Unit III: PopulationsUnit III: PopulationsChapter 8: Understanding PopulationsChapter 8: Understanding Populations8-1 How Populations Change in Size8-1 How Populations Change in Size

PopulationPopulation: all members of a species : all members of a species living in the same place at the same time living in the same place at the same time ((ex. All the water lilies in a pondex. All the water lilies in a pond). ). *Specific species, specific location.**Specific species, specific location.*

3 Key Properties of 3 Key Properties of Populations:Populations:1. Population Size1. Population Size: # of individuals in a population: # of individuals in a population

Very small populations are likely to become extinctVery small populations are likely to become extinct * * InbreedingInbreeding (breeding w/ relatives) is common in (breeding w/ relatives) is common in

small populationssmall populations– unfavorable recessive traits are more likely to be homozygous unfavorable recessive traits are more likely to be homozygous

and therefore expressed.and therefore expressed.– Reduced variability limits the population’s ability to adapt. Reduced variability limits the population’s ability to adapt.

2. 2. Population DensityPopulation Density: # of individuals found in a given : # of individuals found in a given areaarea

Widely spaced populations may rarely or never Widely spaced populations may rarely or never encounter one another, making reproduction rare encounter one another, making reproduction rare

3. 3. Population DispersionPopulation Dispersion: the way the individuals of the : the way the individuals of the population are arranged population are arranged

Randomly spacedRandomly spaced: determined by chance: determined by chance evenly spacedevenly spaced: regular intervals: regular intervals clumpedclumped: bunched together (*most common type of : bunched together (*most common type of

distribution in nature) distribution in nature)

Random Even Clumped

Growth RateGrowth Rate::

Change in pop. size = birth rate – death Change in pop. size = birth rate – death raterate

Can be positive, negative, or zeroCan be positive, negative, or zero

Reproductive PotentialReproductive Potential: the : the maximum # of offspring each maximum # of offspring each member of the population can member of the population can

produceproduce Biotic PotentialBiotic Potential: the fastest rate at which a : the fastest rate at which a species population can grow (limited by species population can grow (limited by reproductive potential)reproductive potential)

Reproductive Potential Increased by:Reproductive Potential Increased by: Producing more offspring at a timeProducing more offspring at a time Reproducing more oftenReproducing more often Reproducing earlier in life (has greatest effect)Reproducing earlier in life (has greatest effect)

– Shortens Shortens generation timegeneration time: avg. time it takes a member : avg. time it takes a member of the population to reach the age when it reproduces.of the population to reach the age when it reproduces. Small organisms = short generation times (bacteria & insects = Small organisms = short generation times (bacteria & insects =

few hours-few days)few hours-few days) Large organisms – long generation time (humans = about 20 Large organisms – long generation time (humans = about 20

years)years) Who has a higher reproductive potential – humans Who has a higher reproductive potential – humans

or insects?or insects?

Exponential GrowthExponential Growth: when : when populations grow faster and fasterpopulations grow faster and faster

Occurs in nature when: plenty of food Occurs in nature when: plenty of food and space, no competition or and space, no competition or predatorspredators

Forms a J-shaped curve when graphedForms a J-shaped curve when graphed constant growth rate, population grows constant growth rate, population grows

quickly as the size of the population quickly as the size of the population increases increases

ex. Human Population Growthex. Human Population Growth

Carrying CapacityCarrying Capacity: maximum : maximum population size an ecosystem can population size an ecosystem can support support Eventually as the population grows the Eventually as the population grows the

growth rate falls (birth rates decline and growth rate falls (birth rates decline and death rates rise)death rates rise)

Why? Competition for food, shelter, Why? Competition for food, shelter, mating sites, and other resources, and mating sites, and other resources, and the accumulation of toxic wastes tends to the accumulation of toxic wastes tends to increase as a population approaches its increase as a population approaches its carrying capacity.carrying capacity.

Forms an S-shaped curve when graphed.Forms an S-shaped curve when graphed. a.k.a. a.k.a. equilibriumequilibrium

Limiting resource / limiting Limiting resource / limiting factorfactor: resources that limit the : resources that limit the size of a population (mainly abiotic size of a population (mainly abiotic factors)factors)

Competition for limiting resources is Competition for limiting resources is often indirect (ex. social dominance often indirect (ex. social dominance or territory)or territory)– TerritoryTerritory: an area defended by 1 or : an area defended by 1 or

more individuals against other more individuals against other individuals (of value for space, shelter, individuals (of value for space, shelter, food, or breeding sites)food, or breeding sites)

