the intersection). The immigration rate curve flat-
tens with increasing isolation, and extinction rate
with increasing area, thereby generating a family
of curves providing unique combinations of rich-
ness and turnover for each combination of area
and isolation (Fig. 4.1). Evolution is subsumed in
the model with the assumption that as an island
becomes ever more isolated, new forms are
increasingly likely to arise as a result of in situradi-
ation rather than immigration. This elegantly sim-
ple core model will be termed here the
equilibrium model of island biogeography
(EMIB), although as others have stressed it is
really the dynamic equilibrialnature of the model
that distinguishes it.
At the outset, it is important to emphasize that
The theory of island biogeographycontained a lot
more than this simple model, indeed it was really
just the starting point within the 1967 volume, and
the authors went on to develop a series of increas-
ingly sophisticated models of modifying effects,
and of properties of effective island colonists, end-
ing the book by feeding these ideas into evolu-
tionary models. As we discuss in a later chapter, at
the time of their collaboration, Wilson had already
published seminal papers on character displace-
ment and the taxon cycle. So although it is fair to
criticize the EMIB for its simplicity (which is also
its strength), it is inaccurate to claim, as some
have, that their theorygave no consideration to
speciation and evolution.
MacArthur and Wilson’s ideas have proved
pivotal to debate in island ecology (and conserva-
tion biology) for several reasons. One is that their
theory has focused attention on two of the most
pervasive, fundamental themes of the subject:
●Do ecological systems operate in an essentially
balanced, equilibrial state, or do they exhibit non-
equilibrium modes of behaviour?
●To the extent that they approximate equilibrial behav-
iour, how inherently dynamic are they over time?
These themes can be found in subfields of biogeog-
raphy as widely spaced as diversity theory,78 SPECIES NUMBERS GAMES: THE MACROECOLOGY OF ISLAND BIOTAS
Sfs Sfl Sns Snl
Number of species presentI nearRateTnsTfsI farPE largeEsmallFigure 4.1A version of MacArthur and Wilson’s (1963, 1967) equilibrium model of island biogeography (EMIB), showing how immigration
rates are postulated to vary as a function of distance, and extinction rate as a function of island area. The model predicts different values for S
(species number), which can be read off the ordinate and for turnover rate (T) (i.e.IorE, as they are identical). Each combination of island area
and isolation should produce a unique combination of SandT. To prevent clutter, only two values for Tare shown.