147
factors at play in island ecosystems
and, in 1967, they outlined a new
theory of island biogeography.
Their theory proposed that each
island reflected a balance between
the rate of new species arriving
there and the rate at which existing
species become extinct. For example,
a habitable but relatively empty
island would have a low extinction
rate since there are fewer species
to become extinct. When more
species arrive, competition for
limited resources increases. At a
certain point, smaller populations
will be outcompeted, and the rate
of species extinction will rise. An
equilibrium point occurs when the
species immigration rate and the
rate of those becoming extinct are
equal; this may remain constant
until a change occurs in either rate.
The theory also proposes that
the rate of immigration depends
on the distance from the mainland,
or another island, and declines with
increased distance. The area of an
island is a further factor. The larger
it is, the lower its rate of extinction,See also: Evolution by natural selection 24–31 ■ Predator-prey equations 44–49
■ Field experiments 54–55 ■ The ecosystem 134 –137ECOSYSTEMS
Island 1Island 3
Island 2MainlandAn island’s size and distance from the mainland both
affect its species richness. Islands closer to the mainland
will receive more random dispersion of organisms; the larger
island gets the most, the island furthest away gets the least.Random dispersal of organisms to islands
because if native species are
pushed out of prime habitat by new
immigrants, they have a better
chance of finding an alternative,
albeit imperfect (“suboptimal”)
habitat. Larger islands are also
likely to have a greater variety of
habitats or microhabitats in which
to accommodate new immigrants.
A combination of variety and lower
rates of extinction produces a
greater species mix than on a small
island—the “species-area effect.”
The actual species in the mix will
change over time, as a result of
colonization and extinction, but will
remain relatively diverse.Monitoring mangroves
In 1969, Wilson and his student
Daniel Simberloff conducted a field
experiment that tested the theory
on six small mangrove islands in
the Florida Keys in the US. They
recorded the species living there,
then fumigated the mangroves to
remove all the arthropods, such as
insects, spiders, and crustaceans.
In each of the next two years, ❯❯Robert H. MacArthur
Born in Toronto, Canada, in
1930, and later relocating to
Vermont in the US, Robert
MacArthur originally studied
mathematics. In 1957, he
received his Ph.D. from Yale
University for his thesis
exploring ecological niches
occupied by warbler species
in conifer forests. MacArthur’s
emphasis on the importance
of testing hypotheses helped
transform ecology from an
exclusively observational
field to one that employed
experimental models as well.
This methodology is reflected
in The Theory of Island
Biogeography, which he
coauthored with Edward O.
Wilson. MacArthur received
awards throughout his career,
and was elected to the
National Academy of Sciences
in 1969. In 1972, he died of
renal cancer. The Ecological
Society of America awards a
biennial prize in his name.Key works1967 The Theory of Island
Biogeography
1971 Geographical Ecology:
Patterns in the Distribution
of SpeciesUS_144-149_Island_biogeography.indd 147 12/11/18 6:25 PM