out, much of this literature is, in essence, concerned
with competition, chasing the elusive question
(Law and Watkinson 1989) of the extent to which
competing species can coexist. The answer remains
elusive in the island evolutionary context because it
is easier to detect functional changes (particularly
morphological changes) in island forms, than it is to
determine the role of interspecific competition in
relation to other potential causes of such change.
The best chance here is to examine contemporary
processes, which may then be inferred to hold a
wider historical relevance. The greatest method-
ological difficulties come when it is the ‘ghost of
competition past’ that is being sought, i.e. when
researchers are setting out to deduce past events
from current pattern (Law and Watkinson 1989).
Notwithstanding these cautions, if we are to
understand how adaptive radiations of organisms,
such as the Galápagos finches or Hawaiian honey-
creepers have come about on oceanic islands, we
have to be able to build explanatory models that
start with the arrival on a remote island of the first
seed eater, the first nectar feeder, or even the first
land-bird species. The founding population thus
can exploit hitherto largely untapped resources: in
short, empty niche space. As the island fills up with
a mix of further colonists and of neoendemic
species the availability of empty niche space must
fluctuate, eventually declining again as the island
ages and subsides.
Ecological–evolutionary responses to the differ-
ential occupancy on remote islands (as compared
with continents) provide interesting insights, which
we now attempt to review, starting with two gen-
eral responses. First, the phenomenon of ecological
release, i.e. of niche expansion by colonist species,
and second, density compensation, the term given
for the general pattern of increased average density
of island species that goes hand in glove with the
lower richness of island ecosystems.
Ecological release
Ecological releaseoccurs when a species in colo-
nizing an island thus encounters a novel biotic
environment in which particular competitors or
other interacting organisms, such as predators, are
absent. The response can take perhaps two main
forms: (1) loss of ‘unnecessary’ features (e.g. defen-
sive traits, bold patterning), and (2) increase in vari-
ation in features such as beak morphology.
Lomolino (1984a) argues that ecological release
attributable to the absence of predators may be
more widespread than generally recognized and,
conversely, that competitive release may be overes-
timated. He cites examples of Fijian fruit bats of the
genusPteropusbeing more diurnal in the absence of
predatory eagles than those on less isolated Pacific
islands, and of the meadow vole Microtus pennsyl-
vanicusbeing essentially indiscriminant of habitat
type on islands without one of its major predators,
Blarina brevicauda.
Sadly, the loss of defensive traits commonly
involves a lack of fear of, and ability to flee from,
humans and terrestrial vertebrate predators:
undoubtedly a contributing factor to the demise of
island species such as the dodos and the many
species of flightless rails extinguished from islands
across the Pacific (Steadman 1997a). The Solomon
Islands rail, Gallirallus rovianae, is not only flightless
but also exemplifies another tendency: the loss of
bold patterning (Diamond 1991a). The loss of elab-
orate or distinctive morphology and a return to
simpler song types both appear common features
of island endemics, especially where islands pos-
sess only one representative of a genus (Otte 1989).
The general interpretation offered for these changes
is that organisms released from competition with
closely related species no longer require such accu-
rate mating barriers, and so these features are grad-
ually lost.
The second form of release in the absence of close
competitors on an island is to free a colonist from
constraining selective pressure, thereby allowing it
to occupy not only different niches but also a wider
array of niches than the continental ancestral form
(Lack 1969; Cox and Ricklefs 1977). This form of
ecological release, the increase in niche breadth,
forms a key part in the reasoning of the competitive
speciation model we consider later and is an impor-
tant part of many scenarios for island evolution
(e.g. adaptive radiation). Island finches provide
classic examples of evolutionary increases in niche
breadth within lineages. In illustration, granivorousECOLOGICAL RESPONSES TO EMPTY NICHE SPACE 173