copyright © 2005 pearson education, inc. publishing as benjamin cummings population ecology...

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Population Ecology

• Definitions

• Population growth

• Population regulation

• Environmental factors that regulate growth

• Human population growth and regulation


• Population ecology is the study of populations in relation to environment

– Including environmental influences on population density and distribution, age structure, and variations in population size


• Dynamic biological processes influence population density, dispersion, and demography

• A population

– Is a group of individuals of a single species living in the same general area


• Density is the result of a dynamic interplay

– Between processes that add individuals to a population and those that remove individuals from it

Figure 52.2

Births and immigration add individuals to a population.

Births Immigration

PopuIationsize

Emigration

Deaths

Deaths and emigration remove individuals from a population.


Potential for population increase

• It is quite high

• A single bacterium can reproduce by fission every 20 min, in 36 hours there will be enough bacteria to form a layer foot deep over the entire world

• A pair of elephants could produce a population of 19 million in 750 years


Geometric/Exponential Models

• Populations in which reproduction is restricted to a particular season of the year have non-overlapping generations. Their growth is modeled using geometric equations (time interval is discrete)

• In populations in which reproduction happens continuously, their growth can be modeled using exponential equations (time interval is continuous)


How do populations grow?

We can envision a population consisting of few individuals living in an ideal, unlimited environment; the only restrictions: inherent physiological limitations due to life history

– The population will increase in size with:

Change in populationsize during time interval

= Births + Immigration− Deaths − Emigration

during time interval


Verbal equation of population growth

For simplicity:


= Births during time interval

− Deaths duringtime interval


Exponential model


= Births during time interval

− Deaths duringtime interval

Let N = population size; t = time, ΔN = change in population size;Δt = change in time


Then now we have:

ΔN/Δt= B−D

Where: B= No. of births in the population during the time intervalD= No. of deaths in the population during the time interval


We can now express births and deaths as the average numberof births and deaths per individual during a specified time interval:b = per capita birth rated = per capita death rate

We can obtain the numbers of births and deaths in apopulation by multiplying the per capita birth rate times the population size and the per capita death rate times the population size.

Hence, we can revise the population growth equation using the per capita rates:

ΔN/Δt = bN − dN


We combine the per capita birth and death rates into the percapita growth rate:

r = b − d

A population grows when r is positive, declines when r isnegative, or stays the same when r = 0


We can now rewrite the equation using the per capita growthrate:

ΔN/Δt = rN

Assuming reproduction happens continuously, we can use differential calculus notation to express the equation as follows:

dN/dt = rN


In a population increasing under ideal environmental conditions,the per capita growth rate may assume the maximum growthrate for the species called intrinsic rate of increase, rmax. The equation for exponential population growth is then:

dN/dt = rmaxN


• Exponential population growth

– Results in a J-shaped curve

Figure 52.9

0 5 10 150

500

1,000

1,500

2,000

Number of generations

Pop

ulat

ion

size

(N

)

dNdt

1.0N

dNdt

0.5N


• The J-shaped curve of exponential growth

– Is characteristic of some populations that are rebounding

Figure 52.10

1900 1920 1940 1960 1980

Year

0

2,000

4,000

6,000

8,000

Ele

phan

t pop

ulat

ion


• The logistic growth model includes the concept of carrying capacity

• Exponential growth

– Cannot be sustained for long in any population

• A more realistic population model

– Limits growth by incorporating carrying capacity


• Carrying capacity (K)

– Is the maximum population size the environment can support


The Logistic Growth Model

• In the logistic population growth model

– The per capita rate of increase declines as carrying capacity is reached


• We construct the logistic model by starting with the exponential model

– And adding an expression that reduces the per capita rate of increase as N increases

Figure 52.11

Maximum

Positive

Negative

0N K

Population size (N)

Per

cap

ita r

ate

of in

cre

ase

(r)


Logistic equation

• Maximum sustainable population size is K

• K − N tells us how many additional individuals can the environment sustain

• (K − N)/K tells us what fraction of K is still available for population growth

• Hence:

dN/dt = rmaxN (K−N)

K


• A hypothetical example of logistic growth

Table 52.3


• The logistic model of population growth

– Produces a sigmoid (S-shaped) curve

Figure 52.12

dNdt

1.0N Exponential growth

Logistic growth

dNdt

1.0N1,500 N

1,500

K 1,500

0 5 10 150

500

1,000

1,500

2,000

Number of generations

Pop

ulat

ion

size

(N

)


How well do these populations fit the logistic population growth model?


