Population Ecology

Population defined

• Population – is a group of individuals of a

single species living in the same general

area.

Population Ecology

• Study of populations in relation to their

environment, including environmental

influences on density and distribution, age

structure, and population size

Population Ecology Concepts:

• The physical environment limits the geographic

distribution.

• On small scales, individuals within populations are

distributed in patterns that may be random, regular, or

clumped; on larger scales, individuals within a population

are clumped.

• Many populations are subdivided into subpopulation

called metapopulation.

• Population density declines with increasing organism

size.

• Commonness and rarity of species are influenced by

population size, geographic range, and habitat tolerance.

Population Characteristics

1. Natality – total number of individuals added to the

population through reproduction over a particular period

of time.

Biotic communities

i.e. Plants, fungi, bacteria – sexual and asexual

Animals – usually sexual reproduction

Human population natality is described in terms of birth rate –

number of individuals born per 1000 individuals per year

2. Mortality – number of deaths in a population over a

particular period of time.

3. Population Growth – rate of increase subtracted by

rate of decline; (immigration + birth rate) – (death rate +

emigration)

Births Deaths

Immigration Emigration

Births and immigration add individuals to a population.

Deaths and emigration remove individuals from a population.

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

• Measuring density of populations is a

difficult task.

– We can count individuals; we can estimate

population numbers.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 52.1

Density

• It is the result of an interplay between

processes that add individuals to a

population (birth and immigration) and

those that remove individuals (death and

emigration)

Parameters that effect size or density of a population:

Figure 1. The size of a population is determined by a balance

between births, immigration, deaths and emigration

Birth Death

Emigration

Immigration

Population (N)

Dispersal

• Interaction may not be symmetrical

• Populations increase and send out many

dispersers

• Small populations have few dispersers

• Individual populations may become extinct

• Population bottlenecks may occur – Population bottlenecks occur when a population’s

size is reduced for at least one generation.

Patterns of Distribution and Dispersal

• Environmental and social factors influence

the spacing of individuals in a population.

• Overall, dispersion depends on resource

distribution.

RANDOM UNIFORM CLUMPED

Uniform Dispersion

• Regular pattern

• A uniform dispersion is one in which

individuals are evenly distributed.

• It may be influenced by social interactions

such as territoriality, the defense of a

bounded space against other individuals.

• There is an antagonistic interaction

between individuals.

Clumped Dispersion

• Individuals in areas of high local

abundance are separated by areas of low

abundance

• Uneven distribution of resources

• Individuals are attracted to a common

resource.

Random Dispersion

• An individual has an equal probability of

occurring anywhere in an area.

• The position of each individual is

independent of other individuals.

• There is a neutral interaction between

individuals and it occurs in the absence of

strong attractions or repulsions.

Age Structure

• The proportion of individuals in each age class

of a population.

• Iteroparous species – individuals that give birth

to few offsprings at several reproductive periods;

exhibit age structure; ex. mammals

• Semelparous species – reproducing only once in

a life time; no age structure; ex. mayflies,

cicadas

Age Structure

• Age structure has a critical influence on a

population’s growth rate

• Classification of ages based on reproductive

stages:

1. Pre- reproductive stage – 0 to 14 years

2. Reproductive stage – 15 to 44 years

3. Post- reproductive stage – 45 years and older

Life tables

• 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.

Survivorship Curves

• A graphic way of representing the pattern

of survival of individuals in a population

from birth to the maximum age attained by

each individuals.

• This is a plot of the number of individuals

in a cohort still alive at each age.

Figure 53.5

Males

Females

1,000

100

10

1

Age (years)

Nu

mb

er

of

su

rviv

ors

(lo

g s

cale

)

0 2 4 6 8 10

Survivorship Curves

Three general types:

• Type I - low death rates during early and

middle life and an increase in death rates

among older age groups

• Type II - a constant death rate over the

organism’s life span

• Type III - high death rates for the young

and a lower death rate for survivors

Figure 53.6

1,000

III

II

I

100

10

1 100 50 0

Percentage of maximum life span

Nu

mb

er o

f su

rviv

ors

(lo

g sc

ale)

Population Growth

• A function of reproduction and immigration.

