Different species of seabirds can coexist on small, isolated islands despite eating the same kind of fish. A researcher at Uppsala University has been involved in developing a mathematical model that can be used to better understand how this ecosystem works.

“Our model shows that coexistence occurs naturally when species differ in their ability to catch fish and to efficiently fly long distances to the area where they catch fish,” says Claus Rüffler, Associate Professor of Animal Ecology at Uppsala University.

Seabirds can breed in very large colonies, sometimes consisting of several hundred thousand pairs. Ecologists working with seabirds have long been interested in what regulates the size of such colonies. Researchers at Uppsala University and the University of Lausanne have developed a mathematical model that studies how the behavioural decisions of seabirds about where to catch fish affect the distribution of fish around a breeding colony, how this in turn regulates the size of bird populations, and how different bird species breeding in the same colony and eating the same resource can coexist.

According to basic ecological theory, two different species cannot exist on the same limiting resource – the better competitor is expected to drive the other to extinction. The researchers wanted to understand what makes the coexistence of seabird species breeding on the same isolated island possible.

“For all species, it would be most beneficial to fish close to the island because it would cost them the least energy. But bird species differ in traits such as wing length and how deep they can dive. Our model shows that different species, all maximising their energy intake, automatically use different distances from the colony,” says Rüffler.

The model predicts that seabirds will divide the waters around a colony into different circular zones, with each species using its own zone to fish in.

“Our model is fundamentally about coexistence and biodiversity. Understanding this is important in itself – we humans have a desire to understand how nature works. However, such an understanding is also crucial for any management strategy for an endangered ecosystem. Also, we believe that our results contribute to ecology more generally because the mechanism for coexistence discovered in our model likely applies to systems other than seabirds,” Rüffler concludes.