Conservation biology and unhealthy changes

David S. Wilcove and Martin Wikelski lament the decline in the numbers of migratory animals.

Those of us living in eastern North America can no longer experience the flocks of millions of passenger pigeons that temporarily obscured the sun as they migrated to and from their breeding grounds. Nor can residents of the Great Plains climb to the top of a hill and gaze down up hundreds of thousands of bison trekking across the prairies, as was possible less than two centuries ago.

If fewer animals is a cause for sorrow you might think that more animals, or other living things, would be a cause for celebration. But no.

Several studies have shown that birds reduce insect populations in temperate forests, thus raising the question of whether ongoing declines in migratory birds pose a threat to the health of our forests and farmlands.

Why is bird abundance good but insect abundance bad? Is it because insects eat vegetation? No. Eating vegetation is good:

Similarly, one wonders how the ecology of the Serengeti would change if its migratory population of wildebeest (exceeding 1 million individuals) were to collapse, given the major role these animals surely play in terms of consuming herbaceous vegetation and redistributing nutrients via their urine and dung.

So, in the end it is not "declines" that worry Wilcove and Wikelski, but changes. Bad luck. We live in a changing world.