Population Biology – an Introduction

Population Biology Population Biology – an Introduction– an Introduction

Mark MayoMark Mayo

Cypress CollegeCypress College

Last update: 11/12/13

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PopulationsPopulations

Population biologyPopulation biology – the study of populations of animals and the study of populations of animals and

plants, plants, – a a populationpopulation being a group of being a group of

interbreeding organisms in a specific interbreeding organisms in a specific region —for example, the members of a region —for example, the members of a fish species on a reeffish species on a reef

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PopulationPopulation– a given population a given population is usually isolatedis usually isolated to to

some degree from others of its species, some degree from others of its species, whether geographically or in terms of whether geographically or in terms of behavioral or anatomical differences, behavioral or anatomical differences, but its boundaries may be vaguebut its boundaries may be vague

– for example, the fish in a lake may also for example, the fish in a lake may also interbreed with the fish of interbreed with the fish of interconnecting waterways.interconnecting waterways.

– a population is a useful, if occasionally a population is a useful, if occasionally artificial, unit for study artificial, unit for study

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Population densityPopulation density = = individuals/unit area individuals/unit area **

12,000 deer per square mile 12,000 deer per square mile ** sometimes only reproductively active sometimes only reproductively active

individuals countedindividuals counted

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Patterns of population Patterns of population dispersaldispersal

clumped clumped ** – like humans (something – like humans (something makes an area favorable: soil, makes an area favorable: soil, weather, rainfall, salinity)weather, rainfall, salinity)

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uniformuniform **– very evenly spaced (creosote – very evenly spaced (creosote bushes in the desert are very, very evenly bushes in the desert are very, very evenly spaced)spaced)– they effectively remove limited water from surrounding they effectively remove limited water from surrounding

areaarea– animals use the shade and protection of the bush to animals use the shade and protection of the bush to

forage for seedsforage for seeds– only successful seeds are in areas where a plant has only successful seeds are in areas where a plant has

dieddied

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random spacingrandom spacing **– no pattern – no pattern wolf spiders in forestswolf spiders in forests ferns in the forestferns in the forest

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Population Size Changes Population Size Changes

Factors that change a population Factors that change a population **– emigrationemigration – loss of individuals from a – loss of individuals from a

populationpopulation– immigrationimmigration – new individuals move in – new individuals move in

and live permanentlyand live permanently– birthbirth– death death

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Types of population growthTypes of population growth

Calculation of population growthCalculation of population growth– per capitaper capita – per head (how many do you – per head (how many do you

produce, how about your neighbor, etc.)produce, how about your neighbor, etc.)– birth ratebirth rate** = number of births/total = number of births/total

population in a specific time periodpopulation in a specific time period example – 100 births/400 rats in one monthexample – 100 births/400 rats in one month

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Calculation of population growthCalculation of population growth– must also factor in deaths so you need a must also factor in deaths so you need a

death rate death rate ** = deaths/total population = deaths/total populationin a specific time in a specific time

periodperiod example – 40 deaths/400 rats in one monthexample – 40 deaths/400 rats in one month

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net reproduction rate net reproduction rate ** per individual per individual

rate or rate or rr:: subtract death rate from birth ratesubtract death rate from birth rate r r ** = br – dr ( = br – dr (this is NOT births – this is NOT births –

deathsdeaths))– our example - 0.25 – 0.10 = 0.15 / our example - 0.25 – 0.10 = 0.15 /

month = rmonth = r

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Types of population growthTypes of population growth G – population growth per unit timeG – population growth per unit time r - net reproduction rate per individual per unit r - net reproduction rate per individual per unit

timetime N – number of individuals in a populationN – number of individuals in a population G = rNG = rN

– our example: G = (0.15) x 400 = 60 our example: G = (0.15) x 400 = 60 newnew individuals/month individuals/month

– After one month the population is G PLUS THE ORIGINAL After one month the population is G PLUS THE ORIGINAL POPULATION POPULATION G+NG+N

– This is the NEW N after one month!!!This is the NEW N after one month!!!

– Compute G + N for all of my homework Compute G + N for all of my homework problems! problems! **

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Zero population growthZero population growth – number of – number of births is balances by the number of births is balances by the number of deaths for a given interval of timedeaths for a given interval of time

G = 0G = 0 births = deathsbirths = deaths

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Graphing Population GrowthGraphing Population Growth

Exponential GrowthExponential Growth – follows a J –shaped curve – follows a J –shaped curve (book examples are much steeper and indicate (book examples are much steeper and indicate faster growth rates) – all are exponential!faster growth rates) – all are exponential!

as you can see in the graph below exponential as you can see in the graph below exponential rates increase on the y axis much faster than on rates increase on the y axis much faster than on the x axis (exponentially more!)the x axis (exponentially more!)

