Chapter 52Chapter 52

Population Ecology: the study of populations in Population Ecology: the study of populations in relation to the environment. This includes relation to the environment. This includes population density & distribution, age structure & population density & distribution, age structure & fluctuations in population size.fluctuations in population size.

Population DensityPopulation Density1.) 1.) Density:Density: the # of individuals per unit area or the # of individuals per unit area or

volume.volume.2.) How measure pop density in a natural 2.) How measure pop density in a natural

population?population?a.) Often impractical to count ALL the a.) Often impractical to count ALL the individuals.individuals.b.) Use samples (count all of a species in small b.) Use samples (count all of a species in small plots & extrapolate to find total number)plots & extrapolate to find total number)c.) Mark & recapture: set traps to capture & c.) Mark & recapture: set traps to capture & mark individuals then release. Set traps again, mark individuals then release. Set traps again, catch both marked & unmarked individuals – use catch both marked & unmarked individuals – use to estimate total # of individuals in population.to estimate total # of individuals in population.

Population Density Population Density cont’d.cont’d.

3.) Density is always changing:3.) Density is always changing:

a.) Births & immigration add to a.) Births & immigration add to population density.population density.

b.) Deaths & emigration remove b.) Deaths & emigration remove individuals.individuals.

Population DispersionPopulation Dispersion

1.) 1.) DispersionDispersion: the pattern of spacing among : the pattern of spacing among individuals within the boundaries of a population. individuals within the boundaries of a population. This can depend on social interactions or just This can depend on social interactions or just how suitable an environment is.how suitable an environment is.

2.) Patterns of dispersion include:2.) Patterns of dispersion include:

a.) Clumpeda.) Clumped

b.) Uniformb.) Uniform

c.) Randomc.) Random

Why these dispersion Why these dispersion patterns?patterns?

1.) 1.) ClumpedClumpeda.) individuals are clumped where there are a.) individuals are clumped where there are good environmental conditions for themgood environmental conditions for themb.) live in groups to hunt & protect each otherb.) live in groups to hunt & protect each other

2.) 2.) UniformUniforma.) usually results from social interactions – a.) usually results from social interactions – defending a territorydefending a territory

3.) 3.) RandomRandoma.) usually occurs in areas where environmental a.) usually occurs in areas where environmental conditions are uniform & dispersion of offspring conditions are uniform & dispersion of offspring random (plants)random (plants)

DemographyDemography

Definition:Definition: the study of the vital statistics the study of the vital statistics (birth & death rates, etc) of a population (birth & death rates, etc) of a population and how they change over time.and how they change over time.

Uses a variety of graphics and calculations Uses a variety of graphics and calculations such as such as life tables, survivorship curves & life tables, survivorship curves & reproductive tablesreproductive tables……

Life TablesLife Tables

1.) Age specific summaries of the survival 1.) Age specific summaries of the survival pattern of a population.pattern of a population.

a.) Usually constructed by following a a.) Usually constructed by following a cohort from birth until all are dead.cohort from birth until all are dead.

b.) b.) CohortCohort: a group of individuals of the : a group of individuals of the same agesame age

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Survivorship CurveSurvivorship Curve1.) 1.) Definition Definition: a plot of the proportion or numbers in a : a plot of the proportion or numbers in a

cohort still alive at each age (you draw this using the info cohort still alive at each age (you draw this using the info from the life table)from the life table)

2.) 2.) Types of survivorship curvesTypes of survivorship curves exhibited by exhibited by populations:populations:a.) a.) Type I:Type I: flat at start (low death rates during early/mid flat at start (low death rates during early/mid life), drops steeply as death rates increase in old age. life), drops steeply as death rates increase in old age. (This is us & many other large mammals that provide (This is us & many other large mammals that provide good parental care)good parental care)b.) b.) Type II:Type II: constant death rate over life span (many constant death rate over life span (many rodents, invertebrates, some plants)rodents, invertebrates, some plants)c.) c.) Type III:Type III: drops sharply at start (high death rates drops sharply at start (high death rates when young) then flattens out (associated with when young) then flattens out (associated with organisms that produce A LOT of offspring w/ little or no organisms that produce A LOT of offspring w/ little or no parental care)parental care)

