Population Ecology

Descriptions and Characterizations of Populations

How can we describe populations?

1. Occupying niche

2. Distribution pattern

3.Survivorship pattern

4.Life Tables/Life History

5.Pattern of growth (models)

6.Genetic traits/alleles

Niche• More than just the habitat• An organisms place or role in the environment• The conditions, resources and interactions

necessary for survival and reproduction• Everything needed for an organism’s existence:

_______________________________________

Define population:

________________

________________

________________Individuals in a populationa.

b.

c.

• Population ecology

_____________________________

_________________________________

• Populations – increase through __________________

______________________________

- decrease through _______________ and ______________________out of an area.

Population ecology is the study of how and why populations change

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• Dispersion patterns can be ______, _______, or _______________.– In a ______________

pattern• resources are

often unequally distributed

• individuals are grouped

in patches.

Density and dispersion patterns are important population variables

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• In a ____________ pattern, individuals are– most likely interacting and– equally spaced in the environment.

Density and dispersion patterns are important population variables

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• In a ______________ pattern of dispersion, the individuals in a population are spaced in an unpredictable way.

Density and dispersion patterns are important population variables

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Life tables track survivorship -the chance of an individual surviving to various ages.

Percentage of maximum life span50 1000

0.1

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III

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Survivorship curves plot survivorship as the proportion of individuals from an initial population that are alive at each age.

Idealized models predict patterns of population growth

2 models:

1.__________________________

2. ___________________________

The rate of population increase under ideal conditions is called exponential growth. It can be calculated using the exponential growth model equation, G = r N, in which

• G is the growth rate of the population,

• N is the population size, and

• r is the per capita rate of increase (the average contribution of each individual to population growth).

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Time (months)

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Exponential growth of rabbits

Eventually, one or more limiting factors will restrict population growth:

• The logistic growth model is a description of idealized population growth that is slowed by limiting factors as the populatio nsize increases.

• To model logistic growth, the formula for exponential growth, r N, is multiplied by an expression that describes the effect of limiting factors on an increasing population size.

• K stands for carrying capacity, the maximum population size a particular environment can sustain.

G = rN

(K N)

K

Year1915 1925 1935 1945

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Growth of a population of fur seals

G rN

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0Time

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(N)

G rN (K N)K

Multiple factors may limit population growth

• The logistic growth model predicts that population growth will slow and eventually stop as population density increases.

• At increasing population densities, density-dependent rates result in– declining births and – increases in deaths.

Number of breeding pairs

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Density (beetles/0.5 g flour)

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• In many natural populations, abiotic factors (examples- ) may affect population size well before density-dependent factors become important.

• These are called Density-independent factors are unrelated to population density. These may include:

• ______________

• ______________

• ______________

Month

Exponentialgrowth

Suddendecline

Apr May Jun Jul Aug Sep Oct Nov Dec

Nu

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Some populations have “boom-and-bust” cycles

• Some populations fluctuate in density with regularity.

• Boom-and-bust cycles may be due to– food shortages or– predator-prey interactions.

Snowshoe hare

Lynx

Year1850 1875 1900 1925

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Predator-Prey Relationship

Evolution shapes life histories

• The traits that affect an organism’s schedule of reproduction and death make up its life history.

• Key life history traits include– age of first reproduction,– frequency of reproduction, – number of offspring, and– amount of parental care.

• Populations with so-called r-selected life history traits– _______________________________________– _______________________________________

• Populations with K-selected traits– _______________________________________– _______________________________________

• Most species fall between these two extremes.

Evolution shapes life histories

• A long-term project in Trinidad– studied guppy populations,– provided direct evidence that life history traits can be

shaped by natural selection, and– demonstrated that questions about evolution can be

tested by field experiments.

Evolution shapes life histories

Pool 1Predator: Killifish;preys onsmallguppies

Guppies:Larger atsexual maturity

Pool 2Predator: Pike-

cichlid;preys on large guppies

Guppies: Smaller atsexual maturity

Hypothesis: Predator feeding preferences caused difference in lifehistory traits of guppy populations.

Pool 3Pools with killifishbut no guppiesprior to transplant

Experiment:Transplantguppies

Control:Guppies from pools withpike-cichlids as predators

Experimental:Guppies transplanted to poolswith killifish as predators

Males Females Males Females

67.5 76.1

161.5185.6

4080

120160200 100

80604020

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mg

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48.558.2

85.7 92.3

Principles of population ecology have practical applications

• Sustainable resource management involves:

_______________________

_______________________

• Example:The cod fishery off Newfoundland– was overfished,

– collapsed in 1992, and

– still has not recovered.

