EcologyEcology

Lecture 7Lecture 7

Ralph KirbyRalph Kirby

Population (Ecology)Population (Ecology)– Group of individuals of the same Group of individuals of the same

species inhabiting the same areaspecies inhabiting the same areaInterbreeding if sexualInterbreeding if sexualLimited in spaceLimited in space

Populations havePopulations have– DensityDensity– Distribution in timeDistribution in time– Distribution in spaceDistribution in space

These are defined byThese are defined by– Rate of birthRate of birth– Rate of growthRate of growth– Rate of deathRate of death

Resulting in age & sex structureResulting in age & sex structure

What is an individual– Easy for most large

sexually reproducing animals

– Not so easy for colonies of animals and many plants

– Ants– Bees– Coral– Trees (Ramets)– Fungi

DistributionDistribution– Area over which a Area over which a

species occursspecies occurs

AbundanceAbundance– Numbers of that Numbers of that

species present in species present in the areathe area

Both vary by Both vary by available habitatavailable habitat– TemperatureTemperature

Red MapleRed Maple– -40-40ooCC

Carolina wrenCarolina wren– Northwards -7Northwards -7ooCC– Westward>52mmWestward>52mm

– When all When all environmental environmental factors within its factors within its range of tolerance, range of tolerance, the organism can the organism can live in its habitatlive in its habitat

Horned larkHorned lark– Avoids forestsAvoids forests– Available Available

territoryterritory

Density can be a key factorDensity can be a key factor– Number of individuals per unit spaceNumber of individuals per unit space– AffectsAffects

growth rategrowth rateResourcesResourcesMatesMatespredationpredation

Thus density controls in partThus density controls in part– Birth rateBirth rate– Growth rateGrowth rate– Death rateDeath rate

Density is difficult to define or measureDensity is difficult to define or measure– What area do you useWhat area do you use– What distribution has the species within the What distribution has the species within the

areaarea– How do you measure the densityHow do you measure the density– Ecological densityEcological density

Some use a factor associated with the habitatSome use a factor associated with the habitat

Distribution Distribution – ClumpedClumped

NormalNormal– UniformUniform

High density and competitionHigh density and competition– RandomRandom

Social behaviorSocial behaviorAvailable nesting sitesAvailable nesting sites

MetapopulationMetapopulation– Multi-Habitat based multiple populationsMulti-Habitat based multiple populations

How do you countHow do you countDirect CountsDirect Counts– Time consumingTime consuming– Needs to be sure you can see all individuals in the areaNeeds to be sure you can see all individuals in the area

QuadratsQuadrats– Organisms must be staticOrganisms must be static– Needs laying of quadrats to be randomNeeds laying of quadrats to be random– Statistical analysis essentialStatistical analysis essential

Mark and recaptureMark and recapture– EstimateEstimate

Assumes equal chance of capture for allAssumes equal chance of capture for allNo deaths or birthsNo deaths or birthsMarked animals random among unmarkedMarked animals random among unmarkedNo loss of marksNo loss of marksNo emigration or immigrationNo emigration or immigrationOther factors include time of capture, stress of capture, sex, age, etc.Other factors include time of capture, stress of capture, sex, age, etc.

Relative abundanceRelative abundance– Using a factor such as tracks, bird song, etcUsing a factor such as tracks, bird song, etc– Get a relative measureGet a relative measure

Disperal – Active and PassiveDisperal – Active and PassiveEmigration – Escaping high densityEmigration – Escaping high densityImmigration – Moving into empty habitatImmigration – Moving into empty habitatMigation – A round trip, perhaps involving matingMigation – A round trip, perhaps involving mating

Age structureAge structure– PrereproductivePrereproductive– ReproductiveReproductive– PostreproductivePostreproductive– Short life spanShort life span

Rapid increaseRapid increase

– Long Life spanLong Life spanSlow increaseSlow increase

Measuring Age Measuring Age structurestructure– Mark and recaptureMark and recapture

Accurate but hard Accurate but hard to doto do

– Special featuresSpecial featuresTeethTeethHorn ringsHorn ringsTree ringsTree rings

Age PyramidsAge Pyramids– Show status of Show status of

populationpopulation

Size/Age can affect the Size/Age can affect the population structurepopulation structure

Sex ratios in populations Sex ratios in populations shifts with ageshifts with age– HumansHumans

More malesMore males

Males have shorter life spanMales have shorter life span– WarWar

– High risk activitiesHigh risk activities

Females need to survive giving Females need to survive giving birth to childrenbirth to children

Therefore sex ratios have Therefore sex ratios have change a lot over the last two change a lot over the last two hundred yearshundred years

– Parturition fever in 19Parturition fever in 19thth Century Century

– First World War in UKFirst World War in UK

– Second World War in USSRSecond World War in USSR

Plants are more Plants are more difficultdifficult– Hard to detrmine Hard to detrmine

ageage– Extremely high Extremely high

loss fo seedlingsloss fo seedlings– Use of yield tableUse of yield table

Probability of survival is age specificProbability of survival is age specific– See death tableSee death table– Probability of death in next yearProbability of death in next year

