POPULATION
ECOLOGY
M.Sc. Botany 4th sem
POPULATION ECOLOGY
• Population - A group of individuals of the
same species occupying a particular space at
particular time.
• Population Ecology – Study of organismenvironment interactions at the population
level.
It is concerned with fluctuations in size and
regulatory factors.
CHARACTERISTICS OF A
POPULATION
Population density
Patterns of Dispersion
Age Structures
Age structure/Diagrams/Population Pyramids
Natality
Mortality
POPULATION DENSITY
It is the number of individuals of a species per unit area in
terrestrial ecosystem or per unit volume in aquatic
ecosystem.
Crude density: the density per unit total space.
Specific/Economic/Ecological density: The density per unit
habitat space i.e. available area or volume that can
actually be colonized by the population.
PATTERNS OF DISPERSION
RANDOM
UNIFORM/REGULAR
CLUMPED
NO PATTERN
UNIFORM SPACING
WITHIN A POPULATION
INDIVIDUALS
AGGREGATE INTO
GROUPS
INDIVIDUALS SPACED IN
A PATTERN LESS,
UNPREDICTABLE WAY
RESULTS FROM
INTERACTIONS AMONG
THEMSELVES
(COMPETITION)
IN RESPONSE TO
SOCIAL AGGREGATION
OR UNEVEN RESOURCE
DISTRIBUTION
PATTERNS OF DISPERSION
CLUMPED DISTRIBUTION IS
MOST COMMON TYPE
BECAUSE:
Suitable physical, biological, and
chemical conditions are patchy,
not uniform.
UNIFORM DISTIBUTION IS
RARE.
It is rare in nature.
Many aniamls form social groups
Many offsprings are not highly
mobile and are forced to live
“where they landed”.
If it does occur, it is usually the
result of fierce competition for
limited resources.
AGE STRUCTURE
It is the proportion of individuals in each age group and is usually shown
graphically
The three age groups are:
• Prereproductive
• Reproductive
• Postreproductive
The relative duration of these age groups in proportion to lifespan varies greatly
with different organisms.
The age structure of a population dictates whether it will grow, shrink or stay the
same size.
AGE STRUCTURE DIAGRAMS/
POPULATION PYRAMIDS
• The graphical representation of the number or percent in each age class.
UPRIGHT BROAD
BASED/TRIANGULAR
BELL
SHAPED/RECTANGULAR
URN
POPULATION/INVERTED
A high
percentage of
young individuals
A moderate
proportion of
young to old
A low percentage
of young
individuals
rapid population
growth (Growing
population)
Relatively stable
population (Aging
Population)
Show decrease in
population (Aged
Population)
AGE STRUCTURE DIAGRAMS/
POPULATION PYRAMIDS
• The graphical representation of the number or percent in each age class.
UPRIGHT BROAD
BASED/TRIANGULAR
BELL
SHAPED/RECTANGULAR
URN
POPULATION/INVERTED
A high
percentage of
young individuals
A moderate
proportion of
young to old
A low percentage
of young
individuals
rapid population
growth (Growing
population)
Relatively stable
population (Aging
Population)
Show decrease in
population (Aged
Population)
AGE STRUCTURE DIAGRAMS/
POPULATION PYRAMIDS
NATALITY
MAXIMUM/ABSOLUTE/POTENTIAL/PHYSIOLOGICAL NATALITY: The theoretical maximum
production of new individuals under ideal conditions, also called fecundity rate.
ECOLOGICAL/RELAIZED NATALITY: The actual population increase under existing
specific conditions, also called fertility rate.
CRUDE BIRTH RATE: Births per 1000.
CLUTCH SIZE: the number of young produced in each occasion.
GENERATION TIME: the average time between birth and reproduction for an individual.
MORTALITY
Mortality: Number of individuals removed through death.
MINIMUM/POTENTIAL MORTALITY: The theoretical minimum loss under ideal or
non-limited conditions.
ECOLOGICAL/REALIZED MORTALITY: The actual loss of individuals under a given
environmental condition.
CRUDE DEATH RATE: Deaths per 1000
VITAL INDEX: Birth-death ratio expressed as percentage.
SURVIVORSHIP CURVE
• Mortality
• Mortality rate
• Survivorship is the percentage of an original population that survives to a given age.
• A survivorship curve is the graphic representation of the number of individuals in a
population that can be expected to survive to any specific age.
• Survivorship curves plot the proportion of individual alive at each stage, thus reflect
species differences in reproduction & mortality.
• Three types of survivorship curve:
• Type – I (Convex)
• Type – II (Constant)
• Type – III (Concave)
TYPES OF SURVIVORSHIP CURVE
TYPES OF SURVIVORSHIP CURVE
Type – I: Convex
Late Loss
High parental care
Humans, annual plants and most
primates have a Type I survivorship
curve. I
n a Type I curve, organisms tend not to
die when they are young or middleaged but, instead, die when they
become elderly.
Species with Type I curves usually have
small numbers of offspring and provide
lots of parental care to make sure those
offspring survive.
TYPES OF SURVIVORSHIP CURVE
• Type – II
• Late Loss
• Many bird species have a Type
II survivorship curve.
• In a Type II curve, organisms die
more or less equally at each
age interval.
• Organisms with this type of
survivorship curve may also
have relatively few offspring
and provide significant parental
care.
TYPES OF SURVIVORSHIP CURVE
• Type – III
• Early loss
• Trees, marine invertebrates, and most fish
have a Type III survivorship curve.
• In a Type III curve, very few organisms
survive their younger years.
• However, the lucky ones that make it
through youth are likely to have pretty long
lives after that.
• Species with this type of curve usually have
lots of offspring at once—such as a tree
releasing thousands of seeds—but don't
provide much care for the offspring.
LIFE HISTORY STRATEGIES
• Life history means how organism survives and reproduces over the period of
time.
• Life history traits are
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Number, size and sex ratio of offspring
Timing of reproduction
Age and size at maturity
Growth pattern
• Organism has to maintain balance between growing, surviving and
reproducing.
• Energy available shoeld be properly allocated between growth, survival and
reproduction.
• So, trade offs are inevitable.
LIFE HISTORY STRATEGIES
• Mcarthur divided organisms into two categories on
the basis of “strategies” of growth, survival and
reproduction:
• R-strategist species,
• K- strategist species,
R-SELECTED SPECIES
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R is the symbol of birth rate, so birth rate is critical factor
Small adults, mature quickly
Early reproductive stage
Reproduce often, with many small offsprings
Little or no parental care and protection of offsprings
Short lived, most offsprings die before reaching reproductive stage
Live in places where resources are temporarily abundant
Adapted to unstable environment
Population size fluctuates wildly above and below K
The species are Opportunistic
Generalist niche
Low ability to compete
Early successional species
K-SELECTED SPECIES
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K is the symbol of carrying capacity, so carrying capacity is a critical factor
Large size, mature slowly, most die old
Later reproductive age
Peproduce slowly, with fewer and larger offsprings
High parental care and protection of offsprings
Long lives, most offsprings survive to reproductive age
Population size fairly stable and usually close to k
Adapted to stable climate
Good competitors
Specialist niche
Late successional species
Examples- Trees, humans