Resource Limitation of Insular Animals 331due to a natural recovery from low population
densities or to the provisioning. Apparently, their
densities were so low at the beginning of the
study that even extra food did not allow them
to reach densities comparable to those expected
based on natural fruit availability. Therefore,
those two populations either were not limited
by food or I could not detect such limitation
with my approach because their population den-
sities were so low, but the other six popula-
tions were strongly limited by the availability
of natural fruit. Reproductive activity was very
low in all populations during the provisioning
period, but survival was enhanced. Experimen-
tal populations consequently began each season
of abundant resources at higher densities rela-
tive to controls and to natural fruit availability,
and each of those six populations reached rel-
atively higher peak densities. Thus, food provi-
sioning during the season of resource scarcity
increased density via increased survival rather
than via increased reproductive output during the
provisioning period.
CONSEQUENCES AND GENERAL
IMPLICATIONS
Descriptive and experimental evidence demon-
strates that frugivore and granivore populations
canbefood-limitedinseasonaltropicalforestsand
that bottom-up limitation is important. I suggest
that the role of bottom-up limitation is particu-
larly strong in insular settings where predators
are scarce or absent and frugivore and granivore
populations largely are released from top-down
pressures. I further suggest that attempts to cat-
egorize herbivore populations as being limited
solely by either bottom-up or top-down processes
are likely to fail because both are no doubt impor-
tant, and their relative importance is likely to
fluctuate seasonally and in response to irregular
climatic fluctuations and therefore is context-
specific. Furthermore, populations are likely at
leastpartlyregulatedbydensity-dependentintrin-
sicprocessessuchasrecruitmentandreproductive
output, particularly when released from top-
down limitation. Such regulation is common
within rodent populations (e.g., Wolff 1997) and
would not be unexpected within dense spiny rat
populations.
Bottom-up and top-down processes have impli-
cations far beyond merely limiting population
growth and densities of herbivores.Tropical forest
fragmentation is creating landscapes of insu-
lar patches of forest that lie within a matrix
of less hospitable habitat such as agricultural
areas, pasturage, urban areas, and water from
the construction of hydroelectric projects. Preda-
tor populations are reduced in or eliminated from
many patches, which increases the importance
of bottom-up limitation, thereby unleashing a
trophic cascade, and the resulting cascades have
important implications for plant recruitment and
diversity (Terborghet al. 2001). Thus, in the
absence of predators, herbivores as diverse as leaf-
cutter ants, monkeys, and rodents reach abnor-
mally high densities and greatly influence plant
diversity, species composition, and forest structure
(Terborghet al. 2001).ACKNOWLEDGMENTS
I thank Craig Bernstein, Nicole Casteel, Jorge
Castillo, Mark Endries, Jennifer Freund, Gregory
Hoch, Thomas Lambert, Scott Mangan, Felix
Matias, Andrew Roper, Gregory Ruthig, Jill
Stahlman, DavidTomblin, and PaulTrebe for field
assistance, the Smithsonian Tropical Research
Institute for logistical support, and two anony-
mous reviewers for comments on a previous
draft of the manuscript. Studies described in
this chapter were supported by the US National
Science Foundation, the National Geographic
Society, the Smithsonian Institution, the Smith-
sonian Tropical Research Institute, and the
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