Type I Type II Type III

Reproductive TableReproductive TableDefinition:Definition: an age specific summary of the an age specific summary of the

reproductive rates in a population. Viewed in reproductive rates in a population. Viewed in terms of females giving rise to more females – terms of females giving rise to more females – b/c only females produce offspring.b/c only females produce offspring.

a.) Usually constructed by measuring the a.) Usually constructed by measuring the reproductive output of a cohort from birth to reproductive output of a cohort from birth to death.death.

b.) Is the product of the proportion of females of b.) Is the product of the proportion of females of an age that are breeding & the # of female an age that are breeding & the # of female offspring of those breeding femalesoffspring of those breeding females

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There are LOTS of There are LOTS of different patterns different patterns of reproduction…of reproduction…Why does one type of pattern evolve in a Why does one type of pattern evolve in a particular population??? That is what “life particular population??? That is what “life history” tries to study…history” tries to study…

Life HistoryLife History

The traits that affect an organism’s schedule of The traits that affect an organism’s schedule of reproduction & survival make up its reproduction & survival make up its LIFE LIFE HISTORY.HISTORY.

Life History Variables:Life History Variables:1.) When does reproduction begin?1.) When does reproduction begin?2.) How often does organism reproduce?2.) How often does organism reproduce?3.) How many offspring are produced during 3.) How many offspring are produced during each reproductive episode?each reproductive episode?

Life history patterns are very diverse…Life history patterns are very diverse…

SemelparitySemelparity

Definition:Definition: “one shot” “one shot” or big bang or big bang reproduction.reproduction.

1.) Individuals 1.) Individuals produce MANY produce MANY offspring in one offspring in one reproductive episode reproductive episode & then typically die.& then typically die.

Why semelparity?Why semelparity?

1.) Favored when the survival rate of 1.) Favored when the survival rate of offspring is low – usually b/c of a highly offspring is low – usually b/c of a highly unpredictable environmentunpredictable environment

a.) High numbers of offspring ensures a.) High numbers of offspring ensures that at least some will survive.that at least some will survive.

IteroparityIteroparity

Definition:Definition: repeated reproduction. Individuals repeated reproduction. Individuals produce a few offspring but have more than produce a few offspring but have more than one reproductive episode.one reproductive episode.

Why iteroparity?Why iteroparity?

1.) Favored in dependable environments 1.) Favored in dependable environments where competition for resources is intense.where competition for resources is intense.

a.) Producing a few, large & well cared a.) Producing a few, large & well cared for offspring increases their chance of survival.for offspring increases their chance of survival.

Life Histories Involve Life Histories Involve Trade-offsTrade-offs

Organisms have limited Organisms have limited resources and resources resources and resources used for reproduction can’t used for reproduction can’t be used for something be used for something else.else.

There is a trade-off b/w There is a trade-off b/w reproduction & survival reproduction & survival which is why life histories which is why life histories are diverse. Example: are diverse. Example: caring for offspring often caring for offspring often negatively impacts parental negatively impacts parental survival!survival!

Concept Check QuestionConcept Check Question

Consider 2 rivers – one is spring fed & has Consider 2 rivers – one is spring fed & has a constant water volume & temperature a constant water volume & temperature and the other drains a desert landscape and the other drains a desert landscape and floods & dries at unpredictable and floods & dries at unpredictable intervals. Which is more likely to support intervals. Which is more likely to support many species of iteroparous animals? many species of iteroparous animals? Why?Why?

Population GrowthPopulation GrowthJust think about this…Just think about this…

Some bacteria can divide every 20 Some bacteria can divide every 20 minutes.minutes.

So if you start w/ 1 bacteria, 20 So if you start w/ 1 bacteria, 20 minutes later you would have 2, minutes later you would have 2, after 40 minutes you would have after 40 minutes you would have 4, after 60 minutes you would 4, after 60 minutes you would have 8 bacteria…etc…have 8 bacteria…etc…

If bacteria continued at this rate If bacteria continued at this rate unchecked, in 36 hours there unchecked, in 36 hours there would be enough bacteria to form would be enough bacteria to form a layer a foot deep over the entire a layer a foot deep over the entire globe! globe!

Oh. My. God.Oh. My. God. Why doesn’t this happen?Why doesn’t this happen?

On to some population growth issues, On to some population growth issues, models, etc…models, etc…

Per Capita Rate of IncreasePer Capita Rate of IncreaseWhat is meant by “per capita?”What is meant by “per capita?”

