326 Gregory H. Adler
for four nights each month, and all captured
spiny rats were uniquely marked for individual
identification. Standard data were recorded for
each rat upon first capture each month (Adler
1994). We assumed that each rat captured on
an island was born on that island, and we esti-
matedthemonthof birthof allcapturedratsbased
on growth curves of individuals captured shortly
after birth (Adler 1994).
Wealsoconductedmonthlycensusesof fruiting
trees and lianas on each island to search for
patterns in the relationships between spiny rat
demographyandresourceabundance.Tofacilitate
these censuses, we marked, measured, and identi-
fied all trees≥10 cm in diameter at breast height
(Adler 2000), and censuses were conducted each
month by walking the entirety of each island and
recording all trees and lianas that were produc-
ing ripe fruit. We included in the censuses only
those trees and lianas whose fruits and seeds
are consumed by spiny rats, based upon feeding
trials of captive individuals (Adler 1995). For ani-
mals that occupy small home ranges (generally
<0.2 ha in the case of spiny rats; Endries and
Adler 2005), censuses that include all fruiting
individuals within an animal’s home range are
more accurate indicators of fruit availability than
sampling along transects or using fruit traps
(Chapmanet al. 1994).
For the remainder of this chapter, I will sum-
marize those studies that are relevant to resource
limitation of spiny rats in an insular setting.
First, I will address descriptive studies that provide
the framework for implicating resources in lim-
iting spiny rat populations on the islands. These
studies rely solely on the monthly censuses of
spiny rats and fruit production. I will conclude
by describing two studies that experimentally test
the role of resources in limiting populations of
the spiny rat. These studies rely on the monthly
censuses of spiny rats and fruit production and
food-provisioning experiments.
Natural history of spiny rats
Proechimys semispinosus is a large echimyid
rodent that is distributed widely throughout low-
land tropical forests from southern Honduras to
northwestern South America (Woods 1993).This
rodent is sufficiently generalized that it is able to
live in many types of forests, including dry and
wet or primary and secondary forests (Gonzalez
and Alberico 1993, Tomblin and Adler 1998,
Adler 2000). Demography varies not only tem-
porally but also spatially, and this demographic
flexibility presumably promotes persistence in het-
erogeneous environments (Adler 1996). Spiny
rats reach their greatest abundance in seasonally
dry secondary forests and are associated statisti-
cally with treefall gaps (Lambert and Adler 2000).
Mean adult male body weight is approximately
350g,withexceptionalmalesreachingover700g
(Adler 1996, 2000, Adleret al. 1998).
This rodent is primarily frugivorous and graniv-
orous and consumes a wide variety of fruits and
seeds (Adler 1995). Mycorrhizal fungi also consti-
tute an important component of its diet (Mangan
and Adler 1999, 2002). Spiny rats actively con-
sume subterranean sporocarps of such fungi even
when fruits and seeds are abundant. Provision-
ing spiny rats with food during the season of fruit
scarcity does not reduce the importance of fungi
in their diet, and spiny rats apparently consume
the fungi according to their availability in the soil
(Mangan and Adler 2002).The spiny rat is strictly
terrestrial (Seamon and Adler 1999, Lambert and
Adler 2000) and therefore is able to consume only
subterranean (hypogeous) sporocarps of mycor-
rhizal fungi and those fruits and seeds that fall to
the ground.
In both mainland and insular forests,
Proechimys semispinosus demonstrates seasonal
fluctuations in density, reproductive output, and
recruitment (Fleming 1971, Gliwicz 1984, Adler
1994, Adler and Beatty 1997), but isolated pop-
ulations frequently are asynchronous in their
fluctuations, despite the similar seasonal climatic
changes (Adler 1994). This asynchrony presum-
ably reflects differences in floristic composition
among isolated forest patches, which fosters dif-
ferences in overall fruiting phenology and conse-
quentdifferencesinfoodavailability(Adler1994).
Such asynchrony contrasts sharply with syn-
chrony often observed with temperate rodent pop-
ulations over large spatial scales (e.g., Van Horne
1981, Adler 1987). In temperate forests, virtu-
ally all plant reproduction occurs during warm