What is a population?

• A set of individuals within a species living in the same place at the same time.

• They form a reproductive group.

Properties of Populations

• Described in terms of size, density, and dispersion.

• Size: The actual number of individuals

• Density: The number of individuals in a specified amount of space.

• Dispersion: How the individuals are spread out over the area.

Each species has its own expected population size, density, and dispersion.

• We must understand an organism's natural history in order to understand if its population is healthy.

Growth Rate

• Calculated as a percentage change in a given amount of time.

• Births - Deaths = Change in Population (doesn't give a lot of information)

• Calculating rates rather than crude numbers provides more specific information about that particular population.

• Can be positive, negative, or zero

How fast can a population grow? (Biotic Potential)

• Limited by the maximum number of offspring that each female in the population can produce. (Reproductive potential)

What can affect reproductive potential?

• Amount of offspring produced at one time. (turtles vs. deer)

• Frequency (pandas vs. carp)

• Timing (mice vs. elephants)

• Viability of offspring

• Length of time females reproduce

Reproducing early shortens generation time.

• The average time it takes a member of a population to reach reproductive age

Small organisms generally have shorter generation times than larger animals.

• 20 minutes old for some bacteria!

Exponential population growth

• Population grows faster and faster over time (sometimes doubling each generation)

• Only happens when there are plenty of resources available, and little to no competition or predators.

What limits population growth?

• Carrying Capacity (a theoretical population)

• Resource Limits

• Competition

Density-Dependent Factors

• Occur more quickly in a crowded population

• Limited resources, predation, disease

Density-Independent Factors

• A certain proportion of the population dies regardless of the population's density.

• Affects all members fairly evenly

• Unpredictable, and generally results in more fluctuation in population size.