Resource Limitation of Insular Animals 323regulation” is relatively rare in complex terrestrial
ecosystems (Polis and Strong 1996). According
to this viewpoint, such populations are limited
by the quantity and quality of food from pri-
mary producers. Thus, not only is the quantity
of available plant material important but also the
extent to which plants are protected mechanically
and chemically.The second viewpoint argues that
herbivorepopulationsarelimitedprimarilybytop-
down processes (Hairstonet al. 1960, Terborgh
1988). Thus, predators ultimately limit herbi-
vore populations (either in density-independent
or density-dependent ways), with bottom-up pro-
cesses playing only a secondary role because
population densities are depressed by predators
and virtually never at the level where food would
become limiting.
INSULAR ENVIRONMENTS
Herbivores isolated on islands frequently show
consistent differences when compared with con-
specifics on the adjacent mainland. In the case
of mammals, population densities are higher and
often more stable on islands (e.g., Adler and
Levins 1994). Individuals often have better short-
term survival probabilities and longer lifespans,
reduced aggressive behavior, and reduced repro-
ductive output (Adler and Levins 1994). Body size
tends to increase in the case of small mammals
and to decrease in the case of large mammals (the
islandrule;VanValen1973,Lomolino1985).The
entire suite of population- and individual-level
changes following isolation has been called the
island syndrome (Adler and Levins 1994). Such
differences often occur very rapidly (i.e., within a
single generation), greatly preceding speciation,
and are likely due initially to reaction norms
(the entire complement of phenotypes expressed
by a single genotype across a complete range of
environmental conditions) rather than to imme-
diate genetic changes (Adler and Levins 1994).
However, initial genetic changes also could occur
concomitantly through the founder effect and
geneticdrift.Thus,inthenovelenvironmentalset-
ting of higher densities, body size increases and
reproductiveoutputdecreases,evenintheabsence
of immediate genetic changes. Over longer time
frames, however, genetic changes, driven by direc-
tional selection, may occur that provide better
adaptation to the novel insular setting (Adler and
Levins 1994).
The cause of the island syndrome is unknown,
but two major hypotheses have been offered
(reviewed by Adler and Levins 1994). First, pre-
dation is greatly reduced on most islands, and
the increased densities are the result of release
from top-down limitation. Because top-down lim-
itation is absent or at least reduced, herbivore
populations are limited by food and intrinsic regu-
latory (density-dependent) mechanisms. Second,
interspecific competitors are often absent from
islands, and the higher island densities result
from competitive release and greater resource
availability for the species that remain. Thus, the
bottom-up versus top-down controversy can be
examined in an insular setting in which there are
fewer predators and competitors. In either case,
individual-level changes (e.g., increased survival
probabilities, increased body size, and reduced
reproductive output) might be expected to occur
in response to increased densities and greater
crowding, and bottom-up limitation, perhaps in
concert with intrinsic regulatory mechanisms
(e.g., density-dependent recruitment and repro-
ductive output), should assume primary impor-
tance for herbivores.TROPICAL ENVIRONMENTS
Throughout most of the tropics, seasonal fluctua-
tions in rainfall and concomitant fluctuations in
other climatic variables such as irradiance, rel-
ative humidity, and soil water potentials impose
a seasonal rhythm on the activities of organ-
isms living within the tropics. Thus, tropical
organisms in both mainland and insular settings
generally show seasonal changes in reproduc-
tive output, behavior, and vitality in response
to seasonal fluctuations in resource abundance
(Fleming 1971, 1992, Bonaccorso 1979, Glanz
et al. 1982, Milton 1982, Russell 1982, Gliwicz
1984,Hentry1994,AdlerandBeatty1997).Sea-
sonal fluctuations also occur with great regularity
(just as temperate regions exhibit seasonal regu-
larity), albeit with some variability among years,