Population Ecology

Definition of Population:

• Group of individuals of a single species living in a specific geographic region at the same time

Study of populations in relation to environment

– Environmental influences on:• population density • age structure• population distribution (dispersion)

• Density is the result of a dynamic interaction of processes that add individuals to a population and

those that remove individuals from itBirths and immigration add individuals to a population.

Births Immigration

PopuIationsize

Emigration

Deaths

Deaths and emigration remove individuals from a population.

How do these factors Contribute to Population Size??

• Births• Deaths• Immigration• Emigration

Clumped Dispersion

– Individuals aggregate in patches– May be influenced by resource availability

and behavior

Uniform Dispersion– Individuals are evenly distributed– May be influenced by social interactions

such as territoriality

Random Dispersion

• Position of each individual is independent of other individuals

(c) Random. Dandelions grow from windblown seeds that land at random and later germinate.

How do wildlife biologists quantify populations?

Defined Populations-complete counts-incomplete counts-indirect counts

Undefined Populations

Mark and Recapture

Life history traits are products of natural selection

• Life history traits are evolutionary outcomes– Reflected in the development, physiology, and

behavior of an organism

• Three things needed quantify life historyi. When reproduction beginsii. How often organism reproducesiii. How many offspring produced

Semelparity: Big Bang– Reproduce a single time and die

– putting all available resources into maximizing reproduction at the expense of future life 

Iteroparity – Repeated Reproduction– produce offspring repeatedly over time– increased parental care along with

enhanced energetic investment per offspring

“Trade-offs” and Life Histories• Organisms have finite resources

The lower survival rates of kestrels with larger broods indicate that caring for more offspring negatively affects survival of the parents.

CONCLUSION

100

80

60

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20

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Reduced brood size Normal brood size Enlarged brood

size

Par

ents

sur

vivi

ng th

e fo

llow

ing

win

ter

(%)

Male

Female

– Which may lead to trade-offs between survival and reproduction

RESULTS

– Kestrels:

• Produce a few eggs?

– Can invest more into each, ensuring greater survival

• Produce many eggs?

– Costly but if all survive, fitness is better

More is Better?• Some plants produce a large number of small

seeds– Ensuring that at least some of them will

grow and eventually reproduce

Fewer is Better?

• Other types of plants produce a moderate number of large seeds

– That provide a large store of energy that will help seedlings become established

Demography

• Study of the vital statistics of a population– And how they change over time

• Death rates and birth rates

• Zero population growth – Occurs when the birth rate equals the death

rate

Exponential Population Growth

Population increase under idealized conditionsNo limits on growth

• Under these conditions– The rate of reproduction is at its maximum,

called the intrinsic rate of increase

Example-understanding growth

Question: I offer you a job for 1 cent/day and your pay will double every day. You will be hired for 30 days. Will you take my job offer?

Answer: If you said YES, you will have made $~21 million dollars for 30 days of work.

How is this possible?????

1ST DAY OF WORK: 1 cent pay/day

30TH DAY OF WORK: ~10.2 million/day

How is this possible?????

Am

ou

nt

of

Pay/D

ay

# of Days

Exponential Growth Model*Idealized population in an unlimited

environment

*Very rapid doubling time; steep J curve

*r=N=(b-d)N tr=instrinsic rate of growth

dNdt rmaxN

Exponential Growth in the Real World

• Characteristic of some populations that are rebounding

1900 1920 1940 1960 1980Year

0

2,000

4,000

6,000

8,000

Ele

phan

t po

pula

tion

–Cannot be sustained for long in any population

Logistic Population Growth

• More realistic model of population growth

• Carrying capacity (K)– Is the maximum population size the

environment can support

• In the logistic population growth model– The per capita rate of increase declines as

carrying capacity is reached

Logistic Population Growth

– Produces a sigmoid (S-shaped) curve

Figure 52.12

dN

dt 1.0N Exponential

growth

Logistic growth

dN

dt 1.0N

1,500 N1,500

K 1,500

0 5 10 150

500

1,000

1,500

2,000

Number of generations

Pop

ulat

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size

(N

)

dNdt

(K N)Krmax N

800

600

400

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0

Time (days)

0 5 10 15

(a) A Paramecium population in the lab. The growth of Paramecium aurelia in small cultures (black dots) closely approximates logistic growth (red curve) if the experimenter maintains a constant environment.

