12 Consumer–resource dynamics
In this chapter we explore those things an animal needs to eat to survive and
reproduce: resources. This leads to a description of the structure and dynamics of
consumer–resource systems, where both the consumers and their resources may
interact in complex ways. We show how to analyze such systems by breaking them
down into their dynamic components. This approach is used to compare several
different systems: kangaroos and plants in Australia, trees, moose, and wolves in
North America, small mammals in northern Europe, and snowshoe hares and lynx
in Canada.A resourceis defined as something that an animal needs, whose consumption by one
individual makes the resource unavailable to another individual. The most obvious
example is food, and to that may be added shelter, water, or nesting sites. By
definition, a resource is beneficial. As the availability of the resources rises, the fecun-
dity and probability of survival of an individual is enhanced.
Food resources are often characterized by two attributes: the amount of food
available to an animal and the suitability of that food to the animal’s requirements.
For example, quality may be described as the percentage of digestible protein in the
food, whereas quantity may be measured as dry mass of food per hectare. This often
leads to a discussion on whether quality or quantity of the food is the most import-
ant to the animal. In most cases the distinction is meaningless. It indicates that the
resource is being measured in the wrong units. If the resource is in fact digestible
protein, then that is what should be measured. The availability of the resource should
be expressed as dry weight of digestible protein per hectare. Its measurement may
entail measuring dry weight of herbage as an intermediate step, but that does not
make herbage the resource.It is necessary at this stage to give a classification of resources because the inter-
action between the resources and the animals that depend upon them can take several
forms. These in turn influence the dynamics of the population in different ways.
The use of a resource may be pre-emptive. An example is the use of nesting holes
by parrots. Individuals are either winners or losers. On the other hand the use of a
resource may be consumptive. All individuals have access to the resource and each
individual’s use of it reduces the level of the resource available to other individuals.
An example is the use of plants by herbivores. We see that both pre-emptive and
consumptive use of a resource removes a component of the resource from use by
other individuals. Consumptive use removes the component permanently whereas
pre-emptive use removes it temporarily.196