– High biotic potential and high rate of

immigration

– Biotic potential is the maximum reproductive

potential of an organism.

• The population growth rate can be

expressed mathematically as:

where N is the change in population size, t is

the time interval, B is the number of births, and D

is the number of deaths

NB D

t

• Births and deaths can be expressed as the

average number of births and deaths per

individual during the specified time interval

where b is the annual per capita birth rate, m

(for mortality) is the per capita death rate, and N is

population size

B bN

D mN

• The population growth equation can be revised

NbN mN

t

• The per capita rate of increase (r) is given by

r b m

• Zero population growth (ZPG) occurs when the

birth rate equals the death rate (r 0)

Population Growth

• Principle: In the presence of abundant

resources, populations can grow at

geometric or exponential rates.

Exponential Growth

• Population increase under idealized conditions

• Under these conditions, the rate of increase is at

its maximum, denoted as rmax

• Equation:

• Results in a J-shaped curve

• Rarely seen in nature.

dN dt

rmaxN

Exponential growth of rabbits

dN dt

rmaxN

• The J-shaped curve of exponential growth also

characterizes some rebounding populations

– For example, the elephant population in Kruger

National Park, South Africa, grew exponentially

after hunting was banned

Year

Ele

ph

ant

po

pu

lati

on

8,000

6,000

4,000

2,000

0 1900 1910 1920 1930 1940 1950 1960 1970

Kruger National Park, South Africa

Logistic Population Growth

• Describes how a population grows more

slowly as it nears its carrying capacity. – Carrying capacity (K) is the maximum population

size the environment can support; varies with the

abundance of limiting resources.

• The per capita rate of increase declines as

carrying capacity is reached.

Logistic Population Growth

• The logistic model starts with the exponential

model and adds an expression that reduces per

capita rate of increase as N approaches K.

• Produces a sigmoid (S-shaped) curve

dN dt

(K N)

K rmax N

Figure 53.10

Time (days) Time (days)

(a) A Paramecium population in the lab

(b) A Daphnia population in the lab

Nu

mb

er

of

Para

meciu

m/m

L

Nu

mb

er

of

Dap

hn

ia/5

0 m

L

1,000

800

600

400

200

0 0 5 10 20 15 0 160 40 60 80 100 120 140

180

150

120

90

60

30

0

Figure 52.11 Population growth predicted by the logistic model

Imposition of limits

New or Changing

Environment

(no competition / limits)

dN/dt = r N (K-N)/K

Environmental Resistance

• Sum of the total environmental limiting

factor (both biotic and abiotic) that prevent

the biotic potential (rmax) of a population

from being realized

• Some of the assumptions built into the

logistic model do not apply to all

populations. • It is a model which provides a basis from which we

can compare real populations.

Severe Environmental Impact

• Other populations have regular boom-and-

bust cycles.

–There are populations that fluctuate

greatly.

Boom and then Bust

Water flee (Daphnia magna) is adapted to exploit new environment: high

growth rate, resistant eggs produced before crash.

Boom and then really Bust

Reindeer introduced to Pribilov island. Initial exponential growth, crash,

complete extinction.

Boom and sort of Bust

Predators were removed from Kaibab plateau. Mule deer population size

increased from 4,000 to hundred thousand, then dropped and stabilzed at 10,000.

Boom but not much Bust

Sheep introduced to Tasmania: rapid initial growth, overshoot, drop,

fluctuation around carrying capacity.

Boom & Bust & Boom & Bust & Boom & Bust

The familiar 10-11 year hare-lynx cycle might not be true. Biased data.

Factors influencing population growth:

1. Sex ratio and age distribution

Sex ratio – relative number of males and females

Number of females are very important since they

determine the number of offsprings produced in a

population.

Polygamous – number of males – less important to

population growth

Monogamous – both sexes important

Age distribution – number of individuals in the pre-

reproductive period is a determinant factor in

population growth rate.

Factors influencing population growth:

2. Carrying capacity

Population size that can be maintained in an

area over time without harming the habitat.

Combination of factors that sets the carrying

capacity of an area is called environmental

resistance.

