21.2 Studying populations page 711

21.2 Studying populations page 711• Key concepts: How do ecologists

determine the size of a population? What causes populations to change in size? What factors limit population growth? • Key terms: Estimate, birth rate, death

rate, immigration, emigration, population density, limiting factor, carrying capacity

Determining Population Size• Some methods of determining population size are direct and indirect

observations, sampling, and mark-and-recapture studies.

• Direct observation – count all of its members• Indirect observation – counting nests, observing what the organisms

use or leave behind• Sampling – count the number of organisms in a small area and

multiply to estimate (approximate) actual population size

Mark-and-recapture• Scientists mark the

organisms, release them, then capture a bunch and see how many have marks and how many don’t. You can use this to estimate the number of organisms in the population.

Changes in population size• Populations change in size

when organisms join the population or when they leave. • Births and deaths• Birth rate: number of births per

population in a certain amount of time• Death rate: “” deaths “” • The MAIN way individuals leave

is by dying

Population statement• If the birth rate is > death

rate, then more organisms are coming in than going out, and population increases. • If the death rate > than birth

rate, then more organisms are going out than coming in, and population decreases.

Immigration and emigration• Immigration: moving into a population• Emigration: moving out of a population

Population Density• Population density – the

number of individuals in an area of a specific size. It can be written as• Population density = (# of

individuals/unit area)

Limiting factors• A limiting factor is an

environmental factor that causes a population to decrease.• For example: some

limiting factors are food and water, space, and weather conditions.

Food and water• Organisms need them ;)• The largest population

that an area can support is called its carrying capacity.

Space• Another limiting factor for both

plants and animals

Weather• Amount of rainfall, temperature

Please complete• Section 2 assessment in your notes – I will check it tomorrow• 1ab, 2abc, 3abc, 4