A team of scientists visits an island in the Caribbean Sea. They find a diverse community of turtles, lizards, birds, and a variety of trees, shrubs, and grasses. No humans live on the island. Several years later, the scientists return to the island. They find a much smaller community, with reduced populations of all of the species they had identified earlier. Some of the animals now appear to be absent from the island. Which is the most likely explanation for the changes that the scientists observed?