• Resource managers use population ecology to determine sustainable yields.

1960 1970 1980 1990 20000

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Resource managers use population ecology to determine sustainable yields

Random mutations may result in new phenotypes within a population

Founder Effect

Natural selection

is simply the logical result of four features of living systems: variation - individuals in a population vary from one another inheritance - parents pass on their traits to their offspring genetically selection - some variants reproduce more than others time - successful variations accumulate over many generations

THE HUMAN POPULATION

The human population continues to increase, but the growth rate is slowing

• The human population– grew rapidly during the 20th century and– currently stands at about 7 billion.

• The demographic transition– is the shift from high birth and death rates– to low birth and death rates, and– has lowered the rate of growth in developed countries.

Population increase

Total population size

Year1500 1550 1600 1650 1700 1750 1800 1850 1900 1950 2000 2050

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Rate ofincrease

Birth rateDeath rate

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Demographic transition in Mexico

• In the developing nations– death rates have dropped,– birth rates are still high, and– these populations are growing rapidly.

The human population continues to increase, but the growth rate is slowing

• The age structure of a population– is the proportion of individuals in different age groups– affects the future growth of the population.

• Population momentum is the continued growth that occurs– despite reduced fertility and – as a result of girls in the 0–14 age group of a

previously expanding population reaching their childbearing years.

36.9 The human population continues to increase, but the growth rate is slowing

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Age structures reveal social and economic trends

• Age-structure diagrams reveal– a population’s growth trends and– social conditions.

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Birth years

80–8475–7970–7465–6960–6455–5950–5445–4940–4435–3930–3425–2920–2415–1910–14

5–90–4

Ag

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85Male Female

1985

before 19011901–19051906–10

1911–151916–20

1921–251926–301931–351936–40

1941–451946–50

1951–551956–601961–65

1971–751976–80

1981–85

1966–70

12 10 8 6 4 2 0 2 4 6 8 10 12Population in millions

Total population size 238,466,283

Birth years Male Female2010

Birth years Male Female2035

12 10 8 6 4 2 0 2 4 6 8 1012Estimated population in millions

Total population size 310,232,863Projected population in millions

Total population size 389,531,156

12 10 8 6 4 2 0 2 4 6 8 1012

before 19261926–30

1931–351936–40

1941–451946–50

1951–551956–60

1961–651966–701971–751976–801981–85

1986–901991–95

1996–20002001–20052006–2010 2031–35

2026–302021–252016–202011–152006–102001–05

1996–20001991–951986–90

1981–851976–801971–751966–701961–65

1956–601951–55

before 1951

Age structures for the United States in 1985, 2010 (estimated), and 2035 (projected)

An ecological footprint is a measure of resource consumption

• The U.S. Census Bureau projects a global population of – 8 billion people within the next 20 years and– 9.5 billion by mid-21st century.

• Do we have sufficient resources to sustain 8 or 9 billion people?

• To accommodate all the people expected to live on our planet by 2025, the world will have to double food production.

• An ecological footprint is an estimate of the amount of land required to provide the raw materials an individual or a nation consumes, including– food,– fuel,– water, – housing, and– waste disposal.

36.11 CONNECTION: An ecological footprint is a measure of resource consumption

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• The United States– has a very large ecological footprint, much greater

than its own land, and– is running on a large ecological deficit.

• Some researchers estimate that– if everyone on Earth had the same standard of living

as people living in the United States, – we would need the resources of 4.5 planet Earths.

Families in India (left) and the United States (right) display their possessions

Ecology Unit Objectives

1. Define a population and population ecology.2. Define population density and describe different types

of dispersion patterns.3. Explain how life tables are used to track mortality and

survivorship in populations.4. Compare Type I, Type II, and Type III survivorship

curves.5. Describe and compare the exponential and logistic

population growth models, illustrating both with examples.

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You should now be able to

6. Explain the concept of carrying capacity.7. Describe the factors that regulate growth in natural

populations.8. Define boom-and-bust cycles, explain why they occur,

and provide examples.9. Explain how life history traits vary with

environmental conditions and with population density.10. Compare r-selection and K-selection and indicate

examples of each.

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You should now be able to

11. Describe the major challenges inherent in managing populations.

12. Explain how the structure of the world’s human population has changed and continues to change.

13. Explain how the age structure of a population can be used to predict changes in population size and social conditions.

14. Explain the concept of an ecological footprint. Describe the uneven use of natural resources in the world.

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