2 Types of Population 2 Types of Population RegulationRegulation

1. Density-Dependent Factors1. Density-Dependent Factors: : cause of death occurs more quickly in a crowded cause of death occurs more quickly in a crowded

population than in a sparse populationpopulation than in a sparse population the effect on the size of the population DEPENDS the effect on the size of the population DEPENDS

upon the density or size of the population upon the density or size of the population (usually biotic factors ex. Disease, parasites, (usually biotic factors ex. Disease, parasites, competition, etc)competition, etc)

2. Density-Independent Factors2. Density-Independent Factors:: a certain portion of a population may die a certain portion of a population may die

regardless of the population’s densityregardless of the population’s density unrelated to population size. affect everyone in unrelated to population size. affect everyone in

the population regardless of population size the population regardless of population size (usually abiotic factors ex. Weather, climate, (usually abiotic factors ex. Weather, climate, natural disasters, etc.)natural disasters, etc.)

8-2 How Species Interact with Each Other8-2 How Species Interact with Each Other NicheNiche: unique roll of a species within an : unique roll of a species within an

ecosystemecosystem Includes:Includes:

– Physical home, environmental factors Physical home, environmental factors necessary for the species’ survivalnecessary for the species’ survival

– All of the species’ interactions w/ other All of the species’ interactions w/ other organismsorganisms

– (where it lives, what it eats, when it (where it lives, what it eats, when it reproduces, etc.)reproduces, etc.)

Niche vs. HabitatNiche vs. Habitat Habitat = organism’s locationHabitat = organism’s location Niche = pattern of use of its habitatNiche = pattern of use of its habitat

CompetitionCompetition – common use of scarce – common use of scarce resourcesresources

Occurs within & between speciesOccurs within & between species Happens when niches Happens when niches overlapoverlap Indirect CompetitionIndirect Competition – 2 species don’t – 2 species don’t

come in direct contact w/ each other (come in direct contact w/ each other (ex. ex. 1 insect species feeds on a plant during 1 insect species feeds on a plant during the day, another species at night)the day, another species at night)

Adaptations to CompetitionAdaptations to Competition Avoiding competition is an advantageous adaptation.Avoiding competition is an advantageous adaptation. Niche RestrictionNiche Restriction – each species uses less of the niche than they – each species uses less of the niche than they

are capable of usingare capable of using– Fundamental NicheFundamental Niche: total niche an organism : total niche an organism couldcould occupy occupy

w/in an ecosystem - entire range of conditions it can toleratew/in an ecosystem - entire range of conditions it can tolerate– Realized NicheRealized Niche: niche a species actually occupies : niche a species actually occupies (it doesn’t (it doesn’t

occupy the entire fundamental niche due to competition)occupy the entire fundamental niche due to competition)

Time To Draw Some Happy Little Time To Draw Some Happy Little Trees….Trees….

Fundamental NiceFundamental Nice Fundamental NiceFundamental Nice Realized NicheRealized NicheSpecies ASpecies ASpecies BSpecies B Species A & BSpecies A & B

Ecological RelationshipsEcological RelationshipsCompetitionCompetition Species ASpecies A = = - - Species BSpecies B = = - - Example:Example: Birds competing for seed at a Birds competing for seed at a

feeder. feeder. DescriptionDescription:2 Species use the same :2 Species use the same

resourcesresources

PredationPredation Species ASpecies A = = + + Species BSpecies B = = - - Example:Example: snakes & mice snakes & mice Description:Description: Predator eats and kills the Predator eats and kills the

prey. prey.

ParasitismParasitism Species ASpecies A = = + + Species BSpecies B = = - - Example:Example: tick & dog ; tapeworm & tick & dog ; tapeworm &

humanhuman Description:Description: Parasite lives in or on the Parasite lives in or on the

host and feeds on the host. host and feeds on the host.

MutualismMutualism Species ASpecies A = = + + Species BSpecies B = = + + Example:Example: Flowering Plant & Honey Bee Flowering Plant & Honey Bee

– Acacia Trees & Ants Acacia Trees & Ants Description:Description: Each species helps the Each species helps the

other (both species benefit) other (both species benefit)

CommensalismCommensalism Species ASpecies A = = + + Species BSpecies B = 0 ? = 0 ? Example:Example: Shark & Remora Shark & Remora

– Birds nesting in treesBirds nesting in trees Description:Description: Species A benefits, species Species A benefits, species

B is unaffected. B is unaffected.

SymbiosisSymbiosis: “living together” – 2 : “living together” – 2 organisms living in close organisms living in close association w/ each otherassociation w/ each other Includes: Parasitism, Mutualism, & Includes: Parasitism, Mutualism, &

CommensalismCommensalism Often results in Often results in coevolutioncoevolution (both (both

species evolve in response to one another species evolve in response to one another – – ex. many flowers match feeding habits ex. many flowers match feeding habits of their pollinatorsof their pollinators