An environmental resistance can be any

limiting factor (raw materials, energy supply,

accumulation of waste products, interactions

among organisms)

Factors influencing population growth:

3. Density-independent factor

population-limiting factor whose intensity in

unrelated to population density

severe storms and flooding

sudden unpredictable severe cold spells

earthquakes and volcanoes

catastrophic meteorite impacts

Factors influencing population growth:

4. Density-dependent factors population-limiting factor whose intensity is linked to

population density.

limiting resources (e.g., food & shelter)

production of toxic wastes

infectious diseases

predation

stress

emigration

As resources become limiting with increasing population size, biotic interaction intensifies.

This decrease fitness of individuals decreasing growth

Biotic interactions whose effects decrease

fitness of individuals:

1. Competition = intra / interspecific

2. Amensalism = 0 -

3. Parasitism = + -

4. Predation = + -

*Fitness – ability to survive and reproduce; relative number of offspring that survive

Niche

• The environmental factor that influence the

growth, survival, and reproduction of a

species.

• Function / role of the organism

• Interspecific competition

Niche

• Competitive Exclusion Principle

– G. F. Gause (1934)

– Two species with identical niches cannot

coexist indefinitely.

– The more effective competitor for limited

resources will have higher fitness and will

eventually exclude all individuals of the

second species.

Niche

• Fundamental niche

– the physical conditions under which a species might

live, in the absence of interactions with other species.

• Realized niche

– actual niche of a species whose distribution is limited

by biotic interaction (competition, predation, disease,

and parasitism)

– may be much smaller than the fundamental niche

Competition and Niches

• Competition can have a significant ecological role and

evolutionary influences on the niches of species.

– Competition restricts the species to their realized niche but they

still retain their capacity to inhabit the fuller range of

environment, fundamental niche.

Biotic interactions whose effects increase

fitness of individuals:

1. Mutualism - + + intimate relationship; one

cannot do without the other; protocooperation -

can live with or without the relationship; ex.

Plants and ants

2. Commensalism – one organism benefits

without affecting the other

Life history

• How natural selection and other evolutionary

forces shape organisms to optimize their survival

and reproduction in the face of ecological

challenges posed by the environment.

• Consist of the adaptations of an organism that

influences aspects of its biology such as the

number of offspring it produces, its survival, and

its size and age at reproductive maturity.

Life history

Concepts:

• Because all organisms have access to limited energy

and other resources, there is a trade-off between the

number and size of offspring.

– Darter species that produce larger eggs produce few eggs.

• Organisms reproduce at an earlier stage when adult

survival is lower; where adult survival is higher,

organisms defer reproduction to a later stage.

– Ex. The survival of adult snakes and lizards increases as their

age at maturity also increases.

• The great diversity of life histories may be classified on

the basis of a few population characteristics.

– r-selection and K-selection

Offspring number versus Size

Life history

Adult Survival and Reproductive Allocation

• Long-lived species delay reproduction

- Advantage: juveniles gain experience before high cost

of reproduction

• Short-lived species reproduce early

- Time is important; delay may mean no offspring

Classification of life history patterns

• r selection

– Refers to per capita rate of increase, r

– Species often colonizing new or disturbed habitats

(pioneer species)

• K selection

– Refers to the carrying capacity, K

– Prominent in situations where species populations are

near carrying capacity

Characteristics favored by r versus K selection

Population attribute r selection K selection

Intrinsic rate of increase, rmax High Low

Competitive ability Not strongly favored Highly favored

Development Rapid Slow

Reproduction Early Late

Body size Small Large

Reproduction Single, semelparity Repeated, iteroparity

Offspring Many, small Few, large

Life history

• r and K selection are end points in a

continuum – correlated with attributes of

the environment and of populations.

r K

Life history

• r selection – characteristics of variable or

unpredictable environment.

– Type III survivorship

• K selection – fairly constant or predictable

environment

– Type I survivorship

Evolution and Natural Selection

• Evolution – change over time; the study of

interplay between heredity and environment.

– Change in genetic composition of a population over

periods of many generations.

– Genetic changes lead to changes in appearance,

functioning or behavior over generations

Evolution Theory

• Charles Darwin

– In 1831 Darwin joined the H.M.S. Beagle as the

naturalist for a circumnavigation of the world; the

voyage lasted five years. It was his observations from

that trip that lead to his proposal of evolution by

natural selection.

