chapter 53 population ecology. copyright © 2008 pearson education, inc., publishing as pearson...

Chapter 53Chapter 53

Population Ecology

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Overview

• Population ecology is the study of populations in relation to environment, including environmental influences on density and distribution, age structure, and population size.

• A population is a group of individuals of a single species living in the same general area.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

I. Population density, dispersion, and demographics

• Density = number of individuals per unit area

• Dispersion = pattern of spacing among individuals within boundaries of population.

• Density is result of processes that add individuals and that remove individuals.

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• Immigration = adding new outside individuals

• Emigration = movement of individuals out

• Sampling techniques used to estimate densities and total population sizes.

– extrapolation from small samples, an index of population size, or the Mark-Recapture Method.

Population dynamics

Births

Births and immigrationadd individuals toa population.

Immigration

Deaths and emigrationremove individualsfrom a population.

Deaths

Emigration

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A. Patterns of Dispersion

• Clumped dispersion = individuals aggregate in patches. (influenced by resource availability and behavior)

• Uniform dispersion = individuals are evenly distributed. (influenced by social interactions such as territoriality)

• Random dispersion = individuals are independent of other individuals. (occurs in the absence of strong attractions or repulsions)

Patterns of dispersion within a population’s geographic range

(a) Clumped

(b) Uniform

(c) Random

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II. Demographics

• Demography = study of vital statistics of a population and how they change

– Death rates and birth rates

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A. Life Tables & Survivorship Curves

• Life table = age-specific summary of survival pattern of population.

• Follows the fate of a cohort (group of individuals of the same age)

• Survivorship curve = graph of data in a life table.

Survivorship curves for squirrels shows relatively constant death rate

Age (years)20 4 86

10

101

1,000

100

Nu

mb

er o

f su

rviv

ors

(lo

g s

cale

)

Males

Females

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• Survivorship curves can be classified into three general types:

– Type I: low death rates early and middle life, increase among older age (humans)

– Type II: the death rate is constant (squirrels)

– Type III: high death rates for the young, slower death rate for survivors (Fish)

Survivorship Curves

1,000

100

10

10 50 100

II

III

Percentage of maximum life span

Nu

mb

er

of

su

rviv

ors

(lo

g s

ca

le)

I

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B. Reproductive Rates

• Reproductive table (fertility schedule) = age-specific summary of reproductive rates in a population. (describes reproductive patterns of a population)

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III. Life history

• Life history comprises the traits that affect its schedule of reproduction and survival:

– age when reproduction begins

– How often it reproduces

– How many offspring are produced each time

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

IV. “Trade-offs” and Life Histories

• Organisms have finite resources, which may lead to trade-offs between survival and reproduction.

– Animals small#, dandelions large# and coconuts more stored energy

Variation in the size of seed crops in plants

(a) Dandelion

(b) Coconut palm

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V. Exponential model

• Study population growth in an idealized situation.

– helps understand the capacity of species to increase and the conditions that may facilitate this growth.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

• Zero population growth = birth rate = death rate.

• Formula: Nt

rN

N = population size, t = time, and r = per capita rate of increase = birth – death

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

VI. Exponential Growth

• Exponential population growth is population increase under idealized conditions.

– rate of reproduction is at maximum (intrinsic rate of increase)

• Results in a J-shaped curve

• Exponential Growth is not sustainable.

Exponential Growth Model

Number of generations

0 5 10 150

500

1,000

1,500

2,000

1.0N =dNdt

0.5N =dN

dt

Po

pu

lati

on

siz

e (N

)

The J-shaped curve of exponential growth characterizes some rebounding populations

8,000

6,000

4,000

2,000

01920 1940 1960 1980

Year

Ele

ph

ant

po

pu

lati

on

1900

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VII. Logistic model

• Exponential growth cannot be sustained for long in any population.

• Carrying capacity (K) = max population size the environment can support.

• In the logistic population growth model, the rate of increase declines as carrying capacity is reached.

• Logistic model of population growth produces a sigmoid (S-shaped) curve.

• Some populations overshoot K before settling down to a relatively stable density.

Logistic Growth model

2,000

1,500

1,000

500

00 5 10 15

Number of generations

Po

pu

lati

on

siz

e (

N)

Exponentialgrowth

1.0N=dN

dt

1.0N=dN

dt

K = 1,500

Logistic growth1,500 – N

1,500

The growth of laboratory populations fits an S-shaped curve which hovers around the Carrying Capacity of the area.

