Population Dynamics

• Changes in a population– How do we define a

population?

• Population– Individuals of a

single species– Occupy the same

area– Rely on the same

resources– Affected by same

environmental factors

– Interact and breed

Why does it matter?

• How long does it take bacteria to proliferate into the millions?– E. coli can double in less than 20 minutes

• Can reach many millions in a few hours

If the environment is not ideal

• How does our body affect bacterial growth?– with wbc, release specific

protein, stimulates a hormone that raises body temp

• How do treatments (antibiotics) affect bacterial growth?– Alters or poisons their

environment…halts growth

Why does it matter? …more examples

• Insect pests?– Can wipe out

crops

• Introduced species can destroy an entire ecosystem.

e.g. Caulerpa taxifolia

What about human populations?

• Linear growth– Example--

2,4,6,8,10,12,14,....– Produces a linear graph.– Doubling occurs relatively

slowly.

• Exponential growth– Example--2, 4, 8, 16, 32,

64, 128....– Produces an exponential

(logarithmic) graph--a "J" curve.

– Doubling occurs rapidly.– Populations tend to grow

exponentially, when there are no limits to growth.

Limiting factors…

• produce environmental resistance

• converts a "J" curve to an "S" curve= logistic growth

e.g. space and hunting restricted elephant seals

& sets the carrying capacity (environmt. support)

Population may stabilize if resources are renewable, and growth was not too severe.

Population may crash if resources are non-renewable or if growth outpaces renewal.

Limiting factors

• Density dependent– Factors that limit

growth more severely as the population increases

– Biotic– E.g. food, nutrients,

disease, etc.

• Density independent– Factors that effect

population in a consistent way whether there are few or many individuals.

– Abiotic– E.g. weather, climate,

etc.

What is the human carrying capacity for earth?

Assuming current trends…

10-12 billion

Low range est.– At U.S. dietary

standards....

1.2 billion– At U.S. energy

consumption....

< 1 billion

What is the human carrying capacity for earth?

What is the human carrying capacity for earth?

• High range estimates… 45 billion or more if the following conditions met.

– Cultivating all arable land– Mass conversion to nuclear power & renewable

resources • what do we do with spent fuel?

– Expansion of mining, perhaps to crustal depths of 1 mile.

• These high estimates assume technologies that do not yet exist.

– Some estimate that the carrying capacity could be as much as 157 billion if the population shifted to a grain diet.

World population depends largely on…

• Regional differences– Sweden very stable

• Birth rates– E.g. 28 births per 1000

people

• Death rates– E.g 11 deaths per 1000

people

Today’s rates?Zero population growth (ZPG)

birth rates = death rates