0198566123.pdf

Area is the devil’s own variable.

(Anon.)

Coastal islands are notorious for their accumulation, at all
seasons, of a staggering variety of migrant, stray, and sex-
ually inadequate laggard birds. In the absence of specific
information on reproductive activity, it is therefore
unwarranted to assume tacitly that a bird species, even if
observed in the breeding season, is resident.

(Lynch and Johnson 1974, p. 372)

Having established the biogeographical signifi-
cance of islands, we now move on to the theories
that have been developed from their study. We
take our lead from Robert H. MacArthur and
Edward O. Wilson’s (1967) classic monograph The
theory of island biogeography, by working from
a consideration of ecological patterns and
processes, through island evolutionary processes
to the emergent evolutionary outcomes.
In this first ecologicalchapter, we focus on a
research programme within ecological biogeogra-
phy in which MacArthur and Wilson provided the
seminal influence. This programme centres around
species numbers as the principal response variable,
and is concerned with: the nature of species–area
relationships; the factors determining the number
of species found on an island; and the rates of
species turnover in isolates. MacArthur and
Wilson’s (1963, 1967) contribution represented a
bold attempt to reformulate (island) biogeography
around fundamental processes and principles,
relating distributional pattern to population ecolog-
ical processes, and at its heart lay a very simple, but
powerful concept. Their dynamic, equilibrium
model postulates that the number of species of a

given taxon found on an island will be the product

of opposing forces leading respectively to the gain

and loss of species and resulting in a continual

turnover of the species present on each island

through time (Box 4.1). Although first proposed as

a zoogeographic model (MacArthur and Wilson

1963), the model was promoted in their 1967 mono-

graph as a general model covering all taxa and all

forms of island.

Their theory was developed in large measure to

account for apparent regularities in the form of

species–area relationships, and invokes varying

rates of species gain and loss as a function of isola-

tion and area respectively. This they captured in

the famous graphical model, in which immigration

rate declines exponentially and extinction rate rises

exponentially as an initially empty island fills up

towards its equilibrial richness value (shown by

77

CHAPTER 4

Species numbers games: the

macroecology of island biotas

Box 4.1 The first formal statement of the

dynamic equilibrium model

‘We start with the statement that

∆s=M+G–D

where sis the number of species on an island.

Mis the number of species successfully

immigrating to the island per year.

Gis the number of new species being added

per year by local speciation (not including

immigrant species that mainly diverge to

species level without multiplying), and

Dis the number of species dying out per year.’

From MacArthur and Wilson (1963, p. 378)