310 Jo hn Terborg hand Kennet hFeeley
resulted in the distortion of many biological
interactions. Since vertebrate and invertebrate
consumers interact with plants as pollinators,
seed dispersers, herbivores, etc., distortions in the
structure of the animal community can propa-
gate to the producer level via multiple pathways.
As we investigated some of these pathways, it
became clear that the strength of the differ-
ent pathways was highly dependent on spatial
scale in a discontinuous fashion as key species
(pollinators, dispersers, predators, etc.) entered or
dropped out of the system. We shall now present
a synopsis of the evidence that led us to these
conclusions.
METHODS
Physical setting
Lago Guri is located in east-central Venezuela
on the lower Caroní River near its confluence
with the Orinoco River. The dam was completed
in 1986 and raised the water to 270 m above
sea level from a base of 120 m, inundating an
area of 4300 km^2 (Morales and Gorzula 1986).
Due to the hilly terrain, inundation resulted in
the transformation of hundreds of forested hill-
tops into isolated land-bridge islands (<0.1 ha to
1000 ha; Alvarezet al. 1986).
Lago Guri and a broad watershed protection
zone surrounding it are administered by EDELCA
(ElectrificacióndelCaroní),theVenezuelanenergy
company that operates the hydroelectric station.
Access to the lake is strictly controlled and no
hunting is allowed, though some illegal poaching
still occurs, primarily for capybara.
Mean annual rainfall at the dam is 1100 mm,
but is somewhat higher 60–80 km to the south
where we conducted our research. The habitat of
all study islands was tropical dry forest supporting
50–70 species of trees per hectare (Peetz 2001,
Feeleyet al. 2005). Much of the canopy is facul-
tatively deciduous, such that many crowns keep
their leaves in wet years but may lose them for
several months during dry years (Huber 1986).
Our research built upon two salient fea-
tures of the Guri experiment: the availability of
replicate islands in each of three discrete size
classes – small (S), medium (M), and large (L) –
and the high degree of consistency in animal
community composition within each size class
(Table 18.1). Thus, islands were not assigned to
classes on the basis of areaper se, but by the con-
sistency with which they retained less complete
to more complete animal communities (Terborgh
et al. 1997a). Nearly all of the study islands
had gentle topography formed of well-drained,
clay-rich, oxisols (three islands were rocky and
relatively steep).Animal communitiesOur research began in 1993, some 7 years after
the lake level reached its final stage. We sur-
mise that non-volant species inhabiting the Guri
landscape prior to inundation were forced to
swim or migrate upslope as the water level rose,
thereby becoming concentrated on hilltops that
eventually became islands. After 7 years, most
individual animals that survived the inundation
would have died, except for long-lived species
such as monkeys and tortoises.The populations of
short-lived species either became self-sustaining
or died out soon after isolation. This inference
is supported by the fact that three-quarters or
more of all vertebrate species present on the
mainland were already absent from S and M
islands when we undertook the first surveys in
1993 (Terborghet al. 1997a,b). Although popu-
lations of some species subsequently disappeared
from some islands (Feeley 2005a), most non-
volant species recorded on the S and M islands in
1993 were still present in 2003 when the project
ended. Thus, community composition remained
reasonably consistent throughout the period of
the research.
Small islands ranged in size from 0.25 to
2.5 ha and supported predators of inver-
tebrates (birds, lizards, amphibians, spiders,
etc.), pollinators (bees, wasps, lepidoptera, hum-
mingbirds), seed predators (rodents), and gen-
eralist herbivores (common iguanas, Iguana
iguana; red howler monkeys, Alouatta senicu-
lus– several islands; leaf-cutter ants,Attaspp.,
Acromyrmexsp.), but few, if any, seed dispersers
and no predators of vertebrates.