Trophic cascades
A trophic cascade is the “progression of indirect effects by
predators across successively lower trophic levels” (Estes et al.
2001). In terrestrial ecosystems, top-down and bottom-up
effects can occur simultaneously, although their relative
strength varies, and interactions among trophic levels can be
complex. Here we study top-down processes and associated
trophic interactions that potentially have broad ecosystem effects.
Although our main purpose is to explore nonlethal effects
on ecosystems, we first describe several studies that
emphasize the importance of cascading lethal effects.
Predators obviously can influence the size of prey species
populations through direct mortality, which, in turn, can influence
total foraging pressure on specific plant species or entire
plant communities. For example, at the continental scale,
Messier (1994) examined 27 studies of wolf–moose (Alces alces)
interactions and generally found that wolf predation
limited moose numbers to low densities (< 0.1 to 1.3 moose
per square kilometer [km2
], excluding Isle Royale studies),
which resulted in low browsing levels in northern North
America, especially in areas where wolves and bears both
prey on moose. Comparing total deer (family Cervidae) biomass
in areas of North America with and without wolves, Crête
(1999) suggested that the extirpation of wolves and other
predators has resulted in unprecedentedly high browsing
pressure on plants in areas of the continent where wolves have
disappeared.
On a smaller scale, islands provide settings for studying
predator–prey population dynamics. For example, McLaren
and Peterson (1994) studied relationships between wolves,
moose, and balsam fir (Abies balsamea) in the food chain on
Michigan’s Isle Royale. As a result of suppression by moose
herbivory, young balsam fir on Isle Royale showed depressed
growth rates when wolves were rare and moose densities
were high. McLaren and Peterson concluded that the Isle
Royale food chain appeared to be dominated by top-down
control in which predation determined herbivore density
through direct mortality and hence affected plant growth
rates. Terborgh and colleagues (2001) studied forested hilltops
in Venezuela that were isolated by the impounded water of a
large reservoir. When predators disappeared from the
islands, the number of herbivores increased, and the reproduction
of canopy trees was suppressed because of increased
herbivory in a manner consistent with a top-down theory. On
the islands without predators, Terborgh and colleagues found
few species of saplings represented because of a lack of recruitment,
even though many more species of trees made up
the overstory.
Changes in prey behavior due to the presence of predators
are referred to as nonlethal effects or predation risk effects
(Lima 1998). These behavioral changes reflect the need for herbivores
to balance demands for food and safety, as described
by optimal foraging theory (MacArthur and Pianka 1966).
They include changes in herbivores’ use of space (habitat
preferences, foraging patterns within a given habitat, or
both) caused by fear of predation (Lima and Dill 1990). Such
behaviorally mediated trophic cascades set the foundation for
an “ecology of fear” concept (Brown et al. 1999) and provide
the basis for this study. Ecologists are now beginning to appreciate
how predators can affect prey species’ behavior,
which in turn can influence other elements of the food web
and produce effects of the same order of magnitude as those
resulting from changes in predator or prey populations
(Werner and Peacor 2003). Interestingly, Schmitz and
colleagues (1997) indicate that the effects of predators on the
behavior of prey species may be more important than direct
mortality in shaping patterns of herbivory.
Predation risk can also have population consequences for
prey by increasing mortality, according to the “predationsensitive
food” hypothesis (Sinclair and Arcese 1995). This
hypothesis states that predation risk and forage availability
jointly limit prey population size, because as food becomes
more limiting, prey take greater risks to forage and are more
likely to be killed by predators as they occupy riskier sites.
Wolves have been largely absent from most of the United States
for many decades; hence, little information exists on how adaptive
shifts in ungulate behavior caused by the absence or
presence of wolves might be reflected in the composition
and structure of plant communities.