Population Ecology

I. General InfoA. A population includes all the members of the

same species that live in one place at one timeB. Population density is the # of

individuals/unit area (ex. 180 tree frogs in 9 km2 = ?)

C. Factors Affecting Population Size1. Deaths2. Births3. Immigration – movement of individuals into

an area4. Emigration – movement of individuals out

of an area

Birth Immigration

EmigrationDeath

Populationsize

II. Population GrowthA. Organisms will reproduce and

populations will increase when:1. Abundant space and food is available2. Protected from predators and disease3. Adequate mates

B. Two Types of Population Growth:

1. Exponential Growtha. Occurs under ideal conditions with

unlimited resources ex:b. Constant doubling of the populationc. Not sustainable WHY?

Growth of Aphids

Exponential growth

Steady population

size

Peak populationsize

Rapid decline

Steady population

size

2. Logistic Growtha. As resources become scarce, a

population’s growth slows or stopsWhere do you see this on the graph?

Nu

mb

er

of

Yeast

Cel l

s

Time (hours)

Carrying capacity

b. When an organism is introduced into a new environment it takes a while to establish itself (lag phase)

c. Once the population is established, growth accelerates.

d. When the population approaches the carrying capacity, various biotic and abiotic factors keep the population in balance.

e. Carrying capacity is the maximum number of individuals an ecosystem can support (Draw this graph)

Carrying Capacity

Lag phase

*

PopulationGrowth

can be

represented bycharacterized by characterized by represented by

which cause a

Exponentialgrowth

Logisticgrowth

Fallinggrowth rate

S-shapedcurve

Limits ongrowth

No limits on growth

J-shapedcurve

Constantgrowth rate

Unlimitedresources

III. Limits to Growth

A. Limiting factors are ecological factors that cause population growth to decrease

B. Two Types of Factors:1. Density Dependent Factors

a. Operate strongly on large, high density populations

b. Include competition for limited resources, predation, crowding, parasitism and disease

c. Oh Deer! Lab

60

50

40

30

20

10

01955 1960 1965 1970 1975 1980 1985 1990 1995

2000

1600

1200

800

400

0

2400

Moose Wolves

2. Density Independent Factorsa. Affect all populations regardless of size b. Include human activities and natural

disasters like fire, volcanic eruptions, severe storms, drought, or other climate conditions.

IV. Human Population Growth

A. Background Info1. Earth is approximately 4.6 billion years

old2. Modern humans have only been on

Earth ~150,000-200,000 yrs3. For most of human history, limiting

factors kept population levels low (scarce food, disease, etc.); high death rates and high birth rates

4. In the last 500 years, improved agriculture, medicine, and sanitation increased population growth

Agriculturebegins

Plowingand irrigation

Bubonicplague

IndustrialRevolutionbegins

B. Future Population Growth1. Current human population is ~7 billion

2. Overpopulation = when the # of people in an area exceeds the resources and capacity of the environment necessary to sustain human activities

3. Scientists estimate that the Earth’s carrying capacity for humans is 7.7-12 billion people

4. More people = more competition for resources more human conflicts

5. If the growth rate does not slow down, resources will be depleted, the environment will be degraded and people’s standard of living will be greatly reduced