2.6.1-.2 Populations

Population Dynamics

A look at the factors that tend to increase or decrease the size of a population.

The population size of a species in a given space at a given time is determined by the interplay between BIOTIC POTENTIAL and ENVIRONMENTAL RESISTANCE.

Biotic potential = growth rate with unlimited resources.

Environmental resistance = all the factors acting jointly to limit population growth.

POPULATION SIZE

Growth factors(biotic potential)

Favorable lightFavorable temperatureFavorable chemical environment (optimal level of critical nutrients)

Abiotic

BioticHigh reproductive rateGeneralized nicheAdequate food supplySuitable habitatAbility to compete for resourcesAbility to hide from or defend against predatorsAbility to resist diseases and parasitesAbility to migrate and live in other habitatsAbility to adapt to environmental change

Decrease factors(environmental resistance)

Too much or too little lightTemperature too high or too lowUnfavorable chemical environment (too much or too little of critical nutrients)

Abiotic

BioticLow reproductive rateSpecialized nicheInadequate food supplyUnsuitable or destroyed habitatToo many competitorsInsufficient ability to hide from or defend against predatorsInability to resist diseases and parasitesInability to migrate and live in other habitatsInability to adapt to environmental change

Four variables change population size:

1. NATALITY = birth rate

2. MORTALITY = death rate

3. IMMIGRATION = rate of organisms moving in

4. EMIGRATION = rate of organisms moving out

Nu

mb

er

of

ind

ivid

uals

Time

Carrying capacity

K species;experienceK selection

r species;experiencer selection

K

REPRODUCTIVE STRATEGIES

Opportunistic or r-Selected Species

cockroach dandelion

Many small offspringLittle or no parental care and protection of offspringEarly reproductive ageMost offspring die before reaching reproductive ageSmall adultsAdapted to unstable climate and environmental conditionsHigh population growth rate (r)Population size fluctuates wildly above and below carrying capacity (K)Generalist nicheLow ability to competeEarly successional species

Fewer, larger offspringHigh parental care and protection of offspringLater reproductive ageMost offspring survive to reproductive ageLarger adultsAdapted to stable climate and environmental conditionsLower population growth rate (r)Population size fairly stable and usually close to carrying capacity (K)Specialist nicheHigh ability to competeLate successional species

elephant saguaro

Competitor or K-Selected Species

SURVIVORSHIP CURVES

Population density affects population growth.

DENSITY INDEPENDENT FACTORS = affect a populations’ size regardless of its population density.

1. Weather

2. Earthquakes

3. Floods

4. Fires

. . . Natural disasters

R-strategists populations are most affected by these.

DENSITY DEPENDENT FACTORS = affect a populations’ size depending on its population density.

1. Predation

2. Disease

3. Availability of food and water

4. Space

Negative Feedback!!

INTERNAL FACTORS = might include density-dependent fertility or size of breeding territory.

EXTERNAL FACTORS = might include predation and disease.

Species interactions influence population growth and carrying capacity = SYMBIOSIS

Competition for resources.

High

Low

Rela

tive p

op

ula

tion

den

sit

y

0 2 4 6 8 10 12 14 16 18

Days

Each species grown alone

Parameciumaurelia

Parameciumcaudatum

High

LowRela

tive p

op

ula

tion

den

sit

y

0 2 4 6 8 10 12 14 16 18Days

Both species grown together

Parameciumaurelia

Parameciumcaudatum

Resource Portioning

PREDATION

PREY POPULATION

PREDATOR POPULATION

Span worm Bombardier beetle

Viceroy butterfly mimicsmonarch butterfly

Foul-tasting monarch butterfly

Poison dart frog When touched, the snake caterpillar changes shape to look like the head of a snake

Wandering leaf insect

Hind wings of io mothresemble eyes of a much larger animal

Avoiding predators

Parasitism

Mutualism

Oxpeckers and black rhinoceros Clown fish and sea anemone

Shark and ramora

Cleaning station

Sabertooth blenny

Cleaner blenny

Commensalism

Herbivory

Population Growth

change in the size of a population over time.

POPULATION = a group of interbreeding organisms (same species) that live in the same place at the same time and compete for the same resources.

Resources = food, water, shelter, mates, and so on . . .

• resources pop. size

• resources pop. size

Populations change in response to environmental stress or changes in environmental conditions.

1. In size = # of individuals2. Density = # of individual / specific space3. Age distribution = proportions / age

group4. Dispersion =

Clumped(elephants)

Uniform(creosote bush)

Random(dandelions)

No population can grow indefinitely!

Every environment has a CARRYING CAPACITY = the maximum number of individuals of a given species that can be sustained indefinitely in a given space.

2.0

1.5

1.0

.5

Nu

mb

er

of

sh

eep

(m

illion

s)

1800 1825 1850 1875 1900 1925Year

Factors that affect carrying capacity:

1. Competition with/in and between species.

2. Natural and human caused catastrophes.

3. Immigration and emigration.4. Seasonal fluctuations in food, water,

shelter, and nesting sites.

A population that has few if any resource limitations grows exponentially.

EXPONENTIAL GROWTH starts out slowly and then proceeds faster and faster as the population increases.

Time (t)

Pop

ula

tion

siz

e (N

)

“J” population growth curve

Bacteria population

8 1024

24 hourslater

LOGISTIC GROWTH involves initial exponential growth and then there is a steady decrease in growth as the population encounters environmental resistance and approaches carrying capacity and levels off.

“S or sigmoid” population

growth curve

Time (t)

Pop

ula

tion

siz

e (N

)

K

Time (t)

Pop

ula

tion

siz

e (N

)K

Exponential phase

Transitional phase

Plateau phase

2,000

1,500

Nu

mb

er

of

rein

deer

1910 1920 1930 1940 1950

Year

1,000

500

Kaibab Plateau

Kaibab Plateau