preying mostly on terns. Whereas rats could only prey on tern chicks left alone at the nest, cats could also kill adult terns, sometimes at twice the rates they could consume them. Since terns and other seabirds are long-lived species, their population growth is more dependent on adult survival than breeding success. Therefore, the impact of cats on seabird colonies surpasses the direct effect of rats.

Eradication of cats will not likely lead to a strong increase in the rat population (a.k.a. mesopredator release), as rat densities are more driven by bottom-up control of resource limitation during the dry season. Without rats, however, the predation pressure of cats on native wildlife may possibly increase (a.k.a hyperpredation).

ANY PROBLEMS?

Prodding through animal excrement or guts sounds fun, but what you can learn from it has its restrictions. These kind of diet analyses can only provide information on “what” was eaten, not “how much”. Also, not every kind of food is detectable; the data could not provide any information on egg consumption, as their contents are gooey and quickly digested. However, the video surveillance data suggests that rats visited nests more frequently when the parents were not present.

I think that, based on this study and many similar cases, eradication of rats and particularly cats will indeed be beneficial for the seabirds and other endemic species. However, I do wonder if seabird survival and breeding success is more strongly determined by changes in their food distribution due to climate change, overfishing and plastic pollution.

SO WHAT?

Can’t we just kill both cats and rats? That would be ideal, yes, but for financial and logistical reasons this is not always an immediate solution. Poisoning the rats may reduce the number of cats through secondary poisoning, but can be equally harmful to native wildlife. The empirical results and examples from other systems discussed by Ringler et al. indicate that rat eradication should only be prioritized if simultaneous eradication of rats is possible. Studies like this are incredibly useful for biodiversity conservation as they facilitate the decision-making in management policies.