– Galapagos Island

– Published the book ‘Origin of Species’ (1859)

• Alfred Wallace

– South East Asia

• Genetic studies show

all arise from a single

ancestral species.

Darwin’s Finches

Natural Selection

• Natural selection – process where there is

differential reproduction and survival of

individuals carrying alternative inherited traits

– Results in differential representation of genotypes in

the future generation

– Genotype: genetic constitution of an organism

Charles Darwin found out how natural

selection leads to adaptive evolution:

1. Organisms beget like organisms.

2. There are chance variations between individuals in a

species. Some variations are heritable.

3. More offspring are produced each generation than can

be supported by the environment.

4. Some individuals are better suited to their environment

and reproduce more effectively.

Industrial Melanism

in Peppered Moth

Industrial melanism

- adaptive melanism caused

by anthropogenic alteration

of the natural environment in

terms of industrial pollution.

Peppered Moths

Types of Selection

1. Directional selection – drives a feature in one

direction.

2. Stabilizing selection – favors intermediate traits;

preserving the status quo

3. Disruptive selection – traits diverge in two or more

directions;

Directional Selection

• Occurs where one extreme phenotype has an advantage over all other phenotypes.

• Population's trait distribution shifts toward the other extreme.

Stabilizing Selection

• Acts against extreme phenotypes

Disruptive Selection

• Favors two or more extreme phenotypes over the average phenotype in a population.

• Result is a bimodal, or two-peaked, curve in which the two extremes of the curve create

their own smaller curves

Genetic variation and Natural Selection

• Genetic variation - range (variance) of phenotypes;

different chromosomal arrangements (cytogenetics); DNA

sequence differences among individuals.

• Genetic variation within a population is absolutely

necessary for natural selection to occur.

– If all individuals are identical within a population then their fitness

will all be the same.

–Same fitness Natural selection will not occur

Sources of Genetic Variation

• Mutation: inheritable changes in a gene or a chromosome

• Point mutation

• Chromosome mutation

– deletion, duplication, inversion, translocation

• Genetic recombination

• Sexual reproduction

Two individuals produce haploid gametes (egg or sperm) – that combine to

form a diploid cell or zygote.

– Reassortment of genes provided by two parents in the offspring

– Increases dramatically the variation within a population by creating new combinations of existing genes.

• Asexual reproduction: less variation (only mutation)

Evolution is a change in gene frequencies

• Evolution is a change of gene frequencies within a

population (or species) over time.

– Gene frequency: allele frequency; the frequency of occurrence of

an allele in relation to that of other alleles of the same gene in a

population

– Hardy-Weinberg Principle: in a population mating at random in the

absence of evolutionary forces, allele frequencies will remain

constant

p = frequency of one allele (A)

q = frequency of the alternative allele (a)

p2 = frequency of genotype A

q2 = frequency of genotype a

2pq = frequency of individual Aa

Hardy – Weinberg Principle

• Gene frequencies will remain the same in successive

generations of a sexually reproducing population if the

following five conditions hold:

1. Random mating

2. No mutations

3. Large population size

4. No immigration

5. No selection

Genetic drift

• Changes in the gene frequencies in a small population

due to chance or random events.

• Reduces genetic variation in a population over time by

increasing the frequency of some alleles and reducing or

eliminating the frequency of others.

– One allele can become common in a population in the expense of

the alternative allele

• Usually caused by bottleneck events and founder effect.

Bottleneck event

• Severe reduction in a population

size

• Northern elephant seals – reduced genetic variation probably because

of a population bottleneck humans inflicted

on them in the 1890s.

– hunting reduced their population size to as

few as 20 individuals at the end of the 19th

century. Their population has since

rebounded to over 30,000 - but

their genes still carry the marks of this

bottleneck: they have much less genetic

variation than a population of southern

elephant seals that was not so intensely

hunted.

Founder effects

• Occurs when a small number of individuals, representing

only a small fraction of the total genetic variation in a

species, starts a new population.

