"In the 1980s, an ecologist named Thomas Lovejoy conducted an unusual experiment in Brazil’s Amazon rain forest. As loggers moved in with chain saws to clear trees for cattle pasture north of Manaus, he asked them to leave untouched several small “islands” of forest to see how the animals within them fared.
The results were unsettling. Even in the largest protected forest fragments, 250 acres in size, the number of bird species living beneath the canopy declined by half within 15 years. The smaller isolated populations were far more likely to succumb to disease or climate fluctuations or demographic bad luck than a larger population would have.
That experiment helped establish a grim rule of thumb among biologists: When a species becomes isolated in a small disconnected patch of habitat, unable to breed with larger populations elsewhere, it runs a much higher risk of going extinct locally. And since many of the world’s forests are increasingly fragmented, carved up by roads and farms, it seems inevitable that many species within those remaining patches will soon vanish forever."