Population EcologyPopulation Ecology

The struggle between the biotic potential of a species The struggle between the biotic potential of a species and the environmental resistance that it encountersand the environmental resistance that it encounters

Population EcologyPopulation Ecology

Population Population – all of the individuals of a particular – all of the individuals of a particular species that live together in one place at one species that live together in one place at one time.time.

Population ecologyPopulation ecology explores how explores how biotic and abiotic factors influence biotic and abiotic factors influence the density, distribution, size and the density, distribution, size and age structure of populationsage structure of populations

Population EcologyPopulation Ecology

• Principles of population ecology Principles of population ecology may be used to:may be used to:– manage wildlife, fisheries, and manage wildlife, fisheries, and

forests for sustainable yieldforests for sustainable yield– reverse the decline of threatened or reverse the decline of threatened or

endangered speciesendangered species– reduce pest populations reduce pest populations – IPM = Integrated Pest ManagementIPM = Integrated Pest Management

• Integrated pest management (IPM) Integrated pest management (IPM) uses a combination of biological, uses a combination of biological, chemical, and cultural methods to chemical, and cultural methods to control agricultural pestscontrol agricultural pests

• count all the individuals in a count all the individuals in a population population

• estimate by samplingestimate by sampling

• mark-recapture method mark-recapture method depends on likelihood of depends on likelihood of recapturing the same recapturing the same individualindividual

How are How are populationspopulationsmeasured?measured?

Population Population EcologyEcologyThree fundamental Three fundamental characteristics of the characteristics of the organisms in a populationorganisms in a population

• DensityDensity• DispersionDispersion• DemographyDemography

Population SizePopulation Size… … is the number of individuals present is the number of individuals present

at a given time.at a given time.

The passenger pigeon was once North America’s most numerous bird, but it is now extinct.

Population DensityPopulation Density… … is the number of individuals per unit is the number of individuals per unit area.area.

In the 19th century, the flocks of passenger pigeons showed high population density.

• Increases by births Increases by births and immigrationand immigration

• Decreases by deaths Decreases by deaths and emigrationand emigration

Population DispersionPopulation DispersionThe dispersion pattern of spacing among The dispersion pattern of spacing among individuals within the boundaries of the individuals within the boundaries of the populationpopulation

Population DispersionPopulation Dispersion

• ClumpedClumped– Individuals in patches or groupsIndividuals in patches or groups– Usually resource relatedUsually resource related– Example: Bluestripe snappers (Example: Bluestripe snappers (Lutjanus Lutjanus

kasmirakasmira))– Schooling of some fish is a protective Schooling of some fish is a protective

strategystrategy– Herd mentality – protection of youngHerd mentality – protection of young

Population DispersionPopulation Dispersion• Uniform Uniform

– Often the result of antagonistic Often the result of antagonistic interactions interactions

– Animals that defend territories oftens Animals that defend territories oftens show a uniform pattersshow a uniform patters

– Example: Cape gannets (Example: Cape gannets (Morus capensisMorus capensis))– These birds space their nests out evenlyThese birds space their nests out evenly– Plants - alleleopathyPlants - alleleopathy

Population DispersionPopulation Dispersion

• Random DispersionRandom Dispersion– Unpredictable spacing Unpredictable spacing – Not usual in nature Not usual in nature

because there is usually because there is usually a reason for a pattern of a reason for a pattern of spacingspacing

Carrying CapacityCarrying Capacity• refers to the refers to the sizesize of a of a

populationpopulation that can that can live indefinitelylive indefinitely in an in an environment without environment without doing that doing that environment any environment any harmharm.  This applies to .  This applies to all organisms. all organisms.

• # of individuals in a # of individuals in a population that an population that an environment can environment can supportsupport..

• HuntingHunting• Amount of space suitable for Amount of space suitable for

breedingbreeding• Food availability - limited food Food availability - limited food

supplysupply• PreditorsPreditors• The buildup of toxic wastesThe buildup of toxic wastes• Increased diseaseIncreased disease

What limits the carrying What limits the carrying capacity of an ecosystem?capacity of an ecosystem?

Limiting Factor:Limiting Factor: is any biotic or abiotic resource that limits a is any biotic or abiotic resource that limits a populations size.populations size.

