Population Ecology

AP Chap 53

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

Fig. 53-1

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

Every population has geographic boundaries.

• 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 density is often determined by sampling techniques

Population size can be estimated by

1. Direct counting

2. Random sampling based on sample plots (quadrats)

3. Indexes such as tracks, nests, burrows, fecal droppings, etc.

4. Mark and recapture method

Quadrat sampling

Mark-recapture Method

Mark-Recapture Formula for estimating population size

• Estimate of Total Population  = 

(total number recaptured) x (number marked)

(total number recaptured with mark)

In a mark-recapture study, an ecologist traps, marks and releases 25 voles in a small wooded area. A week later she resets her traps and captures 30 voles, 10 of which are marked. What is her estimate of the vole population in that area?

How does population density change?

• Addition – birth, immigration

• Removal – death, emigration

Fig. 53-3

Births

Births and immigrationadd individuals toa population.

Immigration

Deaths and emigrationremove individualsfrom a population.

Deaths

Emigration

Patterns of Dispersion

• Environmental and social factors influence spacing of individuals in a population• In a clumped dispersion, individuals aggregate in patches• A clumped dispersion may be influenced by resource availability and behavior

Fig. 53-4a

(a) Clumped

UNIFORM

• A uniform dispersion is one in which individuals are evenly distributed

• It may be influenced by social interactions such as territoriality

Fig. 53-4b

(b) Uniform

RANDOM

• In a random dispersion, the position of each individual is independent of other individuals

• It occurs in the absence of strong attractions or repulsions

Fig. 53-4c

(c) Random

Demographics• Demography is the study of the vital statistics (death and birth rates)

of a population and how they change over time• Death rates and birth rates are of particular interest to

demographers• A life table is an age-specific summary of the survival pattern of a

population• It is best made by following the fate of a cohort, a group of individuals

of the same age

Table 53-1

The life table of Belding’s ground squirrels reveals many things about this population

Survivorship Curves

• A survivorship curve is a graphic way of representing the data in a life table

• The survivorship curve for Belding’s ground squirrels shows a relatively constant death rate

Fig. 53-5

Age (years)20 4 86

10

101

1,000

100

Nu

mb

er o

f su

rviv

ors

(lo

g s

cale

)

Males

Females

• Survivorship curves can be classified into three general types:– Type I: low death rates during early and

middle life, then an increase among older age groups

– Type II: the death rate is constant over the organism’s life span

– Type III: high death rates for the young, then a slower death rate for survivors

Fig. 53-6

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

More parental care, better health care

Predation, accidents,disease at all levels

High mortalityof many offspring

What type of survivorship curve?

Type 2

Type 3

Type 1

Table 53-2

Reproductive tables focus on female reproductivity.

Life history traits are products of natural selection

• An organism’s life history comprises the traits that affect its schedule of reproduction and survival:– The age at which reproduction begins

– How often the organism reproduces

– How many offspring are produced during each reproductive cycle

• Species that exhibit semelparity, or big-bang reproduction, reproduce once and die

• Species that exhibit iteroparity, or repeated reproduction, produce offspring repeatedly

• Highly variable or unpredictable environments likely favor big-bang reproduction, while dependable environments may favor repeated reproduction.

“Trade-offs” and Life Histories

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

Examples:

• brood size vs parental life span

• number of seeds and chance of gemination and growth

Fig. 53-8

MaleFemale

100

RESULTS

80

60

40

20

0Reduced

brood sizeNormal

brood sizeEnlarged

brood sizePar

ents

su

rviv

ing

th

e fo

llo

win

g w

inte

r (%

)

Fig. 53-9

(a) Dandelion

(b) Coconut palm

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

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

In what way might high competition for limited resources in a predictable

environment influence the evolution of life history traits? Semelparity or

iteroparity

Selection would most likely favor iteroparity, with fewer, larger, better-provisioned or cared-for offspring.

How do we model population growth?

