Ecology I

Population Ecology

Why is it important? •assessing health of population•determining endangered or threatened status•predicting population dynamics

Population size… is the number of individuals present at a given time.

The passenger pigeon was once North America’s most numerous bird, but it is now extinct.

Population density… is the number of individuals per unit area.

In the 19th century, the flocks of passenger pigeons showed high population density.

Population distribution…is the spatial arrangement of

individuals.

a) Randomb) Uniformc) Clumped

Population growthPopulations grow, shrink, or remain stable, depending on rates of birth, death, immigration, and emigration.

(crude birth rate + immigration rate) –

(crude death rate + emigration rate)

= growth rate

Two Modes of Population Growth

1. Exponential growth2. Logistic growth

Represent two different growth patterns or Strategies of populations

Exponential Growth

Also known as a J-curveGrowth is a fixed

percentage of the whole (e.g., 10% per day or year)

Population is growing at its full biotic potential

Exponential growth in a growth curve

Population growth curves show change in population size over time.

Scots pine shows exponential growth

Biotic PotentialMaximum possible growth rate of a

population.Absence of limitations to growth

Example:2 cats plus all their kittens make how

many cats in 10 years? 2 litters/year 2.8 surviving kittens per litter Breeds for 10 years (“breeding life”)

(Drum roll…)(Drum roll…)

ANSWER:80,399,780 cats!!!

Do you need a cat?

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

BioticPotential

J curve

Time

Po

pu

lati

on

Carrying Capacity

Carrying Capacity:

MAXIMUM number of individuals that can be supported

• in a given AREA.

• FOREVER.

Can Populations Grow Forever?

OscillationsOscillations::•Populations Populations overshootovershoot the the carrying capacity carrying capacity and and experience a experience a dieback dieback •Can be regular or irregularCan be regular or irregular

•Overshoot Overshoot When a population When a population surpasses the carrying surpasses the carrying capacity of its environment or capacity of its environment or population explosionpopulation explosion

•Dieback Dieback Population crashPopulation crash

Malthusian Growth

Thomas Malthus:18th century

economistBelieved human

populations tend to grow until they exhaust their resources and become subject to famine, disease or war

Population growth: OscillationsSome populations fluctuate continually above and below carrying capacity, as with this mite.

Population growth: Dampening oscillationsIn some populations, oscillations dampen, as population size settles toward carrying capacity, as with this beetle.

r-Selected Species

r = Growth rateReproduce youngHigh reproductive

ratesMany small

offspringShort-livedFound in unstable

or unpredictable environments

Examples of r-Selected SpeciesWeeds, bacteria, insects, algae, small

mammals

Logistic GrowthAlso known as S-curveGrowth slows as the

population approaches Carrying Capacity

Populations stabilize at carrying capacity

K-Selected SpeciesK = Carrying Capacity

Reproduce later in lifeLow reproductive ratesFewer large offspringLong-livedStable or predictable

environments

Examples of K-Selected Species

Elephants, great apes, hippopotamus, whales, humans (?)

What Determines Population Size or

Carrying Capacity? Environmental ResistanceEnvironmental Resistance

All the limiting factors that tend to reduce population growth rates

BalanceBalance between biotic between biotic potential & environmental potential & environmental resistance determines growth resistance determines growth raterate

Environmentalresistance

Carryingcapacity

Stabilizedpopulation

size

Expo

nent

ial g

row

th

Limiting factors:WaterSpaceFoodPredatorsDisease

LE 4-11

Predator–prey cyclesPopulation dynamics of predator–prey systems sometimes show paired cycles: ups and downs in one drive ups and downs in the other.

Density dependenceOften, survival or reproduction lessens as populations

become more dense.Density-dependent factors that account for the

logistic growth curve Intraspecific competition Interspecific competition Predation Disease Fire (for a population of fire prone plant) Parasitism

Other factors occur regardless of density and are density-independent factors.

Floods Drought Extreme temperatures Pollution Fire Salinity

Exponential Growth Example

0

500

1000

1500

2000

2500

1900 1910 1920 1930 1940 1950 1960

Year

Car

ibou

Pop

ulat

ion

Caribou PopulationCaribou Population

What does this show?•Population overshot the carrying capacity, causing a “die-off”, or sudden reduction in population.

What we observed (actual data):

Exponential Growth ExampleLet’s turn back the clock

to the 1930’s…How might wildlife

managers make the curve level-off to a stable population?

0

500

1000

1500

2000

2500

1910 1920 1930 1940

Year

Car

ibo

u P

op

ula

tio

n

•Hunt them?•Add predators?•Sterilize them?•Move them?•Let them starve?•Import food?

QUESTION: Review

Which is a K-selected species?

a. A dragonfly that lays 300 eggs and flies away

b. An oak tree that drops its acorns each year

c. A bamboo plant that flowers only once every 20 years

d. A human who raises three children

e. A fish on the second trophic level

QUESTION: ReviewHow can you tell that this population growth curve shows exponential growth?

a. Population is increasing.

b. Data points match curve closely.

c. Population is rising by the same number during each interval.

d. Population is rising by the same percentage during each interval.

QUESTION: ReviewThis shows growth ending at a(n) .

a. exponential… carrying capacity

b. intrinsic… equilibrium

c. logistic… carrying capacity

d. runaway… equilibrium

e. logistic… extinction