1,000

Nu

mb

er

of

Pa

ram

eci

um

/ml

The Logistic Model and Real Populations

• The growth of laboratory populations of Paramecium

– Fits an S-shaped curve

Logistic Growth and The Real World

• Some populations overshoot K

– Before settling down to a relatively stable density

180

150

0

120

90

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30

Time (days)

0 16014012080 100604020

Nu

mb

er

of

Da

ph

nia

/50

ml

(b) A Daphnia population in the lab. The growth of a population of Daphnia in a small laboratory culture (black dots) does not correspond well to the logistic model (red curve). This population overshoots the carrying capacity of its artificial environment and then settles down to an approximately stable population size.

What type of feedback loop is this?

Logistic Growth and the Real World

• Some populations

– Fluctuate greatly around K

0

80

60

40

20

1975 1980 1985 1990 1995 2000

Time (years)

Nu

mb

er

of

fem

ale

s

(c) A song sparrow population in its natural habitat. The population of female song sparrows nesting on Mandarte Island, British Columbia, is periodically reduced by severe winter weather, and population growth is not well described by the logistic model.

Natural selection (diverse reproductive strategies)a) Relatively few, large offspring (K selected species)b) Many, small offspring (r selected species)

(r selected species)

(K selected species)

Populations Regulated Biotic and Abiotic Factors

Two general questions we can ask about regulation of population growth

1. What environmental factors stop a population from growing?

2. Why do some populations show radical fluctuations in size over time, while others remain stable?

Competition for Resources

• In crowded populations, increasing population density

– Intensifies intraspecific competition for resources

100 100

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10,000

Ave

rag

e n

um

be

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s p

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rep

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ua

l (lo

g s

cale

)

Ave

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lutc

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ize

Seeds planted per m2 Density of females

0 7010 20 30 40 50 60 802.8

3.0

3.2

3.4

3.6

3.8

4.0

(a) Plantain. The number of seeds produced by plantain (Plantago major) decreases as density increases.

(b) Song sparrow. Clutch size in the song sparrow on Mandarte Island, British Columbia, decreases as density increases and food is in short supply.

• Many vertebrates and some invertebrates are territorial– Territoriality may limit density

Territoriality: Ocean birds

– Exhibit territoriality in nesting behavior

Health

Population density-Can influence the health and survival of organisms

In dense populations, pathogens can spread more rapidly

Fluctuations in Population Size• Extreme fluctuations in population size

– Are typically more common in invertebrates than in large mammals

Figure 52.19

1950 1960 1970 1980Year

1990

10,000

100,000

730,000C

omm

erci

al c

atch

(kg

) of

m

ale

crab

s (l

og s

cale

)

Population Cycles• Many populations undergo regular boom-and-bust

cycles

Year1850 1875 1900 1925

0

40

80

120

160

0

3

6

9

Lynx

pop

ulat

ion

siz

e (t

hous

and

s)

Har

e po

pula

tion

size

(t

hous

and

s)

Lynx

Snowshoe hare

• Influenced by complex interactions between biotic and abiotic factors

Human Populations• No population can grow indefinitely and humans are no

exception

Figure 52.22

8000 B.C.

4000 B.C.

3000 B.C.

2000 B.C.

1000 B.C.

1000 A.D.

0

The Plague

Hum

an

pop

ulat

ion

(bill

ions

)

2000 A.D.

0

1

2

3

4

5

6

Global Carrying Capacity

• Just how many humans can the biosphere support?

