Why
do many more species of birds occur on the island of New
Guinea than on the island of Bali? One answer is that New
Guinea has more than fifty times the area of Bali, and
numbers of species ordinarily increase with available space.
This does not, however, explain why the Society Islands
(Tahiti, Moorea, Bora Bora, etc.), which collectively have
about the same area as the islands of the Louisiade
Archipelago off New Guinea, play host to many fewer species,
or why the Hawaiian Islands, ten times the area of the
Louisiades, also have fewer native birds. Two eminent ecologists, the
late Robert MacArthur of Princeton University and E. 0.
Wilson of Harvard, developed a theory of "island
biogeography" to explain such uneven distributions. They
proposed that the number of species on any island reflects a
balance between the rate at which new species colonize it
and the rate at which populations of established species
become extinct. If a new volcanic island were to rise out of
the ocean off the coast of a mainland inhabited by 100
species of birds, some birds would begin to immigrate across
the gap and establish populations on the empty, but
habitable, island. The rate at which these immigrant species
could become established, however, would inevitably decline,
for each species that successfully invaded the island would
diminish by one the pool of possible future invaders (the
same 100 species continue to live on the mainland, but those
which have already become residents of the island can no
longer be classed as potential invaders). Equally, the rate at which
species might become extinct on the island would be related
to the number that have become residents. When an island is
nearly empty, the extinction rate is necessarily low because
few species are available to become extinct. And since the
resources of an island are limited, as the number of
resident species increases, the smaller and more prone to
extinction their individual populations are likely to
become. The rate at which additional species will establish
populations will be high when the island is relatively
empty, and the rate at which resident populations go extinct
will be high when the island is relatively full. Thus, there
must be a point between 0 and 100 species (the number on the
mainland) where the two rates are equal -- where input from
immigration balances output from extinction. That
equilibrium number of species would be expected to remain
constant as long as the factors determining the two rates
did not change. But the exact species present should change
continuously as some species go extinct and others invade
(including some that have previously gone extinct), so that
there is a steady turnover in the composition of the
fauna. That is the essence of the
MacArthur-Wilson equilibrium theory of island biogeography.
How well does it explain what we actually observe in nature?
One famous "test" of the theory was provided in 1883 by a
catastrophic volcanic explosion that devastated the island
of Krakatoa, located between the islands of Sumatra and
Java. The flora and fauna of its remnant and of two adjacent
islands were completely exterminated, yet within 25 years
(1908) thirteen species of birds had recolonized what was
left of the island. By 1919-21 twenty-eight bird species
were present, and by 1932-34, twenty-nine. Between the
explosion and 1934, thirty-four species actually became
established, but five of them went extinct. By 1951-52
thirty-three species were present, and by 1984-85,
thirty-five species. During this half century (1934-1985), a
further fourteen species had become established, and eight
had become extinct. As the theory predicted, the rate of
increase declined as more and more species colonized the
island. In addition, as equilibrium was approached there was
some turnover. The number in the cast remained roughly the
same while the actors gradually changed. The theory predicts other
things, too. For instance, everything else being equal,
distant islands will have lower immigration rates than those
close to a mainland, and equilibrium will occur with fewer
species on distant islands. Close islands will have high
immigration rates and support more species. By similar
reasoning, large islands, with their lower extinction rates,
will have more species than small ones -- again everything
else being equal (which it frequently is not, for larger
islands often have a greater variety of habitats and more
species for that reason). Island biogeographic theory
has been applied to many kinds of problems, including
forecasting faunal changes caused by fragmenting previously
continuous habitat. For instance, in most of the eastern
United States only patches of the once-great deciduous
forest remain, and many species of songbirds are
disappearing from those patches. One reason for the decline
in birds, according to the theory, is that fragmentation
leads to both lower immigration rates (gaps between
fragments are not crossed easily) and higher extinction
rates (less area supports fewer species). Indications of such changes
in species composition during habitat fragmentation were
found in studies conducted between 1953 and 1976 in a
16-acre nature preserve in Connecticut in which a forest was
reestablishing itself. During that period development was
increasing the distance between the preserve and other
woodlands. As the forest grew back, species such as American
Redstarts that live in young forest colonized the area, and
birds such as the Field Sparrow, which prefer open
shrublands, became scarce or disappeared. In spite of the
successional trend toward large trees, however, two bird
species normally found in mature forest suffered population
declines, and five such species went extinct on the reserve.
The extinctions are thought to have resulted from lowering
immigration rates caused by the preserve's increasing
isolation and by competition from six invading species
characteristic of suburban habitats. Long-term studies of a bird
community in an oak wood in Surrey, England, also support
the view that isolation can influence the avifauna of
habitat islands. A rough equilibrium number of 32 breeding
species was found in that community, with a turnover of
three additions and three extinctions annually. It was
projected that if the wood were as thoroughly isolated as an
oceanic island, it would maintain only five species over an
extended period -- two species of tits (same genus as
titmice), a wren, and two thrushes (the English Robin and
Blackbird). Island biogeographic theory
can be a great help in understanding the effects of habitat
fragmentation. It does not, however, address other factors
that can greatly influence which birds reside in a fragment.
Some of these include whether nest-robbing species are
present in such abundance that they could prevent certain
invaders from establishing themselves, whether the fragment
is large enough to contain a territory of the size required
by some members of the pool of potential residents, or
whether other habitat requirements of species in that pool
can be satisfied. To take an extreme example of the latter,
a grass-covered, treeless habitat in California would not be
colonized by Acorn, Nuttall's, Downy, or Hairy Woodpeckers,
even if it were large and all four woodpeckers are found in
adjacent woodlands. Ecological theory is designed to help us
think about the real world, but it is not a substitute for
an intimate knowledge of nature's ways. SEE: The
Decline of Eastern
Songbirds;
Habitat
Selection. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.