More Limiting FactorsMore Limiting Factors

• TerrestrialTerrestrial– SunlightSunlight– TemperatureTemperature– PrecipitationPrecipitation– Soil nutrientsSoil nutrients– Fire frequencyFire frequency– WindWind– LatitudeLatitude– AltitudeAltitude

• Aquatic/MarineAquatic/Marine– Light penetrationLight penetration

• Water clarityWater clarity

– Water currentsWater currents– Dissolved nutrient Dissolved nutrient

concentrationsconcentrations• Esp. N, P, FeEsp. N, P, Fe

– Dissolved Oxygen Dissolved Oxygen concentrationconcentration

– SalinitySalinity

Limits on Population Growth: Limits on Population Growth: Biotic Potential vs. Environmental ResistanceBiotic Potential vs. Environmental Resistance

No population can increase its size indefinitely.No population can increase its size indefinitely.The intrinsic rate of increase (The intrinsic rate of increase (rr) is the rate at which a ) is the rate at which a

population would grow if it had unlimited resources.population would grow if it had unlimited resources.Carrying capacity (Carrying capacity (KK): the maximum population of a ): the maximum population of a

given species that a particular habitat can sustain given species that a particular habitat can sustain indefinitely without degrading the habitat.indefinitely without degrading the habitat.

Idealized models describe two kinds Idealized models describe two kinds of population growthof population growth

1. exponential growth - 1. exponential growth - the rate of the rate of expansion of a population under ideal expansion of a population under ideal conditionsconditions

2. logistic growth - 2. logistic growth - idealized population idealized population growth slowed by limiting factors as the growth slowed by limiting factors as the population size increasespopulation size increases

How do populations grow?How do populations grow?

• A J-shaped growth curveA J-shaped growth curve– GG = the population growth rate = the population growth rate– rr = the intrinsic rate of increase, or an organism's maximum capacity = the intrinsic rate of increase, or an organism's maximum capacity

to reproduceto reproduce– NN = the population size = the population size

Exponential Growth CurveExponential Growth Curve

• Logistic growthLogistic growth is slowed by population-limiting factors is slowed by population-limiting factors– KK = carrying capacity - maximum population size that an environment = carrying capacity - maximum population size that an environment

can supportcan support– ((KK - - NN)/)/KK accounts for the leveling off of the curve accounts for the leveling off of the curve

Logistic Growth CurveLogistic Growth Curve

Exponential and Logistic Population Exponential and Logistic Population Growth: J-Curves and S-CurvesGrowth: J-Curves and S-Curves

Populations grow Populations grow rapidly with ample rapidly with ample resources, but as resources, but as resources become resources become limited, its growth limited, its growth rate slows and rate slows and levels off.levels off.

What does the logistic growth model suggest about What does the logistic growth model suggest about real populations in nature?real populations in nature?

• A population’s growth rate will be small when the population size is either small or large

• The growth rate will be highest when the population is at an intermediate level relative to the carrying capacity.

• Limiting factors make the birth rate decrease, the death rate increase or both

• Eventually the population will stabilize at the carrying capacity when the birth rate equals the death rate

• These are mathematical models and no population fits either perfectly

Exponential and Logistic Population Exponential and Logistic Population Growth: J-Curves and S-CurvesGrowth: J-Curves and S-Curves

As a population As a population levels off, it often levels off, it often fluctuates slightly fluctuates slightly above and below above and below the carrying the carrying capacity.capacity.

Types of Population Change Types of Population Change Curves in NatureCurves in Nature

Population sizes may stay the same, increase, Population sizes may stay the same, increase, decrease, vary in regular cycles, or change erratically.decrease, vary in regular cycles, or change erratically.StableStable: exhibits dynamic equalibrium.: exhibits dynamic equalibrium.IrruptiveIrruptive: when populations explode and crash.: when populations explode and crash.CyclicCyclic: populations fluctuate up and down: populations fluctuate up and downIrregularIrregular: erratic changes. : erratic changes.

Some factors that limit population growthSome factors that limit population growth

• Competition for resourcesCompetition for resources• As density of song sparrows As density of song sparrows

increase, the number of eggs increase, the number of eggs laid decreases because of food laid decreases because of food shortagesshortages

• Plants grown under crowded Plants grown under crowded conditions tend to be smaller conditions tend to be smaller and less likely to surviveand less likely to survive

• Disease transmission Disease transmission or or accumulation of toxic waste accumulation of toxic waste products can increase mortalityproducts can increase mortality

Some factors that limit population growthSome factors that limit population growth

• AA predator predator may capture more may capture more of a particular kind of prey as of a particular kind of prey as the prey becomes abundantthe prey becomes abundant

• White-footed mice stop White-footed mice stop reproducing at a colony size of reproducing at a colony size of 30-40 even when food and 30-40 even when food and shelter are provided. Stress?shelter are provided. Stress?

