Geoscience Reference
In-Depth Information
such, food or prey items are ranked by profitability and added to the diet as
long as there is an increase in net energy intake. The optimal diet model pro-
vides several useful predictions. If handling times (the time needed to pursue,
capture, and consume) are typically short, the consumer should be a generalist
(use a wide range of foods or prey). On the other hand, if handling times are
long, the consumer should specialize on the most profitable foods. Consider
prey selection by wolves (
Canis lupus
) that are usually in close proximity to
large ungulates, such as moose. The time and energy required to capture a
moose may be considerable. As a result, wolves may specialize on the most
profitable or vulnerable segments of the population (juveniles and older ani-
mals in poor condition). Optimal foraging theory also predicts that a con-
sumer should have a broader diet in an unproductive environment or during
lean periods than in a productive environment or periods of food abundance
(Gray 1987).
Although optimal foraging theory has provided an important platform for
understanding consumer-prey relationships, the successful application of this
theory to understanding diets of free-ranging vertebrates have been limited
(Perry and Pianka 1997). The predictions of this theory are based on a series
of assumptions that may not be justified (Pierce and Ollason 1987; Perry
and Pianka 1997). The first is that the foraging behavior exhibited by present-
day animals was favored by natural selection and continues to enhance the
fitness of animals; the second is that high fitness is achieved by a high rate
of net energy intake (Begon et al. 1996). Numerous field investigations in
the last decade have revealed that diet selection is probably the consequence
of fairly complex interactions of external, internal, and phylogenetic factors
(figure 5.4).
External factors may include prey availability, risk of predation (Lima and
Dill 1990) and social interactions (e.g., competition) (Perry and Pianka 1997).
Internal factors include animal condition or hunger (McNamara and Houston
1984), learned experiences, age, sex and reproductive state, macro- and
micronutrient requirements, and concentration of toxins or distasteful com-
pounds. Phylogenetic factors include morphological constraints (e.g., mouth
shape), sensory limitations, and physiological limitations. With such a com-
plex array factors now known to affect foraging decisions, hindsight is quite
clear: General models will probably fall short in contributing to our under-
standing of foraging patterns. However, recent innovations (e.g., using phylo-
genetic comparative methods) and continued use of manipulative experiments
will undoubtedly advance our ability to identify parameters that are influential
in complex environments.
