Biology Now, 2e

Zika-Busting Mosquitoes ■ 349

For some organisms, if a population exceeds

the carrying capacity of its environment by

depleting its resources, it may damage that envi-

ronment so badly that the carrying capacity is

lowered for a long time. A drop in the carrying

capacity means that the habitat cannot support

as many individuals as it once could. Such habi-

tat deterioration may result in widespread star-

vation and death, causing the population to

decrease rapidly (Figure 19.9).

Not all population changes are due to density.

Density-independent population change occurs

often), and predators may pose a greater risk

(because most predators prefer to hunt abun-

dant sources of food). Disease and preda-

tors obviously increase the death rate. These

changes are also density-dependent. A 2014

study found that the mosquito Culex pipiens,

which spreads the West Nile virus, undergoes

strong density-dependent population changes

arising from competition for resources within

the watery habitats where it lays its eggs. In

other words, larvae compete with each other

for food. But once they become adults, the

population is no longer density-dependent,

because the mosquitoes become airborne and

have wider access to food.

Figure 19.8

Overcrowded conditions result in

density-dependent population change

Overcrowding affects many species. The plantain,

shown here, is a small, herbaceous plant that has

decreased reproduction in crowded conditions.

Q1: What factors may be limiting growth

and reproduction in the plantain’s crowded

conditions?

Q2: Why are overcrowded conditions

considered density-dependent population

changes?

Q3: Relate this example of overcrowded

conditions to the human population growth

shown in Figure 19.6. How do you think

the situations are similar? How are they

different?

Figure 19.9

Habitat destruction results in density-

dependent population change

Q1: In what year did the reindeer’s numbers

begin to rise exponentially?

Q2: In what years was the reindeer’s

population growth logistic?

Q3: How do you predict the graph

(population size) would change if someone

had begun bringing in supplemental food for

the reindeer in 1940? Draw a sketch of your

prediction.

1,000

10,000

100

0

0 10 100

Seeds planted per square meter

Average number of seedsper reproducing individual

The number of seeds

produced per plant

drops dramatically

under increasingly

crowded conditions.

By 1950, only 8

reindeer remained.

(^1) In 1911, 25 reindeer
were introduced to
St. Paul Island off
the coast of Alaska.
The population
rose rapidly to
more than 2,000
individuals.

4

500

1,000

1,500

2,000

0

1915 1925 1935 1945

Year

Number of reindeer

2

The population

crashed

because the

reindeer heavily

overgrazed their

winter food

source, lowering

the carrying

capacity of the

environment.

3