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
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