Basically, “per individual in a population.”Basically, “per individual in a population.”

Applying to population growth:Applying to population growth:

1.) If we ignore imm- & emigration we can define changes 1.) If we ignore imm- & emigration we can define changes in population size as:in population size as:

Change in popChange in pop Births duringBirths during Deaths Deaths duringduring

size during time == timesize during time == time __ __ time time

intervalinterval interval interval interval interval

Per Capita Rate of Increase Per Capita Rate of Increase cont’d…cont’d…Now, let’s make it look mathy…Now, let’s make it look mathy…

∆∆NN = = B - DB - D∆ ∆ tt

∆∆N = change in pop sizeN = change in pop size∆∆t = time interval over which we are measuring t = time interval over which we are measuring pop growthpop growthB = # of births in pop during time intervalB = # of births in pop during time intervalD = # of deaths in pop during time intervalD = # of deaths in pop during time interval

Cont’d…Cont’d…Now, let’s express births as Now, let’s express births as the average # of births per the average # of births per

individual during the time intervalindividual during the time interval – – this is the per this is the per capita birth rate (b).capita birth rate (b).Example: If there are 55 births in one year for a pop of Example: If there are 55 births in one year for a pop of 1,000 individuals, the annual per capita birth rate is 1,000 individuals, the annual per capita birth rate is 55/1,000 = 0.05555/1,000 = 0.055

a.) If we know per capita birth rate (b) we can use the a.) If we know per capita birth rate (b) we can use the formula formula B = bNB = bN to calculate the expected # of births per to calculate the expected # of births per year in a pop of any size.year in a pop of any size.b.) Example: b = 0.055 and the population size is 1,500b.) Example: b = 0.055 and the population size is 1,500

B = bNB = bNB = (0.055)(1,500)B = (0.055)(1,500)B = 82 (births per year)B = 82 (births per year)

Cont’d…Cont’d…

We can calculate the per capita death rate We can calculate the per capita death rate (symbolized by (symbolized by mm for mortality) and use it for mortality) and use it in the same way as well!in the same way as well!

Now our equation can look really official…Now our equation can look really official…

∆∆NN == bN - mNbN - mN∆∆tt

But wait! One more But wait! One more simplification to the simplification to the equation…equation…

The difference b/w the per capita birth rate and the The difference b/w the per capita birth rate and the per capita death rate is the per capita death rate is the per capita rate of per capita rate of increase (r).increase (r).

r = b - mr = b - m

If r > 0 then the pop is growingIf r > 0 then the pop is growing If r < 0 then the pop is decliningIf r < 0 then the pop is declining Zero pop growth occurs when r = 0.Zero pop growth occurs when r = 0.

And now, the final equation And now, the final equation for change in population for change in population size…size…

∆∆NN == rNrN

∆∆tt

So, this equation says that the change in pop So, this equation says that the change in pop size during a time interval = the per capita rate size during a time interval = the per capita rate of increase multiplied by the pop size. of increase multiplied by the pop size.

Exponential GrowthExponential GrowthWhat if a population had unlimited What if a population had unlimited

resources and space and resources and space and mates and no predators???mates and no predators???

Under these ideal conditions, the Under these ideal conditions, the population experiences population experiences exponential growth.exponential growth.

a.) The per capita rate of a.) The per capita rate of increase may reach the increase may reach the maximum rate for the species.maximum rate for the species.b.) This max rate is called the b.) This max rate is called the intrinsic rate of increaseintrinsic rate of increase and is and is symbolized as symbolized as rrmaxmax

c.) If you graph a pop like this c.) If you graph a pop like this over time you will get a over time you will get a characteristic J-shaped curve.characteristic J-shaped curve.d.) Graph at right shows – d.) Graph at right shows – time on X-axis and pop size on time on X-axis and pop size on Y.Y.

Concept Check QuestionConcept Check Question

Where is exponential growth by a plant Where is exponential growth by a plant population more likely – on a newly population more likely – on a newly formed volcanic island or in a mature, formed volcanic island or in a mature, undisturbed rain forest? Why?undisturbed rain forest? Why?

