shelter in the fragments. But gradually as time went on, both the number
and the variety of birds in the fragments started to drop. And then it kept
on dropping.
“In other words,” Cohn-Haft said, “there wasn’t just suddenly this
new equilibrium with fewer species. There was this steady degradation in
the diversity over time.” And what went for birds went for other groups
as well.
ISLANDS—WE are talking about real islands now, rather than “islands” of
habitat—tend to be species-poor, or, to use the term of art, depauperate.
This is true of volcanic islands situated in the middle of the ocean, and it is
also, more intriguingly, true of so-called land-bridge islands that are
located close to shore. Researchers who have studied land-bridge islands,
which are created by fluctuating sea levels, have consistently found that
they are less diverse than the continents they once were part of.
Why is this so? Why should diversity drop off with isolation? For some
species, the answer seems pretty straightforward: the slice of the habitat
they’ve been marooned on is inadequate. A big cat that requires a range of
forty square miles isn’t likely to make it for long in an area of only twenty
square miles. A tiny frog that lays its eggs in a pond and feeds on a hillside
needs both a pond and a hillside to survive.
But if a lack of suitable habitat were the only issue, land-bridge islands
should pretty quickly stabilize at a new, lower level of diversity. Yet they
don’t. They keep on bleeding species—a process that’s known by the
surprisingly sunny term “relaxation.” On some land-bridge islands that
were created by rising sea levels at the end of the Pleistocene, it’s been
estimated that full relaxation took thousands of years; on others, the
process may still be going on.
Ecologists account for relaxation by observing that life is random.
Smaller areas harbor smaller populations, and smaller populations are
more vulnerable to chance. To use an extreme example, an island might
be home to a single breeding pair of birds of species X. One year, the pair’s