Chapter 53

Population Ecology

Population EcologyA population– Is a group of individuals of a single species living in the same

general area

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

Northern Fur Seals

Density and Dispersion

Density– Is the number of individuals per unit area

or volume

Dispersion– Is the pattern of spacing among

individuals within the boundaries of the population

Population DynamicsDensity is the result of a dynamic interplay between processes that add individuals to a population and those that remove individuals from it

Births and immigration add individuals to a population.

Births Immigration

PopuIationsize

Emigration

Deaths

Deaths and emigration remove individuals from a population.

Ins

Outs

Patterns of Dispersion

Environmental and social factors– Influence the spacing of individuals in a

population

A clumped dispersion– Is one in which individuals aggregate in patches– May be influenced by resource availability and

behavior

(a) Clumped. For many animals, such as these wolves, living in groups increases the effectiveness of hunting, spreads the work of protecting and caring for young, and helps exclude other individuals from their territory.

A uniform dispersion– Is one in which individuals are evenly

distributed– May be influenced by social interactions such

as territoriality

(b) Uniform. Birds nesting on small islands, such as these king penguins on South Georgia Island in the South Atlantic Ocean, often exhibit uniform spacing, maintained by aggressive interactions between neighbors.

A random dispersion– Is one in which the position of each individual

is independent of other individuals

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

Demography

Demography is the study of the vital statistics of a population and how they change over time

Death rates and birth rates are of particular interest to demographers

Life Tables

A life table– Is an age-specific summary of the survival

pattern of a population

Life table of Belding’s ground squirrels

Survivorship Curves

A survivorship curve– Is a graphic way of representing the data in a

life table– Shows a pattern of survival

Survivorship curve for Belding’s ground squirrels

1000

100

10

1

Num

ber

of s

urvi

vors

(lo

g sc

ale)

0 2 4 6 8 10

Age (years)

Males

Females

This survivorship curve shows a fairly constant death rate

Survivorship curves (types I, II, and III)

I

II

III

50 10001

10

100

1,000

Percentage of maximum life span

Num

ber

of s

urvi

vors

(lo

g sc

ale)

Reproductive Rates

A reproductive table, or fertility schedule– Is an age-specific

summary of the reproductive rates in a population

Life History

Life history traits – Traits that affect an organism’s schedule of

reproduction and survival

– They are Trade-Offs between survival and reproduction (parental care, # of offspring, etc.)

– They are very diverse

– They are evolutionary outcomes that are reflected in the development, physiology, and behavior of an organism

Life History

Life history entails 3 basic variables– Age at which reproduction begins– Frequency of reproduction– # of offspring produced per reproductive

cycle

Semelparity, or “big-bang” reproduction

– Reproduce a single time and die

Iteroparity, or repeated reproduction– Produce offspring repeatedly over time

Agave

Rabbit

Life History trade-offResearchers in the Netherlands studied the

effects of parental caregiving in European kestrels over 5 years. The researchers transferred chicks among nests to produce reduced broods (three or four chicks), normal broods (five or six), and enlarged broods (seven or eight). They then measured the percentage of male and female parent birds that survived the following winter. (Both males and females provide care for chicks.)

EXPERIMENT

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

40

20

0Reduced

brood sizeNormal brood

sizeEnlarged

brood size

Par

ents

sur

vivi

ng th

e fo

llow

ing

win

ter

(%)

MaleFemale

RESULTS

Some plants produce a large number of small seeds ensuring that at least some of them will grow and eventually reproduce

(a) Most weedy plants, such as this dandelion, grow quickly and produce a large number of seeds, ensuring that at least somewill grow into plants and eventually produce seeds themselves.

Some plants produce a moderate number of large seeds that provide a large store of energy that will help seedlings become established

(b) Some plants, such as this coconut palm, produce a moderate number of very large seeds. The large endosperm provides nutrients for the embryo, an adaptation that helps ensure the success of a relatively large fraction of offspring.

Population Growth

A Couple Questions for Population Ecologists.– 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

Population-Limiting Factors

Competition

Territoriality

Health

Predation

Waste accumulation

Intrinsic factors (factors related to the biology/physiology of the particular species)

Competition for ResourcesIn crowded populations, increasing population density intensifies intraspecific competition for resources

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.

TerritorialityIn many vertebrates and some invertebrates territoriality may limit density

Cheetahs are highly territorial

– Chemical communication can warn other cheetahs of their boundaries

TerritorialityTerritories can be small

Gannets exhibit territoriality in nesting behavior

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– The Wine Example

• Alcohol content of wine generally peaks out at about 13%. Beyond that level the yeast die off from ethanol toxicity

Intrinsic Factors

For some populations– Intrinsic (physiological) factors appear to regulate population

size

– (Such as an increase in aggressive behavior in a crowded colony of animals leading to a decrease in reproduction rate) Stress has a negative impact on the reproductive system.

Population Dynamics

Population dynamics– Focuses on the interactions between biotic and

abiotic factors that cause variation in population size

Extreme fluctuations in population size are typically more common in invertebrates than in large mammals 1950 1960 1970 1980

Year

1990

10,000

100,000

730,000

Com

mer

cial

cat

ch (

kg)

of

mal

e cr

abs

(log

sca

le)

Metapopulations and Immigration

Metapopulation:

Groups of populations

linked by immigration and emigration

This butterfly exists in many small populations. Individuals migrate between patches (populations) and can also colonize unoccupied patches.

Population Cycles

Many populations undergo regular boom-and-bust cycles. Cycles are influenced by interactions between biotic and abiotic factors

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

Human Populations

Human population growth has slowed after centuries of exponential increase

No population can grow indefinitely– humans are no exception

The Global Human PopulationThe human population increased relatively slowly until about 1650 and then began to grow exponentially

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

The rate of growth began to slow approximately 40 years ago

1950 1975 2000 2025 2050Year

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 (zero population growth)– 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.

Is associated with various factors in developed and developing countries

The transition in Sweden was slow. In Mexico the change was abrupt early and has since

slowed.

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

The relative number of individuals at each age

Affects present and future growth trends

Age structure represented in pyramids

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

Global Carrying Capacity

Just how many humans can the biosphere support?– …no agreement in the scientific community on a number.– But, estimates of global population in 2050 are between 8 and 11 billion (currently about 6

billion)– Will technological advances continue to increase the Earth’s capacity for humans.

Ecological Footprint Concept

Ecological footprint– The aggregate land and water area needed to

sustain the people of a nation. What area is needed to produce the resources consumed and absorb wastes generated.

Ecological capacity– the actual resource base of a country

Looking at the ecological footprint 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

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

)

Red dots are countries that were in ecological deficit in 1997

At more than 6 billion people the world is already in ecological deficit

http://www.myfootprint.org/en/