More and more forms of life are becoming extinct, as people destroy habitats, hunt animals and introduce pests. Individual extinctions are often just the start of the problems facing an ecosystem under threat. The elimination of one species can cause disturbances in an ecosystem that could lead to further species being wiped out. Now, mathematicians have developed techniques to predict these flow-on effects, and suggest ways of saving these fragile ecosystems.

Scientists have investigated many cases where the removal of one species causes another species to fail. For example, in the north-western Atlantic Ocean, the overfishing of large sharks meant that they were no longer eating as many cownose rays. The population of these rays skyrocketed, and as a result, they ate more scallops, driving the scallops almost to extinction.

Eliminating one species can unbalance an entire ecosystem, which can cause another extinction. This new extinction can cause even more extinctions, leading to a cascade that can decimate an ecosystem, until it finds a new stable state.

Researchers from Northwestern University in the United States have now developed techniques that can help rangers to reduce the impacts of these cascades, and in some cases, prevent them entirely. The researchers begin with a food web, showing which species rely on each other for food. Food webs are an example of an important mathematical idea called a network. By analysing the links between the species, they can identify which species are the most critical to keep in an ecosystem, and which threaten other species. They can then use the network model to test different intervention strategies that could help reduce the effect of an extinction.

It can be a lot easier and faster to change an ecosystem by removing species, such as by hunting animals, than it is to replace individuals. And with the help of mathematicians and networks, removing some species can stabilise an ecosystem, and hopefully cause less destruction than simply letting the ecosystem sort itself out.

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