CHAPTER
14Reflecting
Questions
Population Ecology
464
In 1988, a species of mollusc never
seen before in North America was
discovered in Lake St. Clair (located
on the river connecting lakes Huron
and Erie). This species was the zebra
mussel (Dreissena polymorpha). A
few individuals of this species had
probably been carried into the lake
a few years earlier, in the ballast of
a ship from the Caspian Sea. Since
then, the zebra mussel has spread
throughout the Great Lakes and into
various rivers and lakes in eastern
Canada and the United States.
Populations of these tiny mussels
become so dense that they can cause
enormous damage to the ecosystems
they invade. As larvae, zebra mussels
attach themselves to hard surfaces,
including the shells of various species
of North American molluscs. The
rapid growth of zebra mussels not
only kills many individuals of these
other mollusc species, but also fills
up their habitat. Efficient consumers
of plankton, zebra mussels have
significantly reduced the food supply
for fish, such as the lake whitefish
(Coregonus clupeaformis). Since few
animals feed on zebra mussels, their
biomass contributes little to the next
higher trophic level. But at the same
time, zebra mussels use up a
considerable amount of energy
contained in the lower levels.
Zebra mussels have also had a
substantial economic impact on
humans whose living is dependent on
healthy fish populations. In addition,
mussels clog water pipes of cities and
towns and encrust boats and docks.What is it about the zebra mussel
that made it possible for its population
to grow so quickly in North America?
Did the same rapid growth occur in
the population of rats imported to
Easter Island? In this chapter, you will
learn about factors that contribute to
the growth or decline of populations.
Why do some grow faster than others?
Why do some appear to remain stable,
while others fluctuate wildly (growing
very large and then “crashing” to a
few individuals)? These questions
interest population ecologists. The
answers are vital to the conservation
of endangered species and the
sustainability of ecosystems.Individual zebra mussels reach only
4 cm in size, but grow in such high
density that they can completely
blanket an area in a short period
of time.How do ecologists
determine the size of
a population?
What factors determine
how fast a population
will grow?
What makes some
populations more
vulnerable to decline
than others?Prerequisite
Concepts
and Skills
Before you begin this chapter,
review the following concepts
and skills:
using examples, explain
the process of adaptation
of organisms to their
environment (Chapter 11,
section 11.1),
using the ecological
hierarchy of living things,
evaluate how a change in
one population can affect
an entire hierarchy
(Chapter 13, section 13.2),
and
using examples of the
energy pyramid, explain
production, distribution,
and consumption of food
resources (Chapter 13,
section 13.3).