350 ■ CHAPTER 19 Growth of Populations
ECOLOGY
when populations are held in check by factors unre-
lated to population density. Density-independent
factors can prevent populations from reaching
high densities in the first place. Year-to-year varia-
tion in weather, for example, may cause conditions
to be suitable for rapid population growth. On the
other hand, poor weather conditions may reduce
the growth of a population directly (by freezing the
eggs of a mosquito, for example) or indirectly (such
as by decreasing the number of food plants avail-
able to an animal).
Other natural disturbances, such as fires and
floods, can also limit the growth of populations
in a density-independent way. Finally, the effects
of environmental pollutants, such as the insecti-
cide DDT, are density-independent; such pollut-
ants can threaten natural populations with
extinction (Figure 19.10).Populations of many species rise and fall
unpredictably over time. These irregular
fluctuations in population size are far more
common in nature than a smooth rise to a
stable population size. In the 1950s, for exam-
ple, Brazil mounted a massive antimosquito
campaign to combat yellow fever—a disease
also transmitted by Aedes aegypti—that
included spraying insecticides and encourag-
ing citizens to get rid of standing water. The
success of the program led officials to declare
in 1958 that Aedes aegypti had been eradi-
cated. If only that had been true. By the 1970s
the mosquitoes had come roaring back, breed-
ing and spreading like crazy—an unwelcome
irregular fluctuation. And now the pests are
resistant to many chemical attacks.
Populations can also exhibit cyclical fluc-
tuations, predictable patterns that occur with
seasonal changes in temperature and precipita-
tion or when at least one of two species is strongly
influenced by the other. The Canadian lynx, for
example, depends on the snowshoe hare for food,
so lynx populations increase when hare popula-
tions rise, and they decrease when hare popu-
lations drop. In this example, the population
cycles are also density-dependent population
changes because each population is affected
by the other’s numbers. They cycle together in
response to each other’s density (Figure 19.11).Friendly Fight
The Oxitec GM mosquitoes—trademarked
“Friendly” Aedes aegypti—were first released in
Brazil in February 2011, under the direction of
biochemist Margareth Capurro of the Univer-
sity of São Paulo. In a densely populated suburb
in northeastern Brazil, the released mosquitoes
reduced the wild population of Aedes aegypti by
85 percent, says Capurro. In 2013, a similar test in
the village of Mandacaru resulted in a 96 percent
reduction of the wild mosquito population in the
area after only 6 months.
“For the first time, we demonstrated that
transgenic mosquitoes can work,” says Capurro,
who is now developing alternate GM mosquito
lines in her own laboratory, attempting to make
the GM insects even more potent against their
wild counterparts. For example, Capurro’s lab is
trying out different genetic mutations to createIn 2010, there were
at least 10,000
breeding pairs in
the lower 48 states.DDT was
banned in 1972.YearBald eagle pairs10,000
9,000
8,000
7,000
6,000
5,000
4,000
3,000
2,000
1,000
0
1960 1970 1980 1990 2000 2010
Figure 19.10
Banning the use of the pesticide DDT removed a density-
independent population limit
DDT poisoning was directly responsible for declining eagle populations by
the middle of the twentieth century. By the early 1960s, population counts
indicated that only 417 breeding pairs of bald eagles remained in the lower
48 states—a huge drop from the estimated 100,000 breeding pairs present in
- Bald eagle populations increased dramatically after DDT was banned.
Q1: In what year did the bald eagle population rise to more than 2,000
breeding pairs?Q2: Give some examples of possible density-dependent limits on bald
eagle populations.Q3: Is the population growth of bald eagles more like logistic or
exponential growth? Explain why you think so.