Population Ecology

Study of populations in relation to environment

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

Definition of Population:

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

Density: A Dynamic Perspective

• Determining the density of natural populations is possible, but difficult to accomplish

• In most cases it is impractical or impossible to count all individuals in a population

– How do wildlife biologists approximate populations?

Estimating Wildlife Population Size

Defined Populations-complete counts-incomplete counts-indirect counts

Undefined Populations

Mark and Recapture

• 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

Patterns of Dispersion• Environmental and social factors influence the

spacing of individuals in a population

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.

Life history traits are products of natural selection

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

behavior of an organism

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

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Reduced brood size Normal brood size Enlarged brood

size

Par

ents

sur

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(%)

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

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8,000

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–Cannot be sustained for long in any population

Logistic Population Growth

• A more realistic population model– Limits growth by incorporating carrying

capacity

Logistic 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

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(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

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of

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eci

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/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

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0 16014012080 100604020

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(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

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1975 1980 1985 1990 1995 2000

Time (years)

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(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.

The Logistic Model and Life Histories

• Life history traits favored by natural selection– May vary with population density and

environmental conditions

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

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Seeds planted per m2 Density of females

0 7010 20 30 40 50 60 802.8

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(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.

• Cheetahs are highly territorial

– Using chemical communication to warn other cheetahs of their boundaries

• 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

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atch

(kg

) of

m

ale

crab

s (l

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cale

)

Metapopulations and Immigration

• Metapopulations– Groups of populations linked by immigration

and emigration

Immigration- Movement Into a Population

• High levels of immigration combined with higher survival can result in greater stability in populations

Figure 52.20

Mandarte island

Small islands

Nu

mb

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fe

ma

les

1988 1989 1990 1991Year

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

cycles

Year1850 1875 1900 1925

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Lynx

pop

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(t

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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

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(bill

ions

)

2000 A.D.

0

1

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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

• One important demographic factor in present and future growth trends– Is a country’s age structure, the relative

number of individuals at each age

• 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

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mo

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(de

ath

s p

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Life

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yea

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