compatibility rules, i.e. that a certain degree of niche
difference is necessary to enable coexistence. After
these various effects were considered, it was appar-
ent that some combinations of species were more
frequent and others less frequent than expected,
suggesting combination rules. In total, seven dis-
tinct assembly rules were derived empirically on the
basis primarily of the distributional data. The inter-
pretations of these patterns rested on the assump-
tion that through competition the component
species are selected and co-adjusted in their niches
and abundances, thereby ‘fitting’ with each other to
form relatively species-stable communities.
Diamond’s analyses drew pointed criticism from
Simberloff and colleagues, who in a twin-pronged
attack questioned both the primacy given to com-
petition and the extent to which the distributional
patterns departed from a random expectation in the
first place. On the first count, Diamond and Gilpin
responded by emphasizing the extent to which
factors other than competition (e.g. predation,
dispersal, habitat controls, and chance) were in fact
integral to the theory. On the second count, they
contested the basis for the null models, demon-
strating that the findings depend heavily on the
biological assumptions that particular authors
build in to their null models.
The debate has continued and reappeared in
other guises. Competition is undoubtedly a force in
shaping island biotas but it is extremely difficult to
distinguish the precise extent of its effects.
Subsequent studies using similar types of distrib-
utional ‘tools’ (e.g. incidence functions) have
demonstrated several common features, often find-
ing a role for island area, but also involving a range
of habitat determinants. Some convincing evidence
of a different form of assembly rule invoking
competitive effects is provided by morphological
overdispersion in assemblages of exotic species
introduced to oceanic islands. These patterns have
been reported for several groups of oceanic islands
and for both passeriform and galliform bird
groups.
Nested distributions are where smaller faunas
(or floras) constitute subsets of the species found in
all richer systems. Nestedness is attributable vari-
ously to ‘passive sampling’, nestedness of habitat,
differential immigration (frequently using the
surrogate of island isolation), and differential
extinction (area). Statistical tests of the degree of
nestedness have been developed and applied to
many island data sets. Nestedness turns out to be a
common feature of insular biotas, varying in
importance depending on the nature of the islands
and the taxon being considered.
Island recolonization constitutes in essence just a
special case of ecological succession. This is illus-
trated by reference to the recolonization of
Krakatau. In this natural experiment, dispersal
attributes, hierarchical relationships across trophic
levels, and habitat changes are each seen as signifi-
cant in structuring island species assembly.
Turnover from the species lists can be attributed
largely to a combination of succession, habitat loss,
in a few cases predation, and to the comings and
goings of ‘ephemerals’, i.e. it is heterogeneous in
nature. For plants, as might be expected from first
principles, it is the rare species that most frequently
fail. The fit with equilibrium theory appears poor:
while some taxa or ‘guilds’ may have reached an
asymptote, overall the system has yet to equilibrate,
if indeed it is destined ever to do so.144 COMMUNITY ASSEMBLY AND DYNAMICS