106 SPECIES NUMBERS GAMES: THE MACROECOLOGY OF ISLAND BIOTAS
important than competition in determining extinc-
tion probabilities (Whittaker 1992).
So, where does all this leave us in evaluating
turnover on islands? The statement in Box 4.1 is of
course a truism: the number of species on an
island at a given point in time has to be a function
of the number previously recorded and the num-
bers gained and lost in the interim, but the extent
to which these rates behave according to the
expectations of the EMIB remains hard to assess.
Although we began in this chapter by discussing
some of the core macroecological relationships
within ecological systems, by focusing on species
turnover, we have inevitably been drawn into dis-
cussing aspects of composition in the latter part of
the chapter. We go on in the next chapter to look
more formally at theories concerned with compo-
sitional pattern, and then pull together both sets of
themes in the final chapter of this section of the
book (Chapter 6).
4.5 Summary
Ecological systems have emergent statistical proper-
ties, detectable for example in regularities of form of
the abundance and number of species, and which
point to the existence of some fundamental govern-
ing ecological rules or processes. The study of sys-
tems in this way is termed macroecology. Island
macroecology began long before the term was
coined, in the early part of the twentieth century. It
gained its core focus with the publication in 1967 of
MacArthur and Wilson’s equilibrium model of
island biogeography (EMIB), which postulates that
species number is the dynamic product of opposing
rates of immigration and local extinction, as inverse
functions respectively of isolation and area.
In this chapter, we first review the building
blocks that lead to the formulation of the EMIB and
then consider how it has fared alongside subse-
quent theoretical developments. Evaluation largely
falls under two heads: spatial patterns and tempo-
ral patterns. We distinguish between several
closely interrelated properties of ecological
systems: species abundance distributions, species–
area curves, species accumulation curves and
island species–area relationships (ISARs). Variation
in the form of ISARs was a central focus of
MacArthur and Wilson’s theory, and a substantial
literature has since developed, demonstrating
varying degrees of fit with the EMIB—implying
that at the least a degree of modification is
required. Variation in form may for instance be
attributable to different variables depending on the
magnitude of variation in independent variables
(area, isolation, elevation, habitat diversity, etc)
and their interaction. Yet, there is also the prospect
that combined area-isolation-climate models can
be developed that can account for the first order
pattern of island species richness globally, at least
for particular taxa.
The EMIB asserts that island richness patterns
are dynamic outcomes—the product of perpetual
turnover in membership and that once at
equilibrium this process continues essentially
unabated. On this score, the evidence appears at
best equivocal, in part because of the enormous
problems involved in accurately quantifying
immigration and extinction rates and thus
measuring turnover. Consideration of evidence
on turnover draws us into discussion of composi-
tional structure in the data, which receives much
fuller consideration in the next chapter on island
assembly theory.