Populations & Limits on Growth

Ch. 5

APES

• Turn in:– Salinity Lab– Predator/Prey Lab– Peppered Moth

Populations

Group of similar individuals/species

Population Distribution

(a) Clumped (elephants)

(b) Uniform (creosote bush)

(c) Random (dandelions)

Why Clumping?

1. Species tend to cluster where resources are available

2. Groups have a better chance of finding resources

3. Protects some animals from predators

4. Packs allow some predators to get prey

How do populations change over time?

Population Ecology # of individuals of a species in an area AND how/why those # change over time Effected by: – Resource competition– Predation– disease

Population Density

# of individuals Area or volume

Determined by external factors (habitat/resources)What is the population density of 200 birds in

Population Density

Population Density

• What is the population density of 200 birds in 20 square miles?

• What about 200 birds in 5 square miles?

• Which area is more densely populated?

Limits on Population GrowthPopulation Increase (+)• Births (b)• Immigration (i)

Population Decrease (-)• Death (d)• Emmigration (e)

Growth Rate (r) (also termed rate of change)

r = b - d• If r (+), population size • If r (-), population size • If r (0), b=d, stationary population

size

Therefore…

• True growth rate includes b, d, i, e

• r = (b-d)+(i-e)

How do populations change over time?

Ideally, population would according to biotic potential

Biotic Potential

Maximum rate at which a population can increase when there are no limits on its rate of growth

Biotic Potential is Influenced by:

1. Reproduction age2. Reproductive

periods3. # of offspring4. Care of offspring

GENERALLY…

Organisms with a high biotic potential:

1. Reproduce early in life2. Have short generation times3. Can reproduce many times4. Have many offspring each time

they reproduce

GENERALLY…

Small organisms have large biotic potentialEX: Housefly – descendants = 5.6 trillion in 13 mo. Bacteria dividing in ½ every 30 min.

Exponential Growth

Results when organisms are growing at biotic potential

Time (t)

Po

pu

lati

on

siz

e (N

)

Exponential Growth

NO POPULATION CAN GROW INDEFINATELY!

Environmental Resistance

All factors acting jointly to limit the growth of a populationPopulation size determined by:

1. Biotic potential2. Environmental resistance

Environmental Resistance

Unfavorable environmental conditions due to resource availability

As environmental conditions deteriorate, b and d

Carrying Capacity (K)

Largest population that can be maintained given fixed resources - S (sigmoid) shaped curve (logistic Growth)

• Reflects influence of Env. Resistance• Populations rarely stays at K• If it rises above K, pop will crash

Time (t)

Po

pu

lati

on

siz

e (N

)

K Carrying capacity

What Happens When Populations Exceed Carrying

Capacity

• Members of populations which exceed their resources will die unless they adapt or move to an area with more resources.

Population Cycles for the Snowshoe Hare and Canada Lynx

Fig. 5-18, p. 118

Minimum Viable Population (MVP)

Estimate of smallest # of individuals necessary to ensure the survival of a population

Below MVP

Extinction = Likely

Species Have Different Reproductive Patterns

1. Asexual reproduction2. Sexual reproduction

Reproductive Strategies

r-strategists• Life based on r

(growth rate -r)• Rapidly increase

numbers• Below carrying

capacities for long periods of time

k-strategists• Life based on

carrying capacity (k)

• Live in a state of equilibrium

• Close to carrying capacity

Figure 9-9Page 196

Nu

mb

er o

f in

div

idu

als

Time

Carrying capacity

K species;experienceK selection

r species;experiencer selection

K

Reproductive Patterns

r strategists (based on study)

• + Live in disturbed environments.• + Ecological generalists.• + Have populations that fluctuate rapidly in size.• + Do not compete well against other species• + Are widely distributed.• + Are slow to respond to ecological opportunities

but live in wide varieties of environments.• + Are short-lived.• + Have many, relatively small young.• + Have short periods of embryonic development.• + Reach adulthood rapidly• + Small sized adults.• + Invest little or no parental care in young.• + Reproduce once per lifetime.• + Early successional species.

r-selected species

Once established – population crash because 1)Changing environment2)Invasion by more competitive

speciesGo through regular BOOM and bust cycles

k strategists (based on study)

• + Live in stable environments.• + Ecological specialists.• + Have populations stable in size.• + Compete well against other species.• + Are restricted in distribution and where they can

live.• + Take rapid advantage of ecological opportunities but

live in specific types of environments.• + Are long-lived.• + Have few, relatively large young.• + Have long periods of embryonic development.• + Reach adulthood slowly.• + Large sized adults.• + Invest intensive parental care in young.• + Reproduce throughout lifetime• + Late successional species.

K-selected species

Do well when pop. size is near KTypically follow logistic growthThrive in constant environment

Many organisms have

reproductive patterns

between the extremes of r &

K, or change from one to the

other depending on environment*

Perc

enta

ge s

urvi

ving

(log

sca

le) 100

10

1

0

Age

Survivorship Curves

Reproductive strategy may give temporary advantage

BUT the availability of suitable

habitat determines ultimate population size

Factors Affecting Population Growth/Population Density

Density Independent Limiting Factors

• Floods• Fires• Hurricanes• Unseasonable

Weather• Habitat Destruction

Density Dependent Limiting Factors

• Competition• Predation• Parasitism• Disease

Fig. 5-13, p. 113

Lower limit of tolerance

Higher limit of tolerance

No organisms

Few organisms Abundance of organisms

Few organisms

No organisms

Po

pu

lati

on

siz

e

Zone of physiological

stress

Optimum rangeZone of

physiological stress

Zone of intolerance

Low Temperature High

Zone of intolerance

Range of Tolerance