How Populations GrowHow Populations Grow

Chapter 5 Chapter 5

Section 1Section 1

Let’s Review!Let’s Review!

What is a population?What is a population? A group of A group of similar organismssimilar organisms that can that can breed breed

and produce fertile offspringand produce fertile offspring Species or not a species?Species or not a species?

Characteristics of PopulationsCharacteristics of Populations Populations Populations often changeoften change due to a variety due to a variety

of of influential factorsinfluential factors There are three important characteristics There are three important characteristics

of a population.of a population.1.1. Geographic Geographic

distributiondistribution –– the range of area the range of area inhabited by a inhabited by a populationpopulation

Characteristics of PopulationsCharacteristics of Populations

2.2. Population density – Population density – the number of the number of individuals per unit areaindividuals per unit area

Population DensityPopulation Density

Can vary tremendously depending on the Can vary tremendously depending on the speciesspecies and its and its ecosystemecosystem

A fewA few saguro cactussaguro cactusA field of sunflowers A field of sunflowers A flock of birdsA flock of birds

Turtles grouped to sunTurtles grouped to sun

Population DensityPopulation Density What are some What are some benefits ofbenefits of living close living close

together or in a together or in a clumped populationclumped population??1.1. Help Help finding foodfinding food

2.2. ProtectionProtection from predators from predators or windor wind

3.3. Easier to findEasier to find species species

PopulationDensityPopulationDensity

What are some What are some drawbacksdrawbacks (negatives) (negatives) of of livingliving in a in a clumpedclumped population? population?

1.1. CrowdedCrowded living areas living areas

2.2. Battles over Battles over food/shelterfood/shelter

3.3. Easier for Easier for disease to spreaddisease to spread

Characteristics of PopulationsCharacteristics of Populations

3.3. Growth rate- the Growth rate- the amount amount by which a by which a population’s size population’s size changes in achanges in a given given timetime

Population GrowthPopulation Growth

Three factors affect population sizeThree factors affect population size

1.1. Number ofNumber of births births

2.2. Number ofNumber of deaths deaths

3.3. Number of Number of individualsindividuals thatthat enter enter oror leave leave

Population GrowthPopulation Growth MigrationMigration- periodic movements of - periodic movements of

populations away from and back to their populations away from and back to their place of origin place of origin

(not what we are talking about here)(not what we are talking about here) Irregular Irregular movementsmovements of some individuals of some individuals

out ofout of an area, an area, intointo a new area, a new area, with no with no returnreturn - affects population - affects population sizesize

Immigration – movement of individuals Immigration – movement of individuals into an areainto an area Population increases in sizePopulation increases in size

Emigration – movement of individuals Emigration – movement of individuals out of an areaout of an area Population decreases in sizePopulation decreases in size

Population GrowthPopulation Growth

Why would an animal immigrate to or Why would an animal immigrate to or emigrate from a population?emigrate from a population? Competitive Competitive pressurespressures on some individuals on some individuals To find food, To find food, better livingbetter living condition, or a mate condition, or a mate

Exponential GrowthExponential Growth

If a population has If a population has plentyplenty of of foodfood and and spacespace and is and is protected from predatorsprotected from predators and and diseasedisease, what will happen to the size , what will happen to the size of the population?of the population? It will increase!It will increase!

Exponential GrowthExponential Growth

Example: Example: BacteriaBacteria reproduce in ~20 reproduce in ~20 minutes by splitting in halfminutes by splitting in half 1 bacteria after 20 min = 1 bacteria after 20 min = 2 bacteria2 bacteria 2 bacteria after 40 min = 2 bacteria after 40 min = 4 bacteria4 bacteria 4 bacteria after 60 min = 4 bacteria after 60 min = 8 bacteria8 bacteria In 180 min (3 hours) = In 180 min (3 hours) = 512 bacteria512 bacteria In one day = In one day = 4,720,000,000,000,000,000,000 bacteria4,720,000,000,000,000,000,000 bacteria

Exponential GrowthExponential Growth Exponential growthExponential growth occurs when the occurs when the

individuals in a population individuals in a population reproduce at a reproduce at a constant rateconstant rate Creates a “J” shaped Creates a “J” shaped

curvecurve Under ideal conditions Under ideal conditions

with unlimited resources with unlimited resources and no predation, and no predation, a a population will grow population will grow exponentiallyexponentially

This is what a graph of bacterial growth would look like if the growth went unchecked

Logistic GrowthLogistic Growth

Do bacteria cover the planet?Do bacteria cover the planet? Exponential growth does Exponential growth does not continuenot continue in in

populations very long.populations very long. As resources become less available, As resources become less available, the the

growth of a population slows or stopsgrowth of a population slows or stops What might cause population growth to What might cause population growth to

stop or slow down?stop or slow down? Predators, less food, less space, Predators, less food, less space,

less resources, loss of habitat, disease, less resources, loss of habitat, disease, etc.etc.

Logistic GrowthLogistic Growth

Logistic growthLogistic growth occurs when a occurs when a population’s growth population’s growth slows or stopsslows or stops following a following a period ofperiod of exponential growth exponential growth Creates an “S” shaped curveCreates an “S” shaped curve

Population growth may slow or stopPopulation growth may slow or stopfor a number of reasons:for a number of reasons: Birthrate Birthrate ↓ ↓ death rate death rate ↑↑

immigration immigration ↓ ↓ emigration emigration ↑↑ At some point, the growth of a population At some point, the growth of a population

will will level offlevel off

Logistic GrowthLogistic Growth

Time (hours)

Nu

mb

er

Logistic GrowthLogistic Growth Occurs after a period of exponential growthOccurs after a period of exponential growth When a population When a population

reaches carrying capacityreaches carrying capacity

Logistic GrowthLogistic Growth Carrying capacity – Carrying capacity – the largest number of the largest number of

individuals an environment can supportindividuals an environment can support Most Most plant and animal populations plant and animal populations follow this follow this

type of type of growthgrowth

Time (hours)

Nu

mb

er o

f Y

east

Cel

lsCarrying capacity

Comparing Exponential and Comparing Exponential and Logistic GrowthLogistic Growth

Limits to GrowthLimits to Growth

Limiting Factor: any factor that Limiting Factor: any factor that causes population growth to stopcauses population growth to stop

Limits the sizeLimits the size of a population of a population Almost anything can be a limiting factor…Almost anything can be a limiting factor… Examples: Examples: competingcompeting for sunlight, space, for sunlight, space,

water, food, etc.water, food, etc. PredationPredation, parasitism, , parasitism,

diseasedisease, natural disasters, , natural disasters, etc.etc.

Limits to GrowthLimits to Growth

As a As a population increasespopulation increases, , so does so does competitioncompetition

Not allNot all individuals will individuals will get what they needget what they need to survive and will dieto survive and will die

The number of The number of births decreasesbirths decreases and the and the number of number of deaths deaths increasesincreases The population The population

gets gets smallersmaller

Limits to GrowthLimits to Growth

Plant Plant roots competeroots compete for space, nutrients, for space, nutrients, and water with the roots of other plantsand water with the roots of other plants

Taller plants Taller plants block out sunblock out sun for shorter for shorter plantsplants