Chapter 4

Populations

Properties of PopulationsPopulation: a group of organisms of 1

species in the same area1) Population Size (usually

estimated)2) Population Density = how

crowded a popltn. isExpressed as # of individual. per area or

volume

Properties of Populations (con’t)

3) Dispersion = the spatial distribution of individuals3 types:A) Even (due to social interactions)B) Clumped (due to resource availability)C) Random (due to seed dispersal by wind or

birds)

Population DynamicsReflects changes in size and composition of

popltn.Birth Rate: # births in 1 period of time

US = ~4.2 million/yearDeath Rate: # deaths in 1 period of time

US = ~2.5 million/yearLife Expectancy: average length of life

US = 72 (male); 79 (female)

Population Dynamics (con’t)Age Structure: distribution of individuals

of different ages in a popltn.As % young increases, growth rate increases

Survivorship Curves: show likelihood of survival at different ages throughout lifetime

Population Growth RateGrowth Rate: the amount by which a

popltn. changes over timeCalculated by:

Growth Rate = Birth rate – Death RateDepends on:

1) birth rate2) death rate3) immigration (moving in)4) emigration (moving out)

Exponential GrowthDescribes a popltn. that grows rapidly after

only a few generationsLarger popltn. = faster growthIn reality, resources will run outLimiting Factor: a factor that restrains

popltn. growth

Logistic GrowthDescribes a realistic popltn. (birth/death

rates vary with popltn. size)Accounts for limiting factorsCarrying Capacity: the maximum # of

individuals the environment can support over time

Population RegulationFactors in place to help control popltn. size2 Types:

1) Density-dependant: depend on popltn. densityi.e. availability of resources

2) Density-independent: popltn. density does not matteri.e. weather, floods, etc.