Population Population EcologyEcology

What is a populationWhat is a population

A group of individuals of a species A group of individuals of a species that live in an area and rely on the that live in an area and rely on the same resources for survival often same resources for survival often interacting and breeding with one interacting and breeding with one anotheranother

Population Population CharacteristicsCharacteristics

Density Density DispersionDispersion DemographyDemography

Life TablesLife Tables Survivorship curvesSurvivorship curves Reproductive ratesReproductive rates

Life Histories-Trade off Life Histories-Trade off between survival and between survival and reproductive traits reproductive traits

When reproduction beginsWhen reproduction begins How often the organism reproducesHow often the organism reproduces How many offspring are produced How many offspring are produced

during each reproductive episodeduring each reproductive episode

Reproduction TypesReproduction Types

Semelparity/Big Semelparity/Big Bang ReproductionBang Reproduction Ex. Pine Looper Ex. Pine Looper

MothMoth

Iteroparity/Iteroparity/Repeated Repeated ReproductionReproduction LizardsLizards

Why choose one over the other?

Studying PopulationsStudying Populations Exponential growth model – the rate of Exponential growth model – the rate of

expansion of a population under ideal expansion of a population under ideal conditionsconditions

Population-limiting factors – hunting, amount Population-limiting factors – hunting, amount of space suitable for breeding, restricted of space suitable for breeding, restricted population growth, food availabilitypopulation growth, food availability

Logistic growth model – idealized populationLogistic growth model – idealized population growth slowed by limiting factors as the growth slowed by limiting factors as the

population size increasespopulation size increases Carrying capacity – the maximum population Carrying capacity – the maximum population

size that an environment can support at a size that an environment can support at a particular time with no degradation to the particular time with no degradation to the habitathabitat

Exponential GrowthExponential Growth

G= growth of population over time (dN/dt)

r=tells if the population is growing (per capita rate of increase) r=0 with ZPG

N= population size

Logistic growth and Logistic growth and exponential growth comparedexponential growth compared

K= carrying capacity

K-N= the number of additional individuals the environment can accommodate

(K-N)/K= fraction of K that is still available for population growth

Growth of a population of Growth of a population of fur sealsfur seals

What does the logistic growth What does the logistic growth model suggest about real model suggest about real

populations in nature?populations in nature? A population’s growth rate will be small A population’s growth rate will be small

when the population size is either small or when the population size is either small or large and highest when the population is large and highest when the population is at an intermediate level relative to the at an intermediate level relative to the carrying capacity.carrying capacity.

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

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

These are mathematical models and no These are mathematical models and no population fits either perfectlypopulation fits either perfectly

Some factors that limit population Some factors that limit population growthgrowth

As density of song As density of song sparrows increase, the sparrows increase, the number of eggs laid number of eggs laid decreases because of decreases because of food shortagesfood shortages

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

Disease transmission Disease transmission or accumulation of or accumulation of toxic waste products toxic waste products can increase mortalitycan increase mortality

Continued……Continued…… A predator may capture A predator may capture

more of a particular more of a particular kind of prey as the prey kind of prey as the prey becomes abundantbecomes abundant

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

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

Continued….Continued…. Some populations Some populations

remain fairly stable in remain fairly stable in size close to carrying size close to carrying capacitycapacity

Most populations Most populations fluctuate as seen at the fluctuate as seen at the leftleft

This graph shows song This graph shows song sparrow populations, sparrow populations, with periodic with periodic catastrophic reductions catastrophic reductions due to severe winter due to severe winter weatherweather

Boom and bust cyclesBoom and bust cycles Hare cycles may be Hare cycles may be

caused by increasing caused by increasing food shortages during food shortages during winter caused by winter caused by overgrazingovergrazing

They may be due to They may be due to predator-prey predator-prey interactionsinteractions

Cycles could be Cycles could be affected by a affected by a combination of food 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 lag prey so they always lag behind prey in behind prey in population growth.population growth.

Regulation of Regulation of PopulationsPopulations Density IndependentDensity Independent

Birth and death rate does not change with Birth and death rate does not change with population densitypopulation density

Density DependentDensity Dependent Death rate that rises as population density risesDeath rate that rises as population density rises

Competition for resourcesCompetition for resources TerritorialityTerritoriality HealthHealth PredationPredation Toxic WasteToxic Waste Physiological factorsPhysiological factors

Population DynamicsPopulation Dynamics Complex interactions between biotic and abiotic Complex interactions between biotic and abiotic

factorsfactors Population CyclesPopulation Cycles

Boom and bust cyclesBoom and bust cycles Snowshoe hare and lynxSnowshoe hare and lynx

Survivorship curvesSurvivorship curves Type I curve – Type I curve –

produce few produce few offspring and give offspring and give them good carethem good care

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

Logistic Model & Life Logistic Model & Life HistoriesHistories

K-Selection K-Selection Occur near carrying Occur near carrying

capacitycapacity Density dependentDensity dependent Low reproductive rates Low reproductive rates

and high careand high care Type I Survivorship CurveType I Survivorship Curve

r-Selection r-Selection High Reproductive RatesHigh Reproductive Rates Unpredictable Unpredictable

environmentsenvironments Density IndependentDensity Independent Type III Survivorship Type III Survivorship

CurveCurve

Life tables – compiled by Life tables – compiled by life insurance agentslife insurance agents

Human carrying capacity Human carrying capacity estimatesestimates

Ecological footprint Ecological footprint with multiple with multiple constraints such as constraints such as food, fuel, water, food, fuel, water, housing, and waste housing, and waste disposal used.disposal used.

Calculates current Calculates current demand on resources demand on resources by each country in by each country in hectares of land per hectares of land per personperson

World ecological World ecological capacity is 1.7 ha per capacity is 1.7 ha per person alive in 1997person alive in 1997