1,000

800

600

400

200

00 5 10 15

Time (days)

Nu

mb

er o

f P

aram

eciu

m/m

L

Nu

mb

er o

f D

aph

nia

/50

mL

0

30

60

90

180

150

120

0 20 40 60 80 100 120 140 160

Time (days)

(b) A Daphnia population in the lab(a) A Paramecium population in the lab

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VIII. Logistic Model and Life Histories

• LH traits favored by natural selection vary with pop density and env connditions.

• K-selection = density-dependent selection, selects for traits that are sensitive to population density.

• r-selection = or density-independent selection, selects for traits that maximize reproduction.

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IX. Population Change and Population Density

• Density-independent populations, birth and death rates do not change with population density.

• Density-dependent populations, birth rates fall and death rates rise with population density.

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X. Density-Dependent Population Regulation

• Density-dependent rates are an example of negative feedback that regulates population growth.

• They are affected by many factors (competition for resources, territoriality, disease, predation, toxic wastes, and intrinsic factors)

Decreased reproduction at high population densities

Population size

100

80

60

40

20

0200 400 500 600300P

erce

nta

ge

of

juve

nile

s p

rod

uci

ng

lam

bs

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A. Territoriality

• Competition for territory may limit density.

Territoriality

(a) Cheetah marking its territory

(b) Gannets

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B. Other Factors

• Dense populations, pathogens can spread more rapidly

• As prey builds up, predators feed preferentially on that species.

• Accumulation of toxic wastes can contribute to density-dependent regulation of population size.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

XI. Population Dynamics

• Population dynamics = interactions between biotic and abiotic factors that cause variation in population size.

Changes in predation pressure can drive population fluctuations

Wolves Moose

2,500

2,000

1,500

1,000

500

Nu

mb

er o

f m

oo

se

0

Nu

mb

er o

f w

olv

es

50

40

30

20

10

01955 1965 1975 1985 1995 2005

Year

Snowshoe hare

Lynx

Nu

mb

er

of

lyn

x(t

ho

us

an

ds

)

Nu

mb

er

of

ha

res

(th

ou

sa

nd

s)

160

120

80

40

01850 1875 1900 1925

Year

9

6

3

0

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XII. Human population

• Humans can not grow indefinitely

• Global population is still growing, the rate of growth began to slow during the 1960s.

• Most of the current global population growth is concentrated in developing countries.

Human population growth

8000B.C.E.

4000B.C.E.

3000B.C.E.

2000B.C.E.

1000B.C.E.

0 1000C.E.

2000C.E.

0

1

2

3

4

5

6

The Plague

Hu

man

po

pu

lati

on

(b

illio

ns)

7

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

A. Regional Patterns

• Regional human population can exist in one of two configurations:

– Zero population growth = High birth rate – High death rate

– Zero population growth =Low birth rate – Low death rate

• The demographic transition is the move from the first state toward the second state.

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B. Age Structure

• Relative number of individuals at each age.

• Age structure diagrams can predict a population’s growth trends.

Age-structure pyramids for the human population of three countries

Rapid growthAfghanistan

Male Female Age AgeMale Female

Slow growthUnited States

Male Female

No growthItaly

85+80–8475–7970–74

60–6465–69

55–5950–5445–4940–4435–3930–3425–2920–2415–19

0–45–9

10–14

85+80–8475–7970–74

60–6465–69

55–5950–5445–4940–4435–3930–3425–2920–2415–19

0–45–9

10–14

10  10 8 866 4 422 0Percent of population Percent of population Percent of population

66 4 422 08 8 66 4 422 08 8

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C. Limits on Human Population

• Ecological footprint = summarizes the land and water area needed to sustain the people of a nation.

• Countries vary greatly in footprint size and available ecological capacity.

• Our carrying capacity could potentially be limited by food, space, nonrenewable resources, or buildup of wastes.

• Estimated CC of humans is 10-15 billion

Review: Population Growth Curve

Number of generations

K = carrying capacityP

op

ula

tio

n s

ize

(N)

rmax NdNdt

=K – N

K

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

You should now be able to:

1. Define and distinguish between the following sets of terms: density and dispersion; clumped dispersion, uniform dispersion, and random dispersion; life table and reproductive table; Type I, Type II, and Type III survivorship curves; semelparity and iteroparity; r-selected populations and K-selected populations.

2. Explain how ecologists may estimate the density of a species.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

3. Explain how limited resources and trade-offs may affect life histories.

4. Compare the exponential and logistic models of population growth.

5. Explain how density-dependent and density-independent factors may affect population growth.

6. Explain how biotic and abiotic factors may work together to control a population’s growth.