Population Biology

CHAPTER 4

Population Dynamics

Population Dynamics is the study of change in populations including growth, decline, births, deaths, internal interactions, and external interactions.

This can be used for any organism, even humans.

Principles of Population GrowthWhat is a population?

_______________________________________Populations grow and die at a

steady pace unless they run out of food, water, shelter, or space or are attacked by diseases or predators.

The Speed of Population Growth

Populations undergo exponential growth, not linear growth.

This results in a J-shaped curve when plotted on a graph.

The population begins small, and reproduces slowly but as it grows, grows in leaps and bounds.

Exponential Growth

If a population undergoes exponential growth it is not limited by anything.

Limiting Factors

A limiting factor is anything that limits the exponential growth of a population.

This can include:The availability of food, water, space, or

shelter.DiseaseCompetitionPredationCatastrophic events

If a population is limited in some way, it becomes an S-shaped curve.

It grows exponentially until it is limited and then it remains relatively constant.

Carrying Capacity

Carrying Capacity is the number of individuals in a population that an environment can support indefinitely.

When a population develops in an area it begins exponential growth until it surpasses the carrying capacity.

It is then limited. Some organisms will not be able to survive and the population will decrease to below the carrying capacity.

Over time the population size will wiggle above and below the carrying capacity.

Reproduction Patterns

Why don’t populations reach their carrying capacity and remain stable?

To explore this we look at an organism’s life-history pattern or reproductive pattern.Mosquitos have a very short life span and

reproduce rapidly and produce many offspring.

Elephants have a long life span, but take years to be able to reproduce and only produce a limited number of offspring.

Rapid Life-History Patterns Organisms with Rapid Life-History

Patterns come from unstable or changing environments.

They tend to be smaller, mature rapidly, reproduce early and in large numbers, and have a short life span.

This allows them to better survive and reproduce in their environment.

Their populations fluctuate wildly depending on environmental conditions.

Slow Life-History PatternsOrganisms with Slow Life-History

Patterns come from stable or relatively changing environments.

They tend to be larger, mature more slowly, reproduce later and in smaller numbers, but have a long life span.

This allows them to better survive and reproduce in their environment.

Their populations stay near carrying capacity.

Density Factors

Density is the number or amount of something in a given space.

Dispersal is how something is spread out in that area.

Size is the number of individuals in a population

The density and dispersal of organisms in an area greatly affects their population size and ability to survive.

Dispersal Patterns

Uniform

Clumped

Random

Reasons for uniform dispersal:

_____________________________________Reasons for clumped dispersal:

_____________________________________Reasons for random dispersal:

_____________________________________

Density-Dependent Factors

These factors increase their effect as population increases

Disease, competition, predators, parasites, water, and food.

Density-Independent Factors

These factors affect populations independently of size.

Usually abiotic factors such as:Volcanic eruptionsTemperatureStormsPrecipitation (floods or droughts)Pesticides or PoisonsCatastrophic Events and Habitat Disruptions

Population Interactions

Some interactions between populations and individuals can affect population size:PredationInterspecific Competition (Different

Populations)Intraspecific Competition (Same

population)Crowding/Stress

Predation

Predation can keep a population limited and contained (a weasel eating chickens) or it can drastically affect a population (locusts eating almost all of a field).

Usually predator and prey population sizes mimic each other and are in balance.

This is a density-dependent factor.

Interspecific Competition This is when members of two different

populations/species compete for the same resources.

A large tree may block the sunlight from a smaller tree.

Lions, cheetahs, and hyenas will all compete for the same zebra (alive or dead).

Grey squirrels and red squirrels compete for the acorns of the oak trees in your yard.

This is a density-dependent factor.

Intraspecific Competition

This is when members of the same population compete for the same resources.

Two lion prides fight for a particular territory.

Two male deer fight to be able to mate with a female.

This is a density-dependent factor.

Crowding/StressWhen population of certain species

become crowded, they may show signs or symptoms of stress.

These can include:Increased aggressionDecrease in parental careDecreased fertilityDecreased resistance to disease

These are all negative and can become limiting factors.

Demography

Demography is the study of the human population and its’:Size,DensityDistributionMovementBirth rateDeath rateLife span

Human and World Population

Human population growth has occurred at an exponential rate since about 1800. Before that it was relatively stable.

This is mostly due to new technology, hygiene, and medicine.

Refrigerators allowed food to be stored safely for longer and new transportation allowed it to be sent to farther away.

Better hygiene and medicine allowed people to live longer and more safely.

Factors Affecting Human Population Growth

Humans are affected differently than other organisms because we can change our environment and surroundings more easily.

We have vaccinated against and eliminated many diseases.

We have developed more efficient methods of growing food.

There is lower infant mortality and better medical care than in past times.

Many have easy access to clean water We have improved our sanitation and hygiene.

Calculating Growth RateBirthrate-the number of live births

per 1000 population in a given year.Death rate-the number of deaths

per 1000 population in a given year.Migration-the movement of

individuals between populationsImmigration-when individuals enter a

populationEmigration-when individuals leave a

population

Birthrate = BRDeath rate = DR Immigration = IEmigration = EPopulation Growth Rate = PGR

(BR + I) – (DR + E) =PGR

However because numbers for immigration and emigration are often not accurate we simplify it to:

BR – DR = PGR

If the Birthrate and the Death rate are equal, then the population growth rate will be zero.

This doesn’t mean that the population is not changing. It just means that the same number of individuals are entering the population (births and immigration) as are leaving the population (deaths and emigration

If the PGR is above zero, then more individuals are entering than leaving the population.

If the PGR is below zero, then more individuals are leaving than entering the population.

What does a PGR of 1.2 mean? What does a PGR of 0.1 mean? What does a PGR of -0.08 mean?

Doubling Time Doubling Time is the amount of time

needed for a population to double in time. It will depend on the current population size and

growth rate.

A country with a quick doubling time is often referred to as a developing country.

A country with a slow doubling time is often referred to as a developed country.

Doubling Time Mini-Lab

Age Structure

Age structure is the proportion of the population that are in different age levels.

At this point, they have been constructed for almost every country of the world.

It will tell you an approximate number of males and females as well as ages.

Types of Age StructuresRapidly growing countries (high PGR)

will have a wide base since most of the population is young.

Countries that have a PGR around 0 will be equal in each age group because there is not much growth.

Countries in population decline will have a wide top portion and it will thin towards the bottom since there are more old people.

Age Structure Mini-Labs

Ecology and Growth

Different governments have different priorities towards their citizens. Some only provide the most basic needs.Some want to maintain the quality of living

of their citizens.Some want to improve the quality of living

of their citizens.

This can come down to what resources are available for the population.

If a population grows too rapidly for the available resources it surpasses its’ carrying capacity.This means that not everyone will

be provided for.This can lead to stress which can

result in conflict, riots, war, a higher crime rate, or other negative consequences.