Chapter 6Population and Community Ecology

Nature exists at several levels of complexity

Natural Selection

Evolution

Ecosystem, Community, Population, Species, Individual??•A forest that includes maple, beech

and hemlock trees.•A bear population preying on a fish

population in a river with high sunlight

•A single blue jay sitting on a telephone wire.

•A pride of lions that lives in a particular region of Africa.

Community

Individual

Population

Ecosystem

Population Ecology• Populations are dynamic• Difference is between inputs and outputs• Why is population ecology important?

Factors that Regulate Population Abundance and Distribution• Population size (N)- the total number of individuals within a defined area at a given time.

• In 1987, California Condor had N = 22• Population ecologists caught all 22, in 2009, N = 300

Factors that Regulate Population Abundance and Distribution

• Population density- the number of individuals per unit area at a given time.

• Helps scientists figure out if species is rare or abundant

• Management zones – typically more hunting/fishing licenses for area with high pop. density

Factors that Regulate Population Abundance and Distribution

• Population distribution- how individuals are distributed with respect to one another.

• Random – no pattern

• Uniform – common in territorial organisms, toxic plants

• Clumped – schooling fish, flocking birds, herd animals

Factors that Regulate Population Abundance and Distribution

• Population sex ratio- the ratio of males to females- Typically 50/50- Helps predict number of offspring in the next

generation

Factors that Regulate Population Abundance and Distribution

• Population age structure- how many individuals fit into particular age categories.- Helps predict how rapidly a population can

grow.

Factors that Influence Population Size

•Density-dependent factors- the size of the population will influence an individual’s probability of survival. - Examples: amount of available food

Factors that Influence Population Size•Gause’s experiment demonstrates

how food supply controls population growth•Limiting resources- Examples: food, waternest sites, nitrogen, phosphorus

•Carrying capacity (K)

Factors that Influence Population Size

•Density-independent factors- the size of the population has no effect on the individual’s probability of survival. - Example: tornado, hurricanes, floods,

fires