Chapter 19

ResourceLimitationof

InsularAnimals:Causes

andConsequences

Gregory H. Adler

OVERVIEW

Populationsof herbivorousanimalsmaybelimitedbymanyfactors,someof whichmaybegroupedaseitherbottom-up
or top-down processes. Bottom-up processes include both quantity and quality of food, while top-down processes
include predation. Ecologists have long been divided over the importance of these two groups of processes in limiting
herbivore populations. Herbivores in tropical forests appear to be at least seasonally limited by bottom-up processes,
as evidenced by seasonal fluctuations in abundance, reproductive activity, and general health, but there is also strong
evidence that populations of such animals are limited by predators. Support for the importance of top-down processes
has been derived from studies of herbivores isolated on islands with few predators, where population densities reach
much higher levels than on adjacent mainland areas. In the absence of strong top-down limitation, herbivore popu-
lations may be limited by food availability, even in seasons of resource abundance, and a trophic cascade may result.
Such cascades may ramify throughout a tropical forest and have implications for plant recruitment and ultimately
forest composition and structure. In this chapter, I outline a series of studies on spiny rats (Proechimys semispinosus)
isolated on small islands in the Panama Canal that either directly or indirectly lend both descriptive and experimental
evidence for the importance of bottom-up and top-down processes in limiting population growth and densities. Insular
populations of spiny rats were censused over a 9-year period to evaluate fluctuations in demography (e.g., population
density, reproductive output, survival, body weight, and age structure). Selected populations also were subjected to
various manipulations to test the importance of food availability in limiting population growth and density. In the
insular setting, spiny rats conform to the general trend of higher densities and reduced reproductive output. I suggest
that spiny rats on the islands largely have been relieved of top-down limitation and instead are more strongly limited
by food availability than their counterparts living within intact forest.

INTRODUCTION

Biologists have expended enormous effort to
identify factors that influence population growth
and ultimately limit population density, either
by density-independent or density-dependent
means. Population limitation includes any factor,
either density independent or density dependent,
that curbs population growth, while popula-
tion regulation includes only those factors that
have a density-dependent effect. In this chapter,

Ispeakprimarilyof populationlimitationbecause,

in most cases, the relationship of limiting fac-

tors to density has not been established. In

the case of herbivores (including frugivores and

granivores), no consensus on the factors that

are most important in limiting density has yet

emerged. Indeed, two major opposing viewpoints

have gained traction as the most likely limiting

factors. One viewpoint argues that bottom-up

processes ultimately limit herbivore populations

(White 1993) and that top-down “community