Logistic GrowthLogistic GrowthEven if a pop experiences exponential Even if a pop experiences exponential

growth for a time, it can’t go on forever – growth for a time, it can’t go on forever – as there are more & more individuals, as there are more & more individuals, there are fewer & fewer resources. There there are fewer & fewer resources. There is a limit to the # of individuals that can is a limit to the # of individuals that can occupy a habitat.occupy a habitat.

Carrying capacity (K):Carrying capacity (K): the maximum the maximum population size that a particular population size that a particular environment can support.environment can support.

Logistic GrowthLogistic GrowthLogistic growth model: as a Logistic growth model: as a

population nears carrying population nears carrying capacity (K), the per capita capacity (K), the per capita rate of increase declines.rate of increase declines.

a.) So, when pop size (N) = K, a.) So, when pop size (N) = K, the pop stops growing.the pop stops growing.

b.) If we graph this model of b.) If we graph this model of pop growth, we get a pop growth, we get a characteristic S-shaped characteristic S-shaped (sigmoid) curve.(sigmoid) curve.

c.) Again, the X-axis should c.) Again, the X-axis should be time on the graph at right…be time on the graph at right…

Logistic growth & Real Logistic growth & Real PopulationsPopulationsEven the logistic growth model Even the logistic growth model

doesn’t fit most natural doesn’t fit most natural populations.populations.

a.) Many pops overshoot their a.) Many pops overshoot their KK

b.) Sometimes small pops b.) Sometimes small pops don’t reproduce that well & don’t reproduce that well & can’t increase quickly --- can’t can’t increase quickly --- can’t find mates, are more find mates, are more vulnerable when alone, etc.vulnerable when alone, etc.

c.) The graph at right shows a c.) The graph at right shows a more complex growth pattern more complex growth pattern but not all pops would fit even but not all pops would fit even that…that…

Concept Check QuestionConcept Check Question

Explain why a population that fits the Explain why a population that fits the logistic growth model increases more logistic growth model increases more rapidly at intermediate size than at rapidly at intermediate size than at smaller or larger sizes.smaller or larger sizes.

Logistic Model & Life Logistic Model & Life HistoriesHistories

1.) Different life histories are favored under 1.) Different life histories are favored under the different conditions in the logistic the different conditions in the logistic growth model:growth model:a.) At high pop density (near K), selection a.) At high pop density (near K), selection favors adaptations that enable organisms favors adaptations that enable organisms to compete & survive w/ limited resources.to compete & survive w/ limited resources.b.) At low pop density adaptations b.) At low pop density adaptations promoting rapid reproduction are favored promoting rapid reproduction are favored – lots of resources available.– lots of resources available.

Logistic Model & Life Histories cont’d…Logistic Model & Life Histories cont’d…

1.) K-selection (density-1.) K-selection (density-dependent selection): dependent selection): selection for life selection for life history traits that are history traits that are sensitive to pop sensitive to pop density.density.

a.) Operates in pops a.) Operates in pops living at or near K.living at or near K.

Logistic Model & Life Histories cont’d…Logistic Model & Life Histories cont’d…

2.) r-selection (density-2.) r-selection (density-independent): independent): selection for traits that selection for traits that maximize r (the rate maximize r (the rate of increase).of increase).

a.) Occurs in pops a.) Occurs in pops living below K, w/ little living below K, w/ little competition.competition.

Concept CheckConcept Check

Would k-selected populations be more Would k-selected populations be more likely to be iteroparous or semelparous? likely to be iteroparous or semelparous? Why?Why?

Regulation of Population Regulation of Population SizeSize

Population size is regulated by biotic & Population size is regulated by biotic & abiotic factors.abiotic factors.

Questions to ask:Questions to ask:

a.) What environmental factors stop a a.) What environmental factors stop a population from growing?population from growing?

b.) Why do some populations fluctuate in b.) Why do some populations fluctuate in size while others are stable?size while others are stable?