• By construction graphs and using mathematical formulas

• If immigration and emigration are ignored, a population’s growth rate (per capita increase) equals birth rate minus death rate

• r = b - d

• Zero population growth occurs when the birth rate equals the death rate

• Most ecologists use differential calculus to express population growth as growth rate at a particular instant in time:

Nt

rN

where N = population size, t = time, and r = per capita rate of increase

Exponential Growth

• Exponential population growth is population increase under idealized. unlimited conditions

• Under these conditions, the rate of reproduction is at its maximum, called the intrinsic rate of increase

• Equation of exponential population growth:

dNdt

rmaxN

Exponential population growth results in a J-shaped curve.

Fig. 53-10

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

)

Fig. 53-11

8,000

6,000

4,000

2,000

01920 1940 1960 1980

Year

Ele

ph

ant

po

pu

lati

on

1900

Elephants in Kruger National Park in S. Africa after they were protectedfrom hunting.

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

But, is this the normal state of population growth?

• 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 per capita rate of increase as N approaches K.

dNdt

(K N)Krmax N

Table 53-3

As N approachesK, rate nears“0”.

• The logistic model of population growth produces a sigmoid (S-shaped) curve

Fig. 53-12

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

K

Fig. 53-13a

1,000

800

600

400

200

00 5 10 15

Time (days)

Nu

mb

er

of

Pa

ram

ec

ium

/mL

(a) A Paramecium population in the lab

These organisms are grown in a constant environment lacking predators and competitors

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

Fig. 53-13b

Nu

mb

er

of

Da

ph

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

Some populations fluctuate greatly and make it difficult to define K.

• Some populations show an Allee effect, in which individuals have a more difficult time surviving or reproducing if the population size is too small

The Logistic Model and Life HistoriesNatural selection shapes the final life history of

individual species. Some members of populations are subject to r-

selection and some to k-selection.When population size is low relative to K, r-selection favors r-strategies:• high fecundity (ability to reproduce), • small body size, • early maturity onset, • short generation time, and • the ability to disperse offspring widely.

Characteristics of r - Selected Opportunists

• Very high intrinsic rate of increase.

• Opportunistic

• Populations can expand rapidly to take advantage of temporarily favorable conditions

• Ex – Bacteria, some fungi, many insects, rodents, weeds, and annual plants.

• In environments that are relatively stable and populations tend to be near K, with minimal fluctuations in population size, K-selection favors K strategies: large body size, long life expectancy, and the production of fewer offspring that require extensive parental care until they mature.

• These populations are strong competitors.

• They are specialists rather than colonists and may become extinct if their normal way of life is destroyed.

Characteristics of K - Selected Species

Population responds slowly, usually with negative feedback control so that constancy is the rule.

Their numbers are controlled by the availability of resources. In other words, they are a density dependent species

Most birds

Most predators

Elephants

Whales

Oaks

Chestnuts

Apple

Coconut

r or k-selected?• Nature is more complex though and most populations

lie somewhere in between these two extremes. • Ex- Gymnosperms and angiosperms are typically

classified as K-strategists but they release many seeds.

• Cod fish are large fish but release large numbers of gametes into the sea with no parental investment. So, cod are considered r-strategists.

K or r-selected ? When a farmer abandons a field, it is

quickly colonized by fast-growing weeds. Are these species more likely to be K-selected or r-selected species?

r

What about the bluegill fish?

Bluegill exhibit one of the most social and complex mating systems in nature. Parental males delay maturation and compete to construct nests in colonies, court females, and provide sole parental care for the young within their nest.

Many factors that regulate population growth are density dependent

• There are two general questions about regulation of population growth:

– What environmental factors stop a population from growing indefinitely?

– 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

Fig. 53-15

(a) Both birth rate and death rate vary.

Population density

Density-dependentbirth rate

Equilibriumdensity

Density-dependentdeath rate

Bir

th o

r d

eath

ra

tep

er c

ap

ita

(b) Birth rate varies; death rate is constant.

Population density

Density-dependentbirth rate

Equilibriumdensity

Density-independentdeath rate

(c) Death rate varies; birth rate is constant.

