Population Ecology

What is a population?

• All the members of a species living in the same place at the same time.

• Organisms usually breed with members of their own population.

How to describe populations• Density: number of individuals per unit

area, or volume. Ex. Number of deer/km2.

• Dispersion: The distribution or arrangement of individuals in space.

• Dispersion may be clumped, even or random.

Growth Rate

• A change in the size of a population over a given period of time = growth rate.

Growth rate = births – deaths

• Populations usually stay about the same size from year to year as factors kill many individuals before they can reproduce.

How fast can a population grow?• Reproductive potential: The maximum

number of offspring that each member of a population can produce.

• Some species have much higher reproductive potential than others.

How many bunnies?

• Rabbits can begin reproducing at 4 - 6 months• Gestation is 31 days• Litters can be up to 14• Females can become pregnant within hours of

giving birth• Lifespan is about 7 years• 95 billion rabbits in 7 years!

Exponential growth

• Occurs only when there is enough:

FoodSpaceLittle or no competitionNo predators

• Populations rarely grow at their reproductive potential

Carrying Capacity

• Carrying capacity is: the maximum population that an ecosystem can support indefinitely.

• Ecosystems change so carrying capacity is difficult to predict or calculate precisely.

Resource Limits

• A species reaches its carrying capacity when it consumes a natural resource at the same rate at which the ecosystem produces the resource.

• This resource is called the limiting resource.

• Examples include: light, nutrients, water.

Density Dependent Population Regulation• Deaths occur more

quickly in a crowded population.

• Limited resources, predation and disease result in higher death rates in dense populations.

• Ex. Emerald Ash Borer possibly going to invade Iowa

Density independent

• A certain proportion of a population dies regardless of population density. Ex. Severe weather