Deeper understanding of species roles in ecosystems

How is an ecosystem affected when one or several species die out, or a new species invades and disturbs the ecological system? In order to predict how complex ecosystems will react to disturbances, it is important to understand how species influence each other. Theoretical ecology approaches such ecological questions with the aid of mathematical models. A new theoretical ecology study, published in the scientific journal Ecology Letters, focuses on marine ecosystems, such as coral reefs, fjords and the open sea. These marine systems contain several habitats of great ecological and economic importance. The scientific analysis of these habitats is, however, complicated because of the number and variety of species that call them home.

One method to deal with this complexity is to describe the system as a food web, where the nodes represent the different species and the links correspond to interactions between them. An interaction may be, for example, that one species feeds on another, that an insect pollinates certain plants, or that a parasite attacks certain hosts.

"Research on ecological networks has focused on how direct interactions like predation are controlled by species traits such as body size. We have taken this idea one step further and looked at how the ecological role of a species, or in other words its pattern of direct and indirect interactions, is controlled by its traits," says Anna Eklöf, senior lecturer in the Department of Physics, Chemistry and Biology and leader of the study.

It is expensive, time-consuming, and difficult to measure who feeds on whom in the natural world and how each species is affected by the feeding patterns of other species. It is, thus, a great advantage if we can take a shortcut and predict the role of a species in an ecosystem based on its known traits without knowing all of its interactions with other species.

"More accurate prediction of the ecological roles of species based on their traits can also help us to predict the roles of species when they are introduced, by accident or design, into new habitats," says Alyssa Cirtwill, postdoc in the Department of Physics, Chemistry and Biology and principal author of the article.

The researchers started by identifying all the possible combinations of interactions for three species in a food web. These patterns are known as "motifs" and are used within many fields of research to analyse networks and to mirror functions within the network.

The researchers defined the ecological role of a species in an ecosystem as the number of times the species appears in different locations in different motifs. They used data from six complex marine food webs, from the tropics to the poles, and looked for correlations between the ecological roles of species and some easily-collected traits. Species' traits and their roles did turn out to be related, and the researchers discovered that the environment in which a species seeks its prey is particularly important.

"You'd expect this, since a predator-prey interaction between two species requires first and foremost that they meet in the same environment. This requirement, however, is not always included in the models used to predict interactions. Our results demonstrate how important it is to include factors that influence the choice of environment in which a species seeks its prey," says Anna Eklöf.

The study has looked at marine ecosystems, and more research is needed to determine whether the roles of species in land-based habitats depend on their traits in the same way.

"It's easy to use our methods in other systems, and we hope that our study will inspire others to look at other systems in the future" says Anna Eklöf.