Life TableLife Table– nnxx

Number in cohortNumber in cohort

– llxxProbability of surviving from birthProbability of surviving from birth

– qqxx

Age specific mortalityAge specific mortality

BirthrateBirthrate– Sex specificSex specific– Age specificAge specific

Net reproductive rate Net reproductive rate depends ondepends on– FecundityFecundity– SurvivorshipSurvivorship

GivenGiven– Age specific mortalityAge specific mortality– Age specific birth rateAge specific birth rate– Project population Project population

growth/declinegrowth/decline

Exponential GrowthExponential Growth– Depends on rate of Depends on rate of

reproduction, reproduction, λλ– If If λλ less than 1, there less than 1, there

is clineis cline

Population growth is Population growth is limited by the limited by the environmentenvironment– Carrying CapacityCarrying Capacity– See reindeerSee reindeer

NonconformityNonconformity– Change in age Change in age

structurestructure– ImmigrationImmigration– EmigrationEmigration– Changes in birth and Changes in birth and

death ratesdeath ratesBut stable once it But stable once it reaches carrying reaches carrying capacitycapacity

Populations should work via positive and negative feedbacksPopulations should work via positive and negative feedbacksDoes not work smoothlyDoes not work smoothly– Depends on environment such as overgrazingDepends on environment such as overgrazing

Population cycles in simpler systemsPopulation cycles in simpler systems– Food availabilityFood availability

Can result in extinctionsCan result in extinctions– Two typesTwo types

Change in environment. Start locallyChange in environment. Start locallyMass extinctions. External event?Mass extinctions. External event?

Three Models for regulation of Three Models for regulation of population densitypopulation densityK represents equilibriumK represents equilibrium– Partial density regulation APartial density regulation A

Birth Rate (B) independent of Birth Rate (B) independent of population densitypopulation densityMortality Rate (M) increases with Mortality Rate (M) increases with densitydensity

– Partial density regulation BPartial density regulation B Birth Rate (B) declines with population Birth Rate (B) declines with population densitydensityMortality Rate (M) independent of Mortality Rate (M) independent of population densitypopulation density

– Full density dependent regulationFull density dependent regulationBirth Rate (B) increases with Birth Rate (B) increases with deceasing population densitydeceasing population densityMortality Rate (M) increases with Mortality Rate (M) increases with increasing population densityincreasing population density

Carrying capacityCarrying capacitySee effect of density for See effect of density for tadpoles and white clovertadpoles and white clover– Maximum number of sustainable Maximum number of sustainable

individualsindividuals– Results in intra-specific Results in intra-specific

competitioncompetition– Two typesTwo types

Scramble competitionScramble competition– Growth and reproduction depressed Growth and reproduction depressed

for all individualsfor all individualsCan result in local extinction or Can result in local extinction or population crashpopulation crash

Contest competitionContest competition– Some individual deny resources to Some individual deny resources to

other individuals resulting in some other individuals resulting in some growing and reproducing better than growing and reproducing better than othersothers

Indirect via exploitation of Indirect via exploitation of resourcesresourcesDirect via interferenceDirect via interference

Population crash Population crash caused by caused by RindepestRindepest

Note exponential Note exponential increase from low increase from low basebase

Grass limited in dry Grass limited in dry seasonseason

Competition related Competition related to rainfall producing to rainfall producing grassgrass

Once in equilibrium, Once in equilibrium, rainfall most rainfall most importantimportant

Similat effect Similat effect for white cloverfor white clover

Effect of Effect of repottingrepottingEffects can Effects can differdiffer– Bison is Bison is

exponentiaexponential curvel curve

– Grizzly Grizzly bear is bear is linear linear effecteffect

Competition Competition can affect can affect reproductionreproduction– harp sealsharp seals

IncreasIncreased age ed age of of sexual sexual maturitymaturity

– maizemaizeLinearLinear

– marsh marsh herbherb

Effects of high density on Effects of high density on reproductionreproduction

StressStressPheromonesPheromonesDiseaseDiseaseIncreased mortality of youngIncreased mortality of youngResults in dispersalResults in dispersal– Sources HabitatSources Habitat– Sink HabitatSink Habitat

Can be dangerousCan be dangerousLack protective coverLack protective coverHigh number of predatorsHigh number of predators

– Move no further than necessaryMove no further than necessary– Move in foraysMove in forays

Social dominance can affect reproduction and dispersionSocial dominance can affect reproduction and dispersionFighting and social position. Both male and female hierarchies are possibleFighting and social position. Both male and female hierarchies are possibleResults in limited matingResults in limited mating– AlphaAlpha– BetaBeta– OmegaOmega

Home RangeCan overlap

TerritoryNo overlapFight to protect

Dominant male getsbest locations

Males and femaleranges can differ

Male and femaleranges overlap

Territory is protectedTerritory is protected– FoodFood– MatingMating– Nesting siteNesting site– Attraction of matesAttraction of mates– Avoidance of suboptimal habitatAvoidance of suboptimal habitat– Needs energyNeeds energy

May not be optimal strategy when May not be optimal strategy when resources are lowresources are low

Other density Other density independent factors independent factors can overrule densitycan overrule density– WeatherWeather– RainfallRainfall– TemperatureTemperature