On an island, there are three kinds of seabirds: puffins, eels and Arctic terns  Among them, eel is the main food for Arctic terns and puffins  In the decade from 1980 to 1990, the eel population dropped from 1 million to 500000
Strangely, in this decade, the number of puffins has dropped from 1 million to 600000, while the number of Arctic terns has dropped from 1 million to about 250000
On other nearby islands with similar conditions, the populations of these two species of seabirds are stable