Chapter 52

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

• Demographics refers to the study of vital statistics especially birth and death rates.

Fig. 53-14

Population, Density and Dispersion• A population is a group of individuals

of a single species living in the same general area

• Density is the number of individuals per unit area or volume

Ex: the number of squirrels per square kilometer or

The number of esterichia coli bacteria per milliliter in a test tube.

Determining Density• In most cases, it is impractical or impossible to

count all individuals in a population

Researchers use different methods to estimate:• One way is to count the number of individuals in

a series of randomly located plots, calculate the average density in the samples, and extrapolate to estimate the population size in the entire area.

• Such estimates are accurate when there are many sample plots and a homogeneous (evenly dispersed) habitat.

The mark-recapture methodCommonly used to estimate wildlife populations.

Individuals are trapped and captured, marked with a tag, recorded, and then released.

•After a period of time has elapsed, traps are set again, and individuals are captured and identified.

•The second capture yields both marked and unmarked individuals.

•From this data, researchers estimate the total number of individuals in the population.

Fig. 53-3

Births

Births and immigrationadd individuals toa population.

Immigration

Deaths and emigrationremove individualsfrom a population.

Deaths

Emigration

Density in a population depends on:

•Immigration is the influx of new individuals from other areas•Emigration is the movement of individuals out of a population

• Dispersion is the pattern of spacing among individuals within the boundaries of the population.. They may be – Clumped

– Uniform

– random

Patterns of Dispersion• Dispersion is clumped when individuals

aggregate in patches.• Plants and fungi are often clumped where soil

conditions favor germination and growth.• Animals may clump in favorable

microenvironments (such as isopods under a fallen log) or to facilitate mating interactions.

• Group living may increase the effectiveness of certain predators, such as a wolf pack.

Fig. 53-4a

(a) Clumped dispersion in the intertidal zone

Video: Flapping Geese (Clumped)Video: Flapping Geese (Clumped)

CLUMPED DISPERSION in a forest

• Dispersion is uniform when individuals are evenly spaced.

• For example, some plants secrete chemicals that inhibit the germination and growth of nearby competitors.

• Animals often exhibit uniform dispersion as a result of territoriality, the defense of a bounded space against encroachment by others.

Fig. 53-4b

(b) Uniform

• In random dispersion, the position of each individual is independent of the others, and spacing is unpredictable.

• Random dispersion occurs where there is neither a strong attraction or repulsion among individuals in a population, or when key physical or chemical environmental factors are relatively homogeneously (evenly) distributed.

• For example, plants may grow where windblown seeds land.

Fig. 53-4c

(c) Random

Video: Prokaryotic Flagella (Video: Prokaryotic Flagella (Salmonella typhimuriumSalmonella typhimurium) (Random)) (Random)

Demography• Demography is the study of the vital

statistics of populations and how they change over time.

• Of particular interest are– birth rates and how they vary among

individuals (specifically females)– death rates.

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

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

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

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

• Survivorship curves can be classified into three general types:

• A Type I curve is relatively flat at the start, reflecting a low death rate in early and middle life, and drops steeply as death rates increase among older age groups.

• Humans and many other large mammals exhibit Type I survivorship curves.

• The Type II curve is intermediate, with constant mortality over an organism’s life span.

• Many species of rodent, various invertebrates, and some annual plants show Type II survivorship curves.

• Prey species that are subject to predation.

• A Type III curve drops off at the start, reflecting very high death rates early in life, then flattens out as death rates decline for the few individuals that survive to a critical age.

• Type III are usually organisms that produce large numbers of offspring but provide little or no parental care.

• Examples are many fishes, long-lived plants, and marine invertebrates.

• Also young that are subject to predation and severe environmental conditions

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

Idealized survivorship curves:

Types I, II, and III

Life history traits • Natural selection favors traits that improve an

organism’s chances of survival and reproductive success.

• In every species, there are trade-offs between survival and traits such as

1. The age at which reproduction begins

2. How often the organism reproduces

3. How many offspring are produced during each reproductive cycle

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

Evolution and Life History Diversity

• Life histories are very diverse• 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

Fig. 53-7

Century Plants grow in arid climates with unpredictable rainfall. After several years it sends up a large flowering stalk, reproduces and then dies.

semelparity

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

Dandelion

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

Coconut palm

how much does an individual gain in reproductive success through one pattern

versus the other?• The critical factor is survival rate of the offspring.• In highly variable or unpredictable environments,

when the survival of offspring is low, semelparity (big-bang) reproduction is favored.

• In dependable environments where competition for resources is intense, iteroparity (repeated reproduction ) is favored.

Per Capita Rate of Increase

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

• 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 = birth – death

Population Growth

Exponential population growth is apopulation increase under idealized conditions• rate of reproduction is at its maximum,

called the intrinsic rate of increase.• Any species regardless of life history is

capable of exponential growth if resources are unlimited

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

)

J shaped curve

Exponential curve

Fig. 53-11

8,000

6,000

4,000

2,000

01920 1940 1960 1980

Year

Ele

ph

ant

po

pu

lati

on

1900

The J-shaped curve of exponential growth characterizes some rebounding populations such as the African Elephant

The Logistic Growth Model (logical)

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

• Limiting resources, food, water, space eventually limit population growth.

• Carrying capacity (K) is the maximum population size the environment can support.

• In the logistic growth model, per capita rate of increase slows down or declines as carrying capacity is reached.

Fig. 53-13

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

Figure 53.13 How well do these populations fit the logistic growth model?

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

S shaped curve

J shaped curve

The Logistic Model and Life Histories

• Life history traits favored by natural selection may vary with population density and environmental conditions.

• K-selection, or density-dependent selection, selects for life history traits that are sensitive to population density

• r-selection, or density-independent selection, selects for life history traits that maximize reproduction

Density- Dependent factors that regulate population growth

Factors that reduce birth rate or increase death rates are density dependent.

• Competition for resources such as food, space, water or essential nutrients intensifies as populations increase.

• Territoriality- available space for territory or nesting may be limited.

• Disease- Increasing densities allow for easier transmission of disease.– Swine flu is a perfect example

• Predation- As prey populations increase, predators may find prey more easily.

Density-dependent regulation provides a negative feedback system that helps reduce birth rates and increase death rates or a population would grow exponentially!

Density Independent Factors

When a birth or death rate does not change with regard to population density it is said to be density independent.

• Natural Disasters will cause an increase in death rate regardless of density.

• Weather and climate such as floods or

Drought are density independent factors.

The human population

Global human populations have grown almost continuously throughout history.

But skyrocketed after the industrial revolution.

Note the dip resulting from the plague in Europe during the 1300 s

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 Global Human Population

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

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

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

Famine in china 1960s about 60 million died

Age Structure Pyramids

• 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

• Each horizontal bar represents a specific age group dividing male and female sides

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

Wide bottom small top Developing nations rapid growth

Columnar structure industrialized nation slow growth

Small bottom wider in the middle stable population

Estimates of Carrying Capacity• The carrying capacity of Earth for humans is

uncertain• The average estimate is 10–15 billion people• A concept termed ecological footprint

examines the total land and water area needed for all resources a person consumes in a population.

• Currently 1.7 hectares (app 4.2 acres) per person is considered sustainable.

• A typical person in the United States has a footprint of 10 hectares (almost 25 acres)