• Populations are regulated by a complex interaction of biotic and abiotic influences

• There are two general questions we can ask

– About regulation of population growth


• What environmental factors stop a population from growing?

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


Population Change and Population Density

• In density-independent populations

– Birth rate and death rate do not change with population density

• In density-dependent populations

– Birth rates fall and death rates rise with population density


Density-Dependent Population Regulation

• Density-dependent birth and death rates

– Are an example of negative feedback that regulates population growth

– Are affected by many different mechanisms


Competition for Resources

• In crowded populations, increasing population density

– Intensifies intraspecific competition for resources

Figure 52.15a,b

100 100

100

0

1,000

10,000

Ave

rag

e n

um

be

r o

f se

ed

s p

er

rep

rod

uci

ng

ind

ivid

ua

l (lo

g s

cale

)

Ave

rag

e c

lutc

h s

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.


Territoriality

• In many vertebrates and some invertebrates

– Territoriality may limit density


• Cheetahs are highly territorial

– Using chemical communication to warn other cheetahs of their boundaries

Figure 52.16


• Oceanic birds

– Exhibit territoriality in nesting behavior

Figure 52.17


Health

• Population density

– Can influence the health and survival of organisms

• In dense populations

– Pathogens can spread more rapidly


Predation

• As a prey population builds up

– Predators may feed preferentially on that species


Toxic Wastes

• The accumulation of toxic wastes

– Can contribute to density-dependent regulation of population size


• Human population growth has slowed after centuries of exponential increase

• No population can grow indefinitely

– And humans are no exception


The Global Human Population

• The human population

– Increased relatively slowly until about 1650 and then began to grow exponentially

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


• Though the global population is still growing

– The rate of growth began to slow approximately 40 years ago

Figure 52.231950 1975 2000 2025 2050

Year

2003

Per

cent

incr

ease

2.2

2

1.6

1.4

1.2

1

0.8

0.6

0.4

0.2

0

1.8


Regional Patterns of Population Change

• To maintain population stability

– A regional human population can exist in one of two configurations


• Zero population growth = High birth rates – High death rates

• Zero population growth = Low birth rates – Low death rates


• The demographic transition

– Is the move from the first toward the second state

Figure 52.24

50

40

20

0

30

10

1750 1800 1850 1900 1950 2000 2050

Birth rateDeath rate

Birth rateDeath rate

Year

Sweden Mexico

Birt

h or

dea

th r

ate

per

1,00

0 pe

ople


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


• Age structure diagrams

– Can predict a population’s growth trends

– Can illuminate social conditions and help us plan for the future


Global Carrying Capacity

• Just how many humans can the biosphere support?


Estimates of Carrying Capacity

• The carrying capacity of Earth for humans is uncertain


Ecological Footprint

• The ecological footprint concept

– Summarizes the aggregate land and water area appropriated by each nation to produce all resources it consumes and to absorb all wastes it generates

– Is one measure of how close we are to the carrying capacity of Earth


• Ecological footprints for 13 countries

– Show that the countries vary greatly in their footprint size and their available ecological capacity

Figure 52.27

16

14

12

10

8

6

4

2

00 2 4 6 8 10 12 14 16

New Zealand

AustraliaCanada

Sweden

WorldChina

India

Available ecological capacity (ha per person)

SpainUK

Japan

GermanyNetherlands

Norway

USA

Eco

log

ica

l foo

tprin

t (h

a pe

r pe

rson

)

copyright © 2005 pearson education, inc. publishing as benjamin cummings population ecology...

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