Population Dynamics

Population Pyramid USA 2000

Population Pyramids -

Kenya

Human Population DynamicsWorld Population Prospects: The 1998 Revision (UN)

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1750 1800 1850 1900 1950 1998 2050

Year

Population (x1000,000)

Oceania

North America

Lat.Am & Car

Europe

Asia

Africa

Population Indicators

Population Size Infant Mortality Growth Rate Life ExpectancyCrude birth rate Age StructureCrude death rate Doubling TimeFertility Access to contraceptives

Per capita spending on health careHIV incidenceNumber of RefugeesFemale LiteracyPercent of population in urban areas

Exponential Population Growth• occurs when resources are not limiting

• causes population size to increase faster & faster with time

• cannot occur forever because eventually some factor limits population growth

The Nature of Population Growth

• How do populations grow? What do you have to know?– Reproduction rate– Food supply – Habitat requirements

• Example: Eagles since the end of DDT – 413 nesting pairs in 1968, over 8,000 nesting pairs today in lower 48 states

POPULATION DYNAMICS

• Some Population Growth Terms:– Exponential Growth - Growth as a percentage

of the whole.– Biotic Potential - Potential of a population to

grow in the absence of expansion limitations.– Carrying Capacity - Number of individuals

that can be indefinitely supported in a given area without environmental degradation

The population change equation

ttEttIttDttBtPttP Δ−Δ+Δ−Δ+=Δ+ )()()()()()(

WhereP(t) is population at time tB(t) is birth rate (births/time)D(t) is death rate (deaths/time)I(t) is immigration rate, and E(t) is emigration rate.

(Note: this is a “difference equation”.)

The Mathematics of Population Growth

Ignoring emigration and immigration, population is only affected by the number of births and deaths:

ttDttBtPttP Δ−Δ+=Δ+ )()()()(

For those comfortable with calculus, this can be transformed into a differential equation:

)()(= tDtBdt

dP-

General Exponential Growth

• If you solve the difference (or differential) equation from the previous slide, you get the exponential growth equation:

tdbt ePP )(

0−=

Exponential Growth Curve

Doubling Time

• When you use this equation to calculate the doubling time, Td , you get:

Note: r = b-d, and is the net growth rate

rTd

2ln=

Using Doubling Time

The annual growth rate in India is 1.3 % annually, while the annual growth rate in China is 0.7% annually. How long will each country take to double its population if current growth rates continue?

rTd

2ln=

(r) Strategies (K) Strategies• Short life• Rapid growth• Pioneers, colonizers• Early maturity• Little parental care.• Little investment in

individual offspring.

• Long life• Slower growth• Late maturity• Fewer large

offspring.• High parental care

and protection.• High investment in

individual offspring.

Growth to a Stable Population

• Logistic Growth - Growth slows as the population approaches carrying capacity.

Limits to Growth

Boom and Bust Cycles

• Exponential growth is graphed as a J curve.

– Carrying Capacity - Number of individuals that can be indefinitely supported in a given area.without environmental degradation

• Overshoot - When a population surpasses the carrying capacity of its environment.

–Dieback

–Oscillations

Population Oscillations

Natural Boom and Bust Cycles

Human fertility: 1975 top,

2005 bottom

Three Viewpoints of Population Growth

• People are the “ultimate resource” and no evidence suggests that population growth is a problem (Julius Simon)

• Human population growth is increasing exponentially while food production is not, leading ultimately to starvation, crime, and misery (Thomas Malthus)

• “There is enough for everyone’s need, but not enough for anyone’s greed” (Mohandas Gandhi) The problem is inequality in distribution of wealth and resources, and in use of resources per person

• Improve social, educational, and economic status of women

• Improve status of children• Accept choice as a valid element in life in general and family planning in particular

• Improve social security and political stability• Improve knowledge of, and access to safe, effective birth control

• Encourage development of national population policies

How can we reduce population growth?

Women’s education and fertility

Leading AIDS rates• Botswana 38.8%

• Zimbabwe 33.7%

• Swaziland 33.4%

• Namibia 22.5%

• Zambia 21.5%

• South Africa 20.1%

• Lesotho 18.1%

Population profile of Botswana