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Graphing Population GrowthGraphing Population Growth

logistical growthlogistical growth – s shaped curve – s shaped curve j shaped at firstj shaped at first slows then levels off as carrying capacity slows then levels off as carrying capacity

is reachedis reached

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Graphing Population Growth Graphing Population Growth **

A – exponential growth A – exponential growth regionregion

B – population that has B – population that has exceeded the carrying exceeded the carrying capacity of the capacity of the environmentenvironment

C – population that is C – population that is dying out possibly due dying out possibly due to over population (B)to over population (B)

D – stable population D – stable population in equilibrium with the in equilibrium with the environmentenvironment

E – carrying capacity of E – carrying capacity of the environment the environment

There are 4 exam questions

on this slide !!!

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Population GrowthPopulation Growth doubling time doubling time ** – period of time needed to – period of time needed to

double the population – troubling for double the population – troubling for humanshumans– 1650 AD – 200 years1650 AD – 200 years– 1960 – 30 years1960 – 30 years– 1975 – 15 years1975 – 15 years– 1987 – 12 years1987 – 12 years– 2012 - 61 years (finally an improvement!)2012 - 61 years (finally an improvement!)

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Population GrowthPopulation Growth Doubling time *Doubling time *

An example of a test-like question…An example of a test-like question…

In 1932, the mule deer population in In 1932, the mule deer population in Montana was 200,000 and after 20 years of Montana was 200,000 and after 20 years of careful wildlife management the population careful wildlife management the population had grown to 394,000. This represents the:had grown to 394,000. This represents the:

– a. polyploidya. polyploidy b. logistical growthb. logistical growthc. doubling timec. doubling time d. exponential growthd. exponential growth

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Population GrowthPopulation Growth

Biotic potentialBiotic potential – maximum – maximum reproductive ratereproductive rate

rarely do conditions permit biotic rarely do conditions permit biotic potentialpotential– sheltershelter– food supplyfood supply– waterwater– mineralsminerals– energyenergy– all of the above and more are collectively all of the above and more are collectively

limiting factors to population growthlimiting factors to population growth

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Population GrowthPopulation Growth

the the carrying capacity carrying capacity ** (E) (E) is the maximum is the maximum number of individuals number of individuals that a particular that a particular environment can environment can supportsupport

sometimes a population sometimes a population can increase beyond the can increase beyond the carrying capacity with carrying capacity with precipitous declines precipitous declines shortly thereafter (C) shortly thereafter (C)

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Density dependent vs. Density dependent vs. independent population controlsindependent population controls

density dependent density dependent ** – these factors – these factors increase or become increase or become more prominent more prominent when the population when the population density increasesdensity increases– diseasedisease– predationpredation– stressstress– pollutionpollution– parasitesparasites– pathogenspathogens

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Density dependent vs. Density dependent vs. independent population controlsindependent population controls

density independent density independent ** – these factors – these factors have their actions have their actions without regard without regard for population for population densitydensity– weather – lightning, weather – lightning,

temperature, temperature, snowfall, droughtsnowfall, drought

– natural disasters andnatural disasters andother catastrophic other catastrophic eventsevents

– seasonal cyclesseasonal cycles– total spacetotal space

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Survivorship CurvesSurvivorship Curves

There are three stages in each curve:There are three stages in each curve:– Dependency (Pre-reproductive) Dependency (Pre-reproductive) – Reproductive Reproductive – Post-Reproductive Post-Reproductive

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The three types of curves are:The three types of curves are:– Type I Type I ** - Most die in post-reproductive stage, - Most die in post-reproductive stage,

i.e. at end of physiological life span i.e. at end of physiological life span (humans, large mammals under favorable (humans, large mammals under favorable conditions like zoos, annual plants)conditions like zoos, annual plants)

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The three types of curves are:The three types of curves are:– Type II Type II ** - Death rate - Death rate

is constant over is constant over lifespan, e.g. birds, lifespan, e.g. birds, rodents, perennial rodents, perennial plantsplants

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The three types of curves are:The three types of curves are:– Type III Type III ** - Most die early in life - fish, - Most die early in life - fish,

invertebrates (such as starfish or invertebrates (such as starfish or oysters), perennial plantsoysters), perennial plants

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Population Biology – an Introduction