Biology 12

Chapter 14 Population Ecology • MHR 489

The third type of symbiotic relationship is
commensalism, in which one partner benefits and
the other is unaffected. There are few (if any)
examples of true commensalism in nature, since
it is unlikely that one participant in any type of
ecological interaction will be unaffected. Barnacles
that “hitchhike” by attaching themselves to whales
are sometimes considered to be the partner that
benefits in a commensal relationship, while the
whale is thought to be unaffected. However,
because barnacles may slightly decrease a whale’s
ability to find food or escape from predators, they
may actually lower the whale’s reproductive
success or chance of survival (and thus reduce the
density of whale populations). Other often-cited
examples of commensalism are the relationships
between cowbirds or cattle egrets and the cattle
they associate with (see Figure 14.27).

As you might predict, populations that are
regulated by density-dependent factors tend to be
more predictable than those regulated by weather,
fire, and other abiotic mechanisms. Ecologists and
wildlife managers make use of this predictability in
various ways, as demonstrated in the Thinking Lab
on page 490.

Figure 14.27Is this relationship commensal? The birds
benefit because they feed on insects flushed out of the
grass by the cows while they graze. Although the cattle are
mostly unaffected by the presence of the birds, they may
obtain some benefit since the birds sometimes eat ticks
and other parasites they find on the cows’ skin.

Population Regulation:

Many Factors at Work

One way to think about the many factors involved

in the growth and regulation of a population’s size

is shown in Figure 14.28. The line labelled (a)

shows the biotic potential of a population — how it

would grow if it could realize its full potential for

growth. The jagged line labelled (b) shows the

regulating effects that the environment has on the

population’s growth. In this case, abiotic factors

produce the sudden dramatic declines in the

curve, while biotic factors produce less dramatic

declines and hold the population roughly to the

environment’s carrying capacity. Since both the

Figure 14.28The relationship between a population’s biotic

potential and the environmental resistance to its growth

Population size

Time

(a) biotic

potential

(b) environmental

resistance

Forestry and farming depend on a mutualistic relationship

between fungi and plant roots. A fungus’s body consists of

many thread-like extensions, which maximize the amount of

surface area exposed to its food source. This is important

since a fungus absorbs the nutrients it needs over its body

surface. When the threads of certain fungal species become

intertwined with a plant’s roots (these associations of roots

and fungal threads are called mycorrhizae), they help the

plant absorb far more water and nutrients than it could on

its own. The fungus also benefits, getting a small portion

of the photosynthetic products the plant makes with these

materials. One of the problems arising from clearcutting is

that removal of all the trees from an area often causes the

death of the soil fungi. This removal of soil fungi, in turn,

may reduce the ability of newly planted seedlings to survive

and grow.

BIO FACT

Use the Electronic Learning Partner to enhance your

understanding of symbiotic relationships, including

parasitism, mutualism, and commensalism.

ELECTRONIC LEARNING PARTNER