Biology Now, 2e

Zika-Busting Mosquitoes ■ 347

Little research has been done on grow th

curves for mosquito populations, primarily

because the insects are small and airborne,

making them hard to track. But we do know

how the human population has changed:

Over the last 500 years, Earth’s human popu-

lation exhibited both logistic and exponen-

tial grow th. At the end of the last ice age, in

approximately 10,000 bce, there were only

5 million people on Earth. With the advent

of agriculture in about 8000 bce, the world

population began to rise logistically, until

about 200 years ago. Then, alongside the use

of fossil fuels and the industrial revolution,

human population growth exploded exponen-

tially. Modern populations have continued to

show exponential grow th to the detriment

of the environment. Current estimates of

the carrying capacity of Earth range from 2

billion to over 1,000 billion people, with the

majority of studies insisting that 9–10 billion

people is the maximum number that Earth

can support. At current population grow th

rates, we will reach this number by the year

2050 (Figure 19.6).

Seeking Change

With human populations increasing in number

and concentrating in urban areas, we can

expect the continued spread of mosquito-borne

illnesses. Thankfully, recent results from field

Exponential growth

Logistic growth

Mosquito larva

carrying capacity

(K)

Time

Population (size)

Adult mosquito populations

have fewer environmental

constraints, so they display

exponential growth

(because K is much higher).

Mosquito larva populations

are constrained by the

amount of standing water

available, so they show

logistic growth as they

approach K.

5 million people

Logistic growth

Exponential growth

Industrial revolution

0

1

2

3

4

5

6

7

8

9

10

11

160,000

BCE

100,000

BCE

10,000

BCE

7000

BCE

6000

BCE

5000

BCE

4000

BCE

3000

BCE

2000

BCE

1000

BCE 1 CE

1000

CE

2000

CE

1800

CE

2100

CE

Number of people (billions)

2050: 9.7 billion

2005: 6.4 billion

1945 : 2.3 billion

1800 : 1 billion

First modern humans

Figure 19.5

Populations can experience

exponential growth or logistic growth

A population that is not constrained by resources

or by the environment can grow exponentially,

whereas one that is constrained by a set carrying

capacity will show logistic growth. M

Figure 19.6

Curves of logistic and exponential

growth in the world human population

The dashed line indicates the United Nations’

estimated carrying capacity of Earth.

Q1: Which form of population growth

displays a J-shaped curve?

Q2: Which form of population growth

displays an S-shaped curve?

Q3: Describe a situation in which a

population initially shows exponential

growth and later shows logistic growth.

Q1: According to this graph, approximately

when did exponential growth begin?

Q2: What milestone corresponds to the

transition from logistic to exponential

population growth?

Q3: What is the UN’s projected carrying

capacity of Earth, and when will we reach it?