• Small population size means that the colony may have:

– reduced genetic variation from the original population

– a non-random sample of the genes in the original population

• Afrikaner population of Dutch settlers in South Africa

– descended mainly from a few colonists

– today, the Afrikaner population has an unusually high frequency

of the gene that causes Huntington’s disease, because those

original Dutch colonists just happened to carry that gene with

unusually high frequency

Selective pressures influence adaptation

• Related species in different

environments

– Experience different

pressures

– Evolve different traits

• Convergent evolution -

unrelated species may

evolve similar traits. – Because they live in similar

environments

Speciation

• The process of generating new species from a single species.

• Concept of species – Morphological species concept

• A species is defined as a morphologically consistent group of organisms than can be distinguished from all other species

– Can fail. So called cryptic species

– Biological species concept • A group of populations whose individuals can interbreed and produce fertile

offspring and cannot interbreed with other species • Reproductive isolation

– Still fails. If you cannot tell the individuals apart morphologically, how can you tell if they are interbreeding or not

– Also, some species can interbreed and produce viable offspring » Bontebok and Blesbok in South Africa

– Genetic species concept • A group of populations whose individuals have a distinct genetic makeup and who

do not interbreed with others groups of populations – Bontebok and Blesbok are genetically distinct as well as being morphologically different.

Mechanisms of Speciation

• Allopatric speciation: geographic speciation; species formation

due to physical separation of populations; allopatric species occupy

area separated by time and space; probably most vertebrates.

• Vicariance: separation of an individual taxon or biota due to the formation of a

physical barrier to gene flow or dispersal

• Sympatric speciation: species form from populations that become

reproductively isolated within the same area; sympatric species

occupy the same place at the same time; plants and insects.

Mechanisms of Speciation

Genetic isolation mechanisms

(reproductive barriers)

• Pre-mating mechanisms - Factors which prevent individuals from mating

• Post-mating mechanisms - Genomic incompatibility, hybrid inviability or sterility

Pre-mating mechanisms

1. Geographic isolation: Species occur in different areas, and are often separated

by barriers.

2. Temporal isolation: Individuals do not mate because they are reproductively

active at different times. This may be different times of the day or different seasons.

The species mating periods may not match up. Individuals do not encounter one

another during either their mating periods, or at all.

3. Ecological isolation: Individuals only mate in their preferred habitat. They do not

encounter individuals of other species with different ecological preferences.

4. Behavioral isolation: Individuals of different species may meet, but one does not

recognize any sexual cues that may be given. An individual chooses a member of

its own species in most cases.

5. Mechanical isolation: Copulation may be attempted but transfer of sperm does

not take place. The individuals may be incompatible due to size or morphology.

6. Gametic incompatibility: Sperm transfer takes place, but the egg is not fertilized.

Post-mating mechanisms

1. Zygotic mortality: The egg is fertilized, but

the zygote does not develop.

2. Hybrid inviability: Hybrid embryo forms, but is not

viable.

3. Hybrid sterility: Hybrid is viable, but the resulting adult

is sterile.

4. Hybrid breakdown: First generation (F1) hybrids are

viable and fertile, but further hybrid generations (F2 and

backcrosses) are inviable or sterile.

Allopatric speciation

1. Geographically isolated

2. The separated populations

diverge (through changes in

mating tactics or use of their

habitat)

3. Reproductively isolated (such

that they cannot interbreed and

exchange genes)

• An ancestral fish population was split into two by the formation of the Isthmus of Panama about 3.5 millions years ago. Since that time, different genetic changes have occurred in the two populations because of their geographic isolation. These changes eventually lead to the formation of different species. The porkfish (Anisotremus virginicus) is found in the Carribean Sea and the Panamic prokfish (Anisotremus taeniatus) is found in the Pacific Ocean.

Speciation via geographic isolation and divergence

Allopatric speciation

Ring species - population of

a single species encircling an

area of unsuitable habitat. As

a result, the species becomes

geographically distributed in a

circular, or ring, pattern over a

large geographic area.

200 years ago, the ancestors of apple maggot flies laid their eggs only on hawthorns,

which are native to America. But today, these flies lay eggs on hawthorns and domestic

apples that were introduced by immigrants and bred there. Females generally choose to

lay their eggs on the type of fruit they grew up in, and males tend to look for mates on the

type of fruit they grew up in. So hawthorn flies generally end up mating with other

hawthorn flies and apple flies generally end up mating with other apple flies. This means

that gene flow between parts of the population that mate on different types of fruit is

reduced.