Density-Dependent Density-Dependent Population RegulationPopulation Regulation

1.) If birth rates and death rates are 1.) If birth rates and death rates are density-dependent, that means as pop density-dependent, that means as pop size (density) increases:size (density) increases:

a.) birth rates decreasea.) birth rates decrease

b.) death rates increaseb.) death rates increase

Density-Dependent Population Density-Dependent Population RegulationRegulation

2.) What are some density-2.) What are some density-dependent regulation factors?dependent regulation factors?

a.) Competition for resourcesa.) Competition for resourcesb.) Territorialityb.) Territorialityc.) Health (diseases are c.) Health (diseases are transmitted more easily in large, transmitted more easily in large, crowded populations)crowded populations)d.) Predationd.) Predatione.) Toxic waste buildup (by-e.) Toxic waste buildup (by-products of metabolic processes products of metabolic processes can build up – limits the amount can build up – limits the amount of microorganisms in small of microorganisms in small spaces)spaces)f.) Intrinsic factors – some pops f.) Intrinsic factors – some pops stop reproducing when they stop reproducing when they reach a certain # (even with reach a certain # (even with unlimited resources). Example: unlimited resources). Example: release stress hormones that release stress hormones that stop mating or increase stop mating or increase aggression, etc.aggression, etc.

Density-Independent Density-Independent Population RegulationPopulation Regulation

Definition: these factors affect population Definition: these factors affect population numbers whether the population is numbers whether the population is crowded or not.crowded or not.

Example: natural disasterExample: natural disaster

Population DynamicsPopulation Dynamics

The study of factors that cause variation in The study of factors that cause variation in population size.population size.

a.) Factors can be biotic or abiotica.) Factors can be biotic or abiotic

b.) Some populations have regular b.) Some populations have regular cycles of growth & decrease.cycles of growth & decrease.

Population Dynamics Population Dynamics cont’d…cont’d…

c.) Predator-prey c.) Predator-prey relationships are relationships are often cyclical.often cyclical.

Human Population Human Population GrowthGrowth

Human Population Human Population GrowthGrowth

1.) In a stable human population, birth rate equals 1.) In a stable human population, birth rate equals death rate. There are 2 ways this can happen:death rate. There are 2 ways this can happen:

a.) ZPG = high birth rate - high death ratea.) ZPG = high birth rate - high death rateb.) ZPG = low birth rate - low death rateb.) ZPG = low birth rate - low death rate

c.) Movement from a to b is called c.) Movement from a to b is called demographic demographic transition.transition.

i.) Happens with an increase in quality of health i.) Happens with an increase in quality of health care & sanitation & increases access to education care & sanitation & increases access to education (usually for women).(usually for women).

Age StructureAge StructureDefinition: the relative number of individuals of each age in Definition: the relative number of individuals of each age in

a populationa population

Infant Mortality & Life Infant Mortality & Life ExpectancyExpectancy

1.) These stats vary widely around the world and 1.) These stats vary widely around the world and are due to many different factors.are due to many different factors.

2.) Think about this…2.) Think about this…

a.) Life expectancy in Afghanistan & Rwanda a.) Life expectancy in Afghanistan & Rwanda is 47 and 39 years respectively…ours is is 47 and 39 years respectively…ours is around 80 years…around 80 years…

b.) While GLOBAL life expectancy increases, it b.) While GLOBAL life expectancy increases, it is decreasing in many countries…is decreasing in many countries…

Human Carrying CapacityHuman Carrying Capacity

““Ecological footprint” concept: the Ecological footprint” concept: the land & water area each nation land & water area each nation needs to produce ALL the needs to produce ALL the resources it consumes & to absorb resources it consumes & to absorb ALL the waste it generates.ALL the waste it generates.

1.) Arable land, pasture, forest (& 1.) Arable land, pasture, forest (& conservation areas), ocean, built-conservation areas), ocean, built-up land & fossil energy land are up land & fossil energy land are distinguished to calculate footprint. distinguished to calculate footprint.

2.) All this land is added up & 2.) All this land is added up & converted to hectares per person converted to hectares per person (1 hectare = 2.47 acres…1 acre is (1 hectare = 2.47 acres…1 acre is about 10 yards short of a football about 10 yards short of a football field, same width)field, same width)

Ecological Footprint Ecological Footprint cont’d…cont’d…You can calculate it for You can calculate it for

individual countries – the individual countries – the footprints vary widely.footprints vary widely.

Each country has an Each country has an “ecological capacity” which “ecological capacity” which is the actual resource base is the actual resource base of the country.of the country.

For US, our footprint is 8.4 ha For US, our footprint is 8.4 ha per person. However, we per person. However, we only have 6.2 ha per only have 6.2 ha per person available --- so, person available --- so, according to this we are according to this we are above our carrying above our carrying capacity.capacity.

http://www.earthday.net/footprint/flash.html