The next element of their theory sketched out, in
mathematical terms, a portrait of the biological
attributes of the superior colonist and how island
environments should ‘select’ for different species
types at early and late stages of the colonization
process. They borrowed from the standard nota-
tion of population models (r, the intrinsic rate of
population increase; K, the carrying capacity), to
describe early colonists as being r-selected—
highly mobile, opportunistic colonizers capable of
rapid growth, maturity and population increase,
and later colonists as K-selected—slower dispers-
ing and growing, but with greater ability to sustain
their populations in resource-limited systems
approaching the environmental carrying capacity.
These ideas influenced Jared Diamond who in his
work on island assembly rules introduced the
termssupertramps,tramps, and sedentary species
for the continuum from highly r-selected (equiva-
lent to pioneer in the succession literature) to K-
selected (late successional) species (see Chapter 5).
Another chapter considered additional effects of
stepping stones, allowing enhanced dispersal
routes and thus altering patterns of biotic exchange
between different biogeographic regions. The later
chapters also incorporated Wilson’s ideas concern-
ing niche shifts and the taxon cycle and radiation
zones. MacArthur and Wilson (1967) thus built up
their analyses from a highly simplified island eco-
logical model, adding increasing evolutionary
detail through the book. In parallel, there is a move-
ment from an essentially stochastic first approxi-
mation to increasingly deterministic ideas. As they
wrote on p. 121, ‘A closer examination of the com-
position and behavior of resident species should
often reveal the causes of exclusion, so that random
processes in colonization need not be invoked.’
Lynch and Johnson (1974, p. 371), in their critical
analysis of problems inherent in testing the EMIB,
argued that for the theory to apply
‘... turnover should reflect the stochastic nature of an
equilibrium condition (i.e. the turnover should not be
attributable to some systematic bias such as ecological
succession, human disturbance of habitats, introduction
of exotic species, etc.)’.
This is a fair assessment in relation to the sim-
plest version of the EMIB, but as we have shown, it
is unnecessarily restrictive in relation to the more
extensive overall body of MacArthur and Wilson’s
theory, in which some structure is acknowledged
and even predicted to occur. This is indicative of a
general problem in assessing MacArthur and
Wilson’s theory: there is scope to define its limits in
rather varied ways. Hence, although individual
components and propositions may be tested and
refuted, when it comes to assessing their legacy in
the round, it is more a matter of assessing whether
their assertion (1967, p. 65) that ‘...only equilib-
rium models are likely to lead to new knowledge
concerning the dynamics of immigration and
extinction...‘ has been borne out, and whether the
research programme they were largely responsible
for initiating continues to pose good questions and
produce interesting insights.4.2 Competing explanations for systematic variation in island species–area relationships
Attempts to evaluate and build on MacArthur and
Wilson’s theory were initially concerned, first, with
the form of ISARs, and second, with the occurrence
of the equilibrial dynamic at the heart of the theory
(Gilbert 1980; Williamson 1988). In the following
sections we consider both aspects. In the dynamic
model (Fig. 4.1), both area and isolation are pre-
dicted to influence island richness, but typically the
analytical approach has been to compute the ISARs
for islands of varying isolation and to compare
their slopes and intersects. Thus, the standard
approach to the problem has been to treat area as
the primary explanatory variable.
MacArthur and Wilson’s EMIB is not the only
explanatory model competing to explain patterns
of variation in island species richness (e.g. Connor
and McCoy 1979; Kelly et al. 1989). Here we list
some alternative ideas that have been proposed
either as alternatives to or as modifiers of the EMIB:
some being of potentially wide application, others
of more limited focus.●Random placement. If individuals are distributed
at random, larger samples will contain more species.
An island can be regarded as a sample of suchSYSTEMATIC VARIATION IN ISLAND SPECIES–AREA RELATIONSHIPS 87