Biology Now, 2e

346 ■ CHAPTER 19 Growth of Populations

ECOLOGY

M

osquitoes cause more human suffering than any other organism

(with the exception of our own species). Here are just a few of the

deadly or debilitating diseases they carry:

● Dengue virus is a huge problem for the population of Brazil. Spread

by the same mosquitoes as Zika, dengue also has no vaccine

or cure, and it causes a disease chillingly known as “breakbone

fever.” Symptoms include continuous joint pain, vomiting, high

fever, and severe rashes. In 2015,

Brazil chalked up a record 1.6

million cases and 863 deaths to

dengue—an 82.5 percent increase

over 2014.

● Malaria, an ancient disease that

has been traced as far back as

2700 BCE, affects an estimated

40 percent of the world’s

population. Hundreds of thousands of deaths are still reported

annually from malaria.

● Yellow fever, against which there is a vaccine, still leads to an

estimated 30,000 deaths per year.

● West Nile virus, which emerged in the 1930s, can invade the

nervous system and cause death.

● Chikungunya is found mainly in Africa and Asia, but cases have been

reported in Florida. The main symptoms are fever, rash, and joint pain

that can last for years. Only one in 1,000 cases results in death.

Mosquito-Borne Diseases

reproductive rate is about 100 years. In stark

contrast, the population doubling time for

mosquitoes can be as little as 30 days in an optimal

environment.

Reaching Capacity

Now back to mosquitoes, and how to get rid of

them. Scientists argue strongly that understanding

mosquito population ecology is a prerequisite for

eliminating mosquito-borne diseases. For exam-

ple, many mosquitoes lay their eggs directly on

the surface of water. That knowledge led to one of

the most important public health efforts to reduce

mosquito-borne diseases: the removal of pools of

standing water. Eliminating these breeding sites

makes it possible to reduce the carrying capacity

of the environment, the maximum population size

that can be sustained in a given area.

In most species, the growth rate of a popu-

lation decreases as the population size nears

the carrying capacity, because resources such

as food and water begin to run out. If there are

only a few locations of standing water in a given

area, for example, a limited number of eggs can

be laid in that area, thereby limiting the size of

the population. Any limiting resource needed

for survival, such as habitat, food, or water, will

determine an environment’s carrying capac-

ity for a specific population. Because different

species have unique needs, the same environ-

ment can have different carrying capacities for

different resident species. For the same reason,

two different environments in which the same

species lives will have different carrying capac-

ities. At the carrying capacity, the population

growth rate is zero.

If a population has no constraints on its

resources, it will experience exponential growth,

which occurs when a population increases by a

constant proportion over a constant time interval,

such as one year. Exponential growth is repre-

sented by a J-shaped growth curve (Figure 19.5).

A population that approaches its carrying capac-

ity because of constrained resources will expe-

rience logistic growth, in which the population

grows nearly exponentially at first but then stabi-

lizes at the maximum population size that can

be supported indefinitely by the environment.

Logistic growth is represented by an S-shaped

growth curve.

isolated in 1947 in the Zika Forest of Uganda,

for which it is named. The first human cases of

Zika were detected in Uganda in 1952, and the

little-studied virus subsequently spread across

central Asia, the South Pacific, and into South

and Central America and the Caribbean.

Humans are no strangers to mosquito-borne

diseases (see “Mosquito-Borne Diseases”). While

Zika and other diseases were once isolated to

specific areas of the globe, global travel and

human population growth, along with climate

change, have increased mosquitoes’ ability to

proliferate and spread disease on a grand scale.

In the second half of the twentieth century, our

planet saw the fastest rate of human popula-

tion growth in its history. Population doubling

time—the time it takes a population to double

in size—is a good measure of how fast a popula-

tion is growing. For example, the doubling time

for the U.S. human population at our current