Resource Limitation of Insular Animals 327months, and availability of resources is entrained
by environmental conditions that prevail across
wide areas, despite differing floristic composition.
Mammalian predators almost never visit the
study islands, and avian predators such as owls
are transient and usually absent for long peri-
ods of time (Adler 1996). The only predators of
spiny rats that are regularly present on the islands
are snakes, but their low metabolic demands no
doubt render them much less important than
mammalian and avian predators in top-down lim-
itation, as in temperate regions (e.g., Lin and
Batzli 1995). Insular populations consequently
demonstrate a pronounced island syndrome, with
densities sometimes reaching at least an order
of magnitude greater than on adjacent mainland
areas(Adler1996).Bodysizeisgreateronislands,
and the largest individual ever recorded (720 g)
was captured on one of the long-term study
islands. Reproductive output also is often greatly
reduced on islands, where the number of births
per adult female in a year is less than 3 (Adler
1996), compared with a maximum reproductive
capacity of 11 births per adult female per year in
mainlandpopulations(Fleming1971).Thebreed-
ing season may be as short as 2 months in a year
(Adler and Beatty 1997) but is often continuous
in mainland populations (Fleming 1971). Most
insular populations show a period of reproductive
quiescence each year, but that period often varies
among islands in timing and duration (Adler and
Beatty 1997).
Anecdotal evidence for resource
limitation
Anecdotalobservationsof spinyratsontheislands
strongly suggest that they are seasonally food-
stressed. As fruiting activity subsides towards the
end of the rainy season and beginning of the
dry season, spiny rats lose weight rapidly (rou-
tinely as much as 25% in 1 month), reproductive
activity ceases, and the vitality of individuals
declines noticeably. Mite infestations and fur loss
are substantial, and many individuals are reduced
to skeletal, depilated ghosts of their former robust
states (personal observations). Once fruit begins
falling again in large quantities in the mid- to late
dry season, the physical decline is reversed, and
individuals rapidly regain their lost weight and fur
and begin breeding.
Statistical patterns further indicate that spiny
rats on the islands are seasonally food-stressed.
Time series analysis reveals a negative relation-
shipbetweenspinyrattrappability(theproportion
of individuals known to be present that are cap-
tured during a monthly census) and the density
of fruiting trees and lianas (Adler and Lambert
1997). Thus, spiny rats more readily enter baited
traps when fruit availability is lower. Mean annual
population density on small islands is positively
related to the mean annual density of fruiting
trees and lianas (Figure 19.1; Adler and Beatty
1997). There is no relationship between the den-
sity of spiny rats and any single species of tree that
produces large crops of edible fruit (Adler 2000),
indicating that spiny rats are not dependent on
a single species of fruiting tree. Similarly, there
is no relationship between spiny rat density and
groups of species of such trees (e.g., palms), with
one notable exception.
The density of spiny rats is positively related
to the density of all species of fig trees (Ficus
spp.). This relationship may be biologically rel-
evant because of the fruiting phenology of fig
trees. These trees reproduce asynchronously, and
individuals can produce figs at any time of the
year, even during the season of resource scarcity
when few or no other species of trees are fruiting.
Spiny rats also avidly consume figs (Adler 1995).
Interestingly, not only spiny rat density but also
demographyingeneralcloselymirrorsfigtreeden-
sity and species composition on the islands (Adler
2000). Thus, islands with higher densities of fig
trees, particularly those species that frequently
produce large figs, had higher densities of spiny
rats,bettersurvivalrates,andlongerbreedingsea-
sons. Fig trees long have been known to provide
important resources for a wide variety of ani-
mal consumers (Janzen 1979). More recently, figs
have been considered to be keystone plant species
for animal consumers and of conservation con-
cern because of the large area occupied by single
breeding units (Nasonet al. 1998).
Resource abundance appears to be a major
force in influencing both reproduction and den-
sityandindrivingpopulationtrajectoriesthrough