8.1 POPULATIONS

CLASSROOM CATALYST

OBJECTIVES• Describe the three main properties of a

population.• Describe exponential population growth.• Describe how the reproductive behavior of

individuals can affect the growth rate of their population.

• Explain how population sizes in nature are regulated.

WHAT IS A POPULATION?• A population is a group of organisms of the

same species that live in a specific geographical area and interbreed.

POPULATION DENSITY

• Density is the number of individuals of the same species in that live in a given unit of

area.

• Measure of how crowded a population is

• Larger organisms

generally have lower

population densities.

• Low population density:

More space, resources;

finding mates can be difficult

• High population density:

Finding mates is easier; tends to be more competition; more infectious disease; more

vulnerability to predators

Northern pintail ducks

POPULATION DISTRIBUTION

• Dispersion is the pattern of distribution of organisms in a

population. A population’s dispersion may be even/spaced,

clumped, or random.

• How organisms are arranged within an area:

• Random distribution:

Organisms arranged in

no particular pattern

• Even/Spaced distribution:

Organisms evenly spaced

• Clumped distribution:

Organisms grouped near resources; most

common distribution in nature

HOW DOES A POPULATION GROW?• A population gains individuals with each new

offspring or birth and loses them with each death.• The resulting population change over time can be

represented by the equation below.

• Growth rate is an expression of the increase in the size of an organism or population over a given period of time. It is the birth rate minus the death rate. It can be +, -, or 0.

BIOTIC/REPRODUCTIVE POTENTIAL

• Reproductive potential is the maximum number of

offspring that a given organism can produce.

• Many factors influence biotic potential, including gestation

time and generation time.

• Organisms with high biotic potential can recover more

quickly from population declines than organisms with low

biotic potential.

POPULATION GROWTH• Populations have the ability to

grow very quickly, however, if they have a perfect environment.

• Exponential Growth• Population growth in which the rate

of growth in each generation is a multiple of the previous generation

• Conditions are never perfect, resources are always limited.

• A population can grow exponentially, but not for long.

CARRYING CAPACITY CONT.

• Carrying Capacity• The number of individuals that can be supported by an

ecosystem• S-Shaped Curve : #Births = #Deaths

RESOURCE LIMITS• A species reaches its carrying capacity when it

consumes a particular natural resource at the same rate at which the ecosystem produces the resource.

• That natural resource is then called a limiting resource.

• The supply of the most severely limited resources determines the carrying capacity of an environment for a particular species at a particular time.

LIMITING FACTORS• Populations can’t grow forever.• Forces that slow growth in a populations are

called limiting factors.• Deaths occur more quickly in a crowded population than in a sparse population.• A certain proportion of a population may die regardless of the population’s

density.

Population

Size

Climate

Water

AvailabilityPredation

Living Space

Food

CompetitionDisease

Parasitism

Natural

DisastersHuman

DisturbanceDensity-

Dependent

Density-

Independent