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?
Top Related