Biology 12

488 MHR • Unit 5 Population Dynamics

project in the Yukon, led by Charles Krebs of the

University of British Columbia, suggests that both

predation and fluctuations in food supply contribute

to the cycling of hare populations.

Symbiotic Relationships

In some cases, the interactions that occur between

members of two different species are so close that

they are said to have a symbiotic relationship.

One member of the pair in such an association

is referred to as the hostand the other as the

symbiont. The host is typically larger and more

independent (that is, it can live on its own) than

the symbiont (which often depends on the host

and the symbiotic relationship for its survival).

There are three general types of symbiotic

relationships. In parasitism, the symbiont is

referred to as a parasite; it benefits from the

relationship, whereas the host is harmed. In terms

of population growth, this relationship is much

like that between a predator and its prey. An

increase in host population density makes it

possible for the parasites to increase in number.

The increase in the number of parasites decreases

Figure 14.25In a laboratory, populations of an Adzuki bean

weevil (Callosobruchus chinensis) and its wasp parasitoid

(Heterospilus prosopidis) cycled for 112 generations or six

years (until the experiment ended). In this case the wasp is

called a parasitoid because it feeds on living tissue (like a

parasite), but kills the larvae of its host, in the process

making it like a predator in some ways.

host survival or reproductive ability, and may

subsequently lead to a decrease in host density.

With fewer hosts available, parasite density then

declines. The result may be cyclical fluctuations of

host and parasite population densities, as shown in

Figure 14.25.

A second type of symbiotic relationship is

mutualism, in which both partners benefit from the

relationship and growth in one population typically

causes an increase in population density of the

associated population. This is shown in Table 14.1,

which summarizes the effects of different symbiotic

relationships on population growth. In this table,

a plus sign (+) indicates an increase in population

density and a minus sign (−) indicates a decrease

in density. In mutualism, for example, both species

benefit from the relationship; positive growth in

one population causes positive growth in the other.

Mutualism is common in nature (as shown in

Figure 14.26). For example, termites and various

mammalian herbivores (such as cattle and sheep)

depend on certain types of bacteria living in their

digestive systems to help them digest the plant

material they eat. The bacteria also benefit, since

they are able to live in a protected, nutrient-rich

environment.

Figure 14.26Examples of mutualism include the

relationships that exist between many types of plant and

animal species. In this case, the animal gets food (pollen or

nectar) while the plant gets help moving its pollen from plant

to plant for reproduction.

Nature of relationship

between populations

Effect of growth in one population

on the other population

competitive

predator/prey

(includes herbivore/plant)

host/parasite

mutualistic

• / – (both are negatively affected)

• / – (one population gains at the
  expense of the other)

• / + (one population gains at the
  expense of the other)

• / + (both are positively affected)

Table 14.1

Interspecific interactions

90

50

10 20 30 40

60 70 80

100 110

400

800

400

800

400

800

Population density

Number of generations

host

parasite

host

host

parasite

parasite