Population Biology

Populations

•Population: a group of organisms of the same species that live within a given area

•Key characteristics:–Growth Rate–Population density –Dispersal Patterns–Life-History Patterns

Ostriches are nomadic, wandering

in small groups.

Aspen trees are quick to pioneer areas that have been disturbed

by fire.

Populations

• Populations—large and small—are DYNAMIC– Meaning, they change over time

• Humans face the same problems as large and small populations in nature

Population Growth

• Exponential Growth

• J-Curve– Unchecked

growth due to unlimited resources

– As a population gets larger it grows faster

– Birth Rate is greater than Death Rate

Population Growth

• Logistic Growth• S-Curve

– Carrying Capacity• Number of

organisms the environment can support

– Carrying capacity limited by resources

– Birth Rate is equal to Death Rate at carrying capacity

Self Check

Which of the following would you expect to observe after a population exceeds its carrying capacity?

D. population growth rate is unaffected by limiting factors

C. deaths exceed births

B. births exceed deaths

A. population increases exponentially

Characteristics of Population Growth

Exponential growth

J curve S curve

Pop

ula

tion

Time

Carrying capacity

The answer is C.

Population Size

• Perils of Small Populations

– low genetic diversity – subject to inbreeding– less likely to adapt to

environmental changes

• Problems being a Large population

– Increase food shortages & diseases

– Decrease in space, clean water

– Live at carrying capacity so can experience huge crash

Factors That Influence Population Size

Population growth rate is determined by: • Natality or Birth rateNatality or Birth rate• Death rateDeath rate

It is also influenced by the number of individuals that enter and leave a population.

• ImmigrationImmigration – move into population – move into population• EmigrationEmigration – move out of population – move out of population

Limits on Population GrowthLimiting Factors- any factor that causes a population to decrease•Density Dependent Limiting Factors

–Depends on the size of the population

• Ex. Food, Water, Shelter, Disease, Competition

•Density Independent Limiting Factors

–Can affect populations regardless of their density

• Ex. Weather, Climate– Floods, Drought, Tornadoes, Fire,

Volcanoes

Water and shelter are critical limiting factors in

the desert.

Fire is an example of a

Density independent

Limiting factor.

Limiting Factors

• Density-dependent factors– Disease– Competition– Predators– Parasites– Food

• Density-independent factors– Volcanoes– Temperature– Storms– Floods– Drought– Habitat disruption

Other population factors

• Predation• Competition

Predator & Prey

• Predator: Organism that eats all or part of another organism

• Prey: an organism that gets eaten by another organism

•Competition: two or more organisms using the same resources

Between different species and the same species

Population Density•Population density is total population size per unit of area.

•Population densities depend on:– Interactions within the environment –Quality of habitat–Density dependent factors –Density independent factors–Birth rate and death rate

Dispersal Patterns Within Populations

Three common patterns of distribution are:

Dispersal Patterns

RandomClumped

Uniform

When Studying Populations…

• The best way to determine population size is to collect an absolute number.– Count up all the individuals

in the population.

• More frequently used is population density.– The number of individuals

per unit area.– Can be measured using a

variety of sampling techniques.

Random Sampling • A method of selecting a

sample from a statistical population in such a way that every possible sample that could be selected has a predetermined probability of being selected (random random samplesample).

• BEST FOR:• Stationary Populations

– Ex. Plants

• Even Dispersal Patterns

Mark and Recapture

• A method of sampling an animal population where animals are caught alive and tagged, and then returned (unharmed) to their habitat

• over time animals from the pop are trapped and those with marks/tags are counted

mathematical formula

estimates the pop size

Patterns in Populations

Reproductive pattern = life-history pattern

Variety of patterns, but TWO extremes

Patterns

• Rapid life-history patterns– Changing or

unpredictable environment

– Small– Mature rapidly– Reproduce early– Short life span

Patterns

• Slow life-history pattern– Large species– Stable environments– Reproduce slowly– Matures slowly– Long life span– Stay at or near carrying

capacity

Reproductive Strategies

Rapid (maximum growth rate, below carrying capacity)– Early reproduction– Short life span– High mortality rate– Little or no parental care– Large investment in

producing large numbers of offspring

– Below carrying capacity– Examples:

• Bony fish• Grasshoppers

Slow (maximizes population size near carrying capacity)– Late reproduction– Long life span– Low mortality rate– Extensive parental care– Greater investment in

maintenance and survival of adults

– At or near carrying capacity

– Examples:• Sharks• Elephants

Survivorship in Populations

Survivorship Curves• Patterns of Mortality

– Populations show three patterns of mortality or survivorship curves:•Type I (low mortality until late in life)

•Type II (constant mortality throughout life)

•Type III (high mortality early in life followed by low mortality for the remaining life span)

Survivorship in Populations

Rapid Life History PatternType III Survivorship

Type III Species: – have high

reproductive rates– tend to occur in

unpredictable environments

– Ex. Fish, Plants

Slow Life History PatternType I Survivorship

Type I Species: – occur near carrying

capacity – experience effects of

population density – have low

reproductive rates, high parental care

– Ex. Humans, Elephants

Human Populations

Age Structures

and

Human Growth

Age Structure

– A population’s age structure indicates the percentage of individuals at each age.

– The right side shows females; the left, males– The x-axis is number is populations size

• Usually in millions

– The y-axis is age ranges usually 0-4, 5-9, 10-14, etc…

Population Age Structure

• Differences in environmental conditions and past history may cause populations to differ in their age distributions.

• The future growth of a population depends on its current age distribution.

Ya’llme

History of Human Population Growth

• The Development of Agriculture– About 10,000 to 12,000 years ago, the

development of agriculture increased the growth rate of the human population.

What happened in the 1600s?

The Population Explosion– Around 1650, improvements in hygiene, diet, and

economic conditions further accelerated population growth.

– After World War II, the human population grew at the fastest rate in history, largely because of better sanitation and medical care in poorer countries.

Advances in Human Technology = Growth

•Human population growth rate has been growing more than exponentially.

•Limited resources eventually will cause human population growth to slow, but global human carrying capacity is not known.

Human Population Growth

1. What is the difference between linear growth and exponential growth

as plotted on a graph?

2. Why don’t populations of organisms grow indefinitely?

3. What is the relation ship of births to deaths in a population

before the population reaches the environment’s carrying capacity?

4. What happens when the population exceeds the carrying capacity?

5. What are some limiting factors that can curb population growth?