• The graph shows aphids which The graph shows aphids which feed on the phloem sap of feed on the phloem sap of plants increase in population in plants increase in population in the summer and then die-off in the summer and then die-off in the fall and winterthe fall and winter

Some factors that limit population growthSome factors that limit population growth

• Some populations remain fairly Some populations remain fairly stable in size close to carrying stable in size close to carrying capacitycapacity

• Most populations fluctuate as Most populations fluctuate as seen at the leftseen at the left

• This graph shows song This graph shows song sparrow populations, with sparrow populations, with periodic catastrophic reductions periodic catastrophic reductions due to severe winter weatherdue to severe winter weather

• Hare cycles may be Hare cycles may be caused by caused by increasing increasing food shortagesfood shortages during during winterwinter caused by caused by overgrazingovergrazing

• They may be due to They may be due to predator-prey interactionspredator-prey interactions

• Cycles could be affected Cycles could be affected by a combination of food by a combination of food resource limitation and resource limitation and excessive predationexcessive predation

• Predators reproduce Predators reproduce more slowly than their more slowly than their prey so they always prey so they always lag lag behindbehind prey in population prey in population growth.growth.

Boom and bust cyclesBoom and bust cycles

Population growth crashPopulation growth crashSome populations that rise too fast and deplete Some populations that rise too fast and deplete resources may then crash, as with reindeer on St. Paul resources may then crash, as with reindeer on St. Paul Island.Island.

St. Paul reindeer, Rangifer tarandus

• This is when a This is when a population is controlled population is controlled by natural events other by natural events other than population than population density.density.

• Natural disasters are Natural disasters are examples of density examples of density independent factorsindependent factors• fires fires • FloodsFloods• EarthquakesEarthquakes• HurricanesHurricanes• VolcanoesVolcanoes• DroughtDrought

Density IndependentDensity Independent

• This occurs when the This occurs when the density of the density of the population controls the population controls the total population of total population of individuals in a species.individuals in a species.• StressStress• DiseaseDisease• Competition for Competition for

resourcesresources• Lack of spaceLack of space

Density DependentDensity Dependent

• Life history traits are Life history traits are products of natural products of natural selectionselection

• Traits that affect an Traits that affect an organism’s schedule of organism’s schedule of reproduction and reproduction and survival make up its life survival make up its life historyhistory

• Life history traits are Life history traits are evolutionary outcomes, evolutionary outcomes, not conscious not conscious decisions by decisions by organismsorganisms

Life HistoryLife History

r- and K- Survivorship Strategiesr- and K- Survivorship Strategies

r-selected speciesr-selected species

• Many offspringMany offspring

• Fast growingFast growing

• No parental careNo parental care

K-selected speciesK-selected species

• Few offspringFew offspring

• Slow growingSlow growing

• Parental careParental care

Terms come from:

r = intrinsic rate of population increase. (Populations that can potentially grow fast, have high r.)

K = symbol for carrying capacity. (Populations tend to stabilize near K.)

Many small offspring

Little or no parental care and protection of offspring

Early reproductive age

Most offspring die before reaching reproductive age

Small adults

Adapted to unstable climate and environmental conditions

High population growth rate (r)

Population size fluctuates wildly above and below carrying capacity (K)

Generalist niche

Low ability to compete

Early successional species

r-Selected SpeciesCockroach

Dandelion

More examples…More examples…

Dandelions and salmon produce many, tiny Dandelions and salmon produce many, tiny offspring with low survivorship probabilitiesoffspring with low survivorship probabilities

Fewer, larger offspring

High parental care and protection of offspring

Later reproductive age

Most offspring survive to reproductive age

Larger adults

Adapted to stable climate and environmental conditions

Lower population growth rate (r)

Population size fairly stable and usually close to carrying capacity (K)

Specialist niche

High ability to compete

Late successional species

K-Selected Species

SaguaroElephant

More examples…More examples…

Coconut palms and kiwis produce a few, big Coconut palms and kiwis produce a few, big offspring with high survivorship probabilitiesoffspring with high survivorship probabilities

Study of vital Study of vital statisticsstatisticsBirth and Death Birth and Death raterate

– Type I curve Type I curve – low death – low death rate early and midlife. rate early and midlife. Death rate increases Death rate increases sharply at older agesharply at older age

– Type II curve Type II curve -probability -probability of survival does not of survival does not change with age; no age change with age; no age biasbias

