| Introduction
Studies of foraging often focus on an herbivore's foraging strategy in the
absence of predation. While many foraging models account for time spent foraging and
energy gained from foraging, few assess the effect predation risk can have on foraging
behavior and efficiency. However, predation can have immediate and profound effects on an
animal's survival, it has the potential to significantly alter the animal's daily
activities. The effects of predation risk on foraging strategies and behavior can be
profound, and thus warrant inclusion in evaluations of foraging ecology.
One of the primary theories of foraging
ecology is the optimal foraging theory. This theory predicts that animals will make
foraging decisions to accomplish one of two main goals: time minimization, or energy
maximization. Time minimization implies that animals are striving to use the least amount
of time possible to acquire some given amount of energy. In contrast, energy maximization
implies that animals are striving to gain as much energy as possible in a given amount of
time. Of these two strategies, only time minimization is compatible with predation risk. A
forager may hurry out in the open to grab a nut before it is spied by a predator, but will
not usually try to gather as many nuts as possible within ten minutes because that is when
the next predator will be coming by. However, foragers, regardless of their basic foraging
strategy, will need to alter their foraging decisions to reduce their predation risk. The
questions then become: how do basic foraging behaviors change under predation risk, and
how do these changes affect foraging efficiency?
For this discussion, it is useful to
have an example species that we can refer to when assessing these questions. Bighorn sheep
(Ovis canadensis) are a good candidate because they have been the subject of many studies
involving behavior and predation risk. Bighorn sheep may be subject to a variety of
predators, including cougars (Felis concolor), coyotes (Canis latrans), and golden eagles
(Aquila chrysaetos). In addition, bighorn sheep employ many different tactics to minimize
predation risk. These tactics include habitat selection, vigilance while foraging, and
group size. Because they are subject to a wide variety of situations, bighorn sheep are an
excellent focus species for a study of predation and foraging behavior.
How Might Sheep Respond to Foraging Behavior
Foraging decisions made under the risk
of predation assume that sheep are aware of many components of their surroundings. First,
decision-making assumes that sheep are aware of their physical environment. This includes
the locations of high or low quality forage, the location of areas where there is little
visual obstruction, and the location of terrain where they might easily escape predators.
In addition, decision-making assumes that sheep have some knowledge about the character
and intensity of predation risk in an area. For example, what predators are prevalent in
an area, what are their densities, and what are their hunting tactics? Sheep behavior
should be flexible to dynamic environments and changing predation risks.
Sheep (and other foragers) can make four
types of foraging decisions: when to eat, where to eat, what to eat, and how to eat. Each
of these four types of decisions may be reflected by different behaviors. For example,
sheep may decide to feed during the hottest part of the day (which is more energetically
costly for them) because this is the time of day during which predators are least active.
Alternately, sheep may decide to feed in a patch of less productive forage because it is
closer to good escape terrain and has a less obstructed view of the surrounding area. Most
studies of sheep foraging ecology have focused on the questions of where to eat and how to
eat.
Studies have shown that bighorn sheep
will feed in less productive areas in order to remain closer to steep, rocky "escape
terrain". This is especially pronounced in ewes with young lambs; lambs are
presumably more susceptible to a wider variety of predators, and thus may need extra
protection. In addition, sheep tend to feed in places where they have a relatively
unobstructed view of their surroundings. As a result, they tend to avoid feeding in areas
dominated by tall vegetation, regardless of whether those areas boast higher-quality
forage.
Most studies involving sheep decisions
about how to eat have involved group size and alert behavior. The more time an individual
sheep spends with its head up scanning for predators, the less time it can spend foraging.
However, a sheep must scan for predators to avoid predation. Sheep avoid this catch-22 by
foraging in larger groups. The more sheep there are in a group, the less time each
individual needs to spend scanning for predators. However, because many individuals are
present, there is usually some individual on the look-out.
How Do These Decisions Affect Foraging Efficiency
The decisions to feed closer to escape
terrain or to spend more time looking around for predators can have profound effects on
foraging efficiency. Studies have shown that the most nutritious, energy-rich forage is
often located far from good escape terrain. Similarly, tall vegetation is often located on
fertile soil with good forage. Therefore, the decision to feed far from tall vegetation or
close to good escape terrain involves a trade-off between foraging efficiency and
predation risk. Similarly, the decision to scan the horizon often precludes constant
feeding. The decision to feed in larger groups may mediate this trade-off, but itself may
affect foraging efficiency because of the effects of competition and other group
interactions.
Decisions such as where to feed or what
group size is optimal are likely learned when lambs are young and can observe the movement
patterns and group size decisions of conspecifics. Parts of these same decisions, however,
may also be genetically influenced. The gregariousness of wild sheep, for example, seems
to be an inherited trait. Thus, while they are genetically inclined to move in groups, the
exact group size that an individual sheep considers optimal may be more of a learned
behavior.
While the specific decisions made under
the threat of predation risk may vary among species, many of the effects are the same.
Foragers must make trade-offs between foraging efficiency and security from predation. The
modifications they make to their behaviors have consequences that may affect their
nutrition or their survival. Althought optimal foraging theory makes an excellent starting
point for examining foraging ecology, it does not fully account for many of the processes
that may impact foraging. Examining how predation affects foraging efficiency is both
warranted and necessary if we are to understand the mechanisms behind the decisions
animals make.
Supplemental Reading
Berger, J. 1978. Group size, foraging, and antipredator
ploys: an analysis of bighorn
sheep decisions. J. Mammal. 80:283-289.
Kie, J. G. 1999. Optimal foraging and risk of predation:
effects on behavior and social
structure in ungulates. J. Mammal. 80:1114-1129.
Lima, S. L. and L. M. Dill. 1990. Behavioral decisions made
under the risk of predation:
a review and prospectus. Can. J. Zool. 68:619-640.
Rachlow, J. L. and R. T. Bowyer. 1998. Habitat selection by
Dall's sheep (Ovis dalli):
maternal trade-offs. J. Zool. 245:457-465.
Websites on Bighorn Sheep Ecology
Bighorn Sheep Fact Sheet: http://imnh.isu.edu/digitalatlas/bio/mammal/Hoofed/bish/sheep.htm
The Bighorn Institute:
http://216.74.126.7/~haul/bighorn/default.htm |