Population density

Density-dependentdeath rate

Equilibriumdensity

Density-independentbirth rate

Bir

th o

r d

eath

ra

tep

er c

ap

ita

So, to determine if the environmental factor is density dependent or

independent….• Density-independent factors may affect

all individuals in a population equally – rainfall, temperature, humidity, acidity,

salinity, catastrophic events

• Density-dependent factors have a greater affect when the population density is higher.

Food supply, disease, parasites, competition, predation

Density-Dependent Population Regulation

• Density-dependent birth and death rates are an example of negative feedback that regulates population growth

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

Fig. 53-17a

(a) Cheetah marking its territory

In many vertebrates and some invertebrates, competition for territory may limit densityCheetahs are highly territorial, using chemical communication to warn other cheetahs of their boundaries

Fig. 53-17b

(b) Gannets

•Oceanic birds exhibit territoriality in nesting behavior

Disease

• 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

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

Intrinsic Factors

• For some populations, intrinsic (physiological) factors appear to regulate population size

Population Dynamics

• The study of population dynamics focuses on the complex interactions between biotic and abiotic factors that cause variation in population size

• Long-term population studies have challenged the hypothesis that populations of large mammals are relatively stable over time

• Weather can affect population size over time

Fig. 53-18

2,100

1,900

1,700

1,500

1,300

1,100

900

700

500

01955 1965 1975 1985 1995 2005

Year

Nu

mb

er o

f sh

eep

Fig. 53-19

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

YearChanges in predation pressure can drive population fluctuations

Population Cycles: Scientific Inquiry

• Some populations undergo regular boom-and-bust cycles

• Lynx populations follow the 10 year boom-and-bust cycle of hare populations

• Three hypotheses have been proposed to explain the hare’s 10-year interval

- winter food supply - predators * - sunspot activity (quality of food) * * affected cycles

Fig. 53-20

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

The human population is no longer growing exponentially but is still increasing rapidly

• No population can grow indefinitely, and humans are no exception

Fig. 53-22

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

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

Fig. 53-23

2005

Projecteddata

An

nu

al p

erc

ent

incr

ease

Year

1950 1975 2000 2025 2050

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

Though the global population is still growing, the rate of growth began to slow during the 1960s

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 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

Fig. 53-24

1750 1800 1900 1950 2000 2050

Year

1850

Sweden MexicoBirth rate Birth rate

Death rateDeath rate0

10

20

30

40

50B

irth

or

dea

th r

ate

per

1,0

00 p

eop

le

The demographic transition in Sweden took about 150 years, from 1810 to 1960. It will take about the same length of time for Mexico.

• The demographic transition is associated with an increase in the quality of health care and improved access to education, especially for women

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

Age Structure

• One important demographic factor in present and future growth trends is a country’s age structure

• Age structure is the relative number of individuals at each age• Age structure diagrams can predict a population’s growth

trends• They can illuminate social conditions and help us plan for the

future

Fig. 53-25

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

Infant Mortality and Life Expectancy

• Infant mortality and life expectancy at birth vary greatly among developed and developing countries but do not capture the wide range of the human condition

Fig. 53-26

Less indus-trialized

countries

Indus-trialized

countries

60

50

40

30

20

10

0 0

20

40

80

Lif

e ex

pec

tan

cy (

year

s)

Infa

nt

mo

rtal

ity

(dea

ths

per

1,0

00 b

irth

s)

Less indus-trialized

countries

Indus-trialized

countries

60

Global Carrying Capacity

• How many humans can the biosphere support?• The carrying capacity of Earth for humans is uncertain• The average estimate is 10–15 billion

Limits on Human Population Size

• The ecological footprint concept summarizes the aggregate land and water area needed to sustain the people of a nation

• It is one measure of how close we are to the carrying capacity of Earth

• Countries vary greatly in footprint size and available ecological capacity

Fig. 53-27

Log (g carbon/year)

13.49.85.8

Not analyzed

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