– Type III curve Type III curve – high – high death rates for very death rates for very young then a period young then a period when death rates are when death rates are lower for those who lower for those who survive to a certain agesurvive to a certain age

Collapse of northern cod fishery –Collapse of northern cod fishery –

• Renewable resource Renewable resource managementmanagement – harvesting – harvesting crops without damaging crops without damaging the resourcethe resource

• Maximum sustainable yieldMaximum sustainable yield – harvest at a level that – harvest at a level that produces a consistent yield produces a consistent yield without forcing a without forcing a population into declinepopulation into decline

• In 1890, a group of Shakespeare enthusiasts released In 1890, a group of Shakespeare enthusiasts released about 120 starlings in New York's Central Park about 120 starlings in New York's Central Park

The Spread of Shakespeare StarlingsThe Spread of Shakespeare Starlings

Today over 100 million starlings are spread over Today over 100 million starlings are spread over North AmericaNorth America

Current

1955

1945

1935 1925

19251935

1915

1905

19251935

1945

1955

Current

•The starling population in North The starling population in North America has some features in common America has some features in common with the global human populationwith the global human population

–Both are expanding and are virtually uncontrolledBoth are expanding and are virtually uncontrolled

–Both are harming other speciesBoth are harming other species

The history of human population growthThe history of human population growth

• Throughout human history Throughout human history parents had many children parents had many children but only two on average but only two on average survived to adulthoodsurvived to adulthood

• Estimates that by 2025 the Estimates that by 2025 the world will have to double world will have to double food production, 2/3 of the food production, 2/3 of the available fresh water on available fresh water on earth will be in use, 60,000 earth will be in use, 60,000 plant species will be lost to plant species will be lost to support the populationsupport the population

• Issues: overgrazing, rivers Issues: overgrazing, rivers running dry, decrease in running dry, decrease in groundwater, energy?groundwater, energy?

Demographic TransitionDemographic Transition

• Going from high birth Going from high birth rates and high death rates and high death rates to low birth rates rates to low birth rates and low death ratesand low death rates

• May take 150 years to May take 150 years to completecomplete

• ““Death rate falls due to Death rate falls due to increased medical care increased medical care and sanitationand sanitation

• Falling birth rate takes Falling birth rate takes longer, thus delaying longer, thus delaying transition.transition.

Population pyramids for Canada and Madagascar.Population pyramids for Canada and Madagascar.

Canada (a) shows a balanced age structure, with relatively even numbers of Canada (a) shows a balanced age structure, with relatively even numbers of individuals in various age classes. Madagascar (b) shows an age distribution individuals in various age classes. Madagascar (b) shows an age distribution heavily weighted toward young people. Madagascar's population growth rate heavily weighted toward young people. Madagascar's population growth rate is nine times that of Canada's. Data from U.N. Population Division.is nine times that of Canada's. Data from U.N. Population Division.

As China's population ages, older people will outnumber the youngAs China's population ages, older people will outnumber the young

Age pyramids show the predicted graying of the Chinese Age pyramids show the predicted graying of the Chinese population between 2005 (a) and 2030 (b). Data from U.N. population between 2005 (a) and 2030 (b). Data from U.N. Population Division.Population Division.

USA Baby Boom 2005USA Baby Boom 2005

The "baby boom" is visible in the 2005 age pyramid for the United States, The "baby boom" is visible in the 2005 age pyramid for the United States, in the age brackets between 40 and 50.in the age brackets between 40 and 50.The nation is experiencing an aging population as baby-boomers grow The nation is experiencing an aging population as baby-boomers grow older. Data from U.N. Population Divisionolder. Data from U.N. Population Division..

TheThe Ecological Footprint Ecological Footprint

(carrying capacity)(carrying capacity) • Amount of productive Amount of productive

land and water needed to land and water needed to support the people in a support the people in a populationpopulation

• Currently 1.7 hectares Currently 1.7 hectares per person is considered per person is considered suitablesuitable

• A typical American has a A typical American has a footprint of 10 hectaresfootprint of 10 hectares

What next? What next?

ConclusionConclusion• Natural selection, speciation, Natural selection, speciation,

and extinction help determine and extinction help determine Earth’s biodiversity.Earth’s biodiversity.

• Understanding ecological Understanding ecological processes at the population processes at the population level is crucial to protecting level is crucial to protecting biodiversity.biodiversity.

• Understanding population Understanding population ecology also helps us ecology also helps us understand human population understand human population growthgrowth..