• Carrying capacity of earth is unknown….

http://www.youtube.com/watch?v=UUOEcNomakw&feature=rec-LGOUT-exp_fresh+div-1r-8-HMhttp://www.youtube.com/watch?v=4B2xOvKFFz4&feature=related

http://www.youtube.com/watch?v=9_9SutNmfFk

• Age structure is commonly represented in pyramids

Figure 52.25

Rapid growth Afghanistan

Slow growth United States

Decrease Italy

Male Female Male Female Male FemaleAge Age

8 6 4 2 0 2 4 6 8 8 6 4 2 0 2 4 6 8 8 6 4 2 0 2 4 6 8Percent of population Percent of population Percent of population

80–8485

75–7970–7465–6960–6455–5950–5445–4940–4435–3930–34

20–2425–29

10–145–90–4

15–19

80–8485

75–7970–7465–6960–6455–5950–5445–4940–4435–3930–34

20–2425–29

10–145–90–4

15–19

Infant Mortality and Life Expectancy

• Infant mortality and life expectancy at birth

– Vary widely among developed and developing countries but do not capture the wide range of the human condition

Figure 52.26

Developed countries

Developing countries

Developed countries

Developing countries

Infa

nt

mo

rta

lity

(de

ath

s p

er

1,0

00

birt

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Life

exp

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yea

rs)

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Search for the Missing Sea Otters

Case Study in Population & Community Ecology

Sea OttersEnhydra lutris

Otter Behavior

http://www.youtube.com/watch?v=YQXKyTWMvpM

http://www.youtube.com/watch?v=dQ2Lrnr0gLc

Population Distribution

GROUP QUESTION: 1) What is happening to sea otter populations at several locations in the Aleutian Islands, Alaska? 2) What factors could be contributing to such a rapid change in the size of sea otter populations? Create a list of possible factors.

Group Question: Read above and eliminate some of the reasons for changes in the relative abundance of sea otters at several locations in the Aleutian Islands, Alaska? What does it have to be?

Could the otters simply have migrated from one part of the region to another? To find out, the researchers analyzed populations over a 500-mile-long stretch of the Aleutians from Kiska to Seguam …. By 1993 otter numbers in that whole stretch had been cut by half. Here the geographical scope of the research effort became critical; a smaller region would not have been large enough to reveal the decline. In 1997, they … found that the population decline had worsened, to about 90 percent ….“That told us for sure it was a very large-scale decline, but we were still trying to understand the cause,” Dr. Estes said …. The researchers … ruled out reproductive failure. Their studies enabled them to keep track of how often otters gave birth and how many young survived, and this revealed that reproduction was continuing to re-supply the population.With other possible causes eliminated, … mortality had to be the explanation. In the past, they had seen temporary declines in otter populations because of starvation, pollution or infectious disease. “In all those cases,” Dr. Estes said, “we find lots of bodies. They get weak and tired and come ashore to die.” This time not a single dead otter was found—a clue, he said, that “something really weird was going on.”

(Excerpted from Stevens, William K. “Search for missing sea otters turns up a few surprises.” New York Times, January 5, 1999.)

Orca or Killer WhaleOrcinus orca

Male vs. Female

Orca prey

Orca predation behavior

http://www.youtube.com/watch?v=Ks40worW_gQ (hunting sea lions)

Orcas & learned hunting behavior? (great white sharks??) :http://www.youtube.com/watch?v=SS6NjdGLVZs&feature=related

https://www.youtube.com/watch?v=ypNg19etJg0

1.Make a list of the types of information about killer whales you believe the scientists might need to test their hypothesis that increased predation by the whales was the cause of the sea otter decline.

2.Describe two experiments that would allow you to test the hypothesis that increased predation by killer whales was the cause of the sea otter decline. Keep in mind the following key components of any good experiment: a control (something to which to compare the treatment), replication (do it more than once), and consideration of confounding factors (what might cause differences other than what you manipulate in your experiment?).

Group Questions

Group question: Interpret the two figures. What do they show? How does this provide evidence that orcas were responsible for the decline in sea otters in Kuluk Bay?

Group question: Why do you think Orcas started eating Sea Otters? Generate a list of possible ideas.

Why did Orcas start eating Sea Otters?

• Fish community of the Bering Sea:– Past:– Ocean perch & herring– High abundance– Very high oil content

Why did Orcas start eating Sea Otters?

• Fish community of the Bering Sea:• Present:

– Pollock – High abundance– Low oil content

Why this change in species composition?

• Where did the herring & perch go???

What were Orcas eating before?

Stellar Sea Lions on Amak Island

Harbor Seals

Population decline

Why care about Sea Otters??

• Habitat:

Giant Kelp Forest

Biodiversity

Sea urchins (Echinoderms)