Population ecology

A population is a group of individuals of the same species living in
the same general area
Population ecology is the study of populations in relation to the
environment
Includes environmental influences on population density and
distribution, age structure, and variations in population size.

Dispersion – the spacing among
individuals within the population
boundaries

• Some significant attributes that
populations possess are –
 Birth rate / natality, population density, death rate/
mortality, sex ratio, age distribution.
 Birth rate or natality refers to the average number of
young ones produced by birth, hatching or
germination per unit time (usually per year). In the
case of humans, it is commonly expressed as the
number of births per 1000 individuals in the
population per year.
• The maximum birth rate that a species can achieve
under ideal environmental conditions called
potential natality. However, the actual birth rate
under the existing conditions is much less. It is
termed as realized natality.
Population Attributes

• Some significant attributes that populations
possess are –
Death rate or mortality is the average number of
individuals that die per unit time (usually per year).
In humans, it is commonly expressed as the
number of deaths per 1000 persons in a population
per year. Lowest death rate for a given species in
most favourable conditions is called potential
mortality, while the actual death rate being
observed in existing conditions is called realized
mortality.
The death rate of a population can be easily
represented by survivorship curve. In this curve,
time is plotted against the number of survivors.
3
Population Attributes

Population density, is the number of
individuals present per unit area or volume at a given
time. For instance, the number of animals per square
kilometre, number of trees per area in a forest, or
number of planktonic organisms per cubic meter of
water.
The population density is determined by four basicprocesses-
natality, mortality, immigration and emigration.
The percentage ratio of natality over mortality is known as
vital index i.e., natality / mortality × 100. It determines the
growth of a population.
Immigration is the number of individuals of the same
species that have come into the habitat from elsewhere
during the time period under consideration.
Emigration is the number of individuals of the population
who left the habitat and gone elsewhere during the time
period under consideration. 4
Population Growth

Age distribution : Various age groups in a
population determine its reproductive status.
The three ages referred to as ecological ages in
a population are – pre-reproductive,
reproductive and post-reproductive.
Population with more young members grow
rapidly, while the declining populations have a
large proportion of older individuals.
5
Population Growth
AGEPYRAMIDS

A life table follows fate of a
cohort (age-class of
organisms) from birth to
death.

Survivorship Curves
Data in a life table can be
represented graphically by a
survival curve.
Curve usually based on a
standardized population of
1000 individuals and the X-axis
scale is logarithmic.

There are three kinds of survivorship curves–
Convex curve (TYPE - I) : When organisms have completed
their potential life span and died in old age then the curve is
convex, the curves goes horizontal till potential life span and
then declines rapidly. e.g., Man, rabbit and many mammals.
Diagonal curve (TYPE – II) : If the death rate of different age
groups of organisms are equal, then the curve is represented or
plotted as a straight line.
e.g., Hydra, mice and many adult birds.
Concave curve (TYPE - III) : This kind of curve is mostly
found in such organisms who die before their potential life
span. e.g., Fish, Oysters and Invertebrates.
Sex ratio is the ratio of males to females in a population.
6
I
II
III
50 100
0
1
10
100
1,000
Percentage of maximum life span
Number
of
survivors
(log
scale)

Reproductive Table
The table tallies the
number of females
produced by each age
group.
Product of proportion of
females of a given age that
are breeding and the
number of female offspring
of those breeding females.

Life history diversity
Some species exhibit semelparity, or “big-
bang” reproduction. These species
reproduce once and die (bamboo, salmon,
century plant).
Semelparous reproduction:
Semelparous reproduction often an adaptation to
erratic climatic conditions.
Suitable breeding conditions occur rarely and
organisms devote all their resources to reproduction
when conditions are good (e.g. century plant).
Century Plant

Iteroparous reproduction
Some species exhibit iteroparity, or repeated reproduction
and produce offspring repeatedly over time.
E.g. humans, cats, birds.
Iteroparous reproduction occurs when organisms have good
prospects of reproducing in the future (i.e., they are long-
lived).
Characteristic of larger organisms and those that experience
more stable environmental conditions.

Quantity
vs.
Quality of offspring
Organisms face tradeoffs between
the number and quality of young
they can produce because they
have only a limited quantity of
resources to invest.
The choice is basically between a
few large or many small
offspring.

Population growth
Occurs when birth rate exceeds death rate.
Organisms have enormous potential to increase their populations if
not constrained by mortality.
Any organism could swamp the planet in a short time if it reproduced
without restraint.

7
Population Growth
(A) Sigmoid or S-shaped
growth curve
It is formed of five phases :
Lag phase: In this phase, individuals adapt themselves to the
new environment, so there is no or very little increase in
population.
Positive acceleration phase: It is the period of slow increasein
population in the beginning.
Logarithmic or exponential phase : It is the period ofrapid rise
in population due to the availability of food and the requirements
of life in plenty and no competition.
Negative acceleration phase : In this again, there is a slow rise in
population as the environmental resistance increases.
Stationary (Plateau) phase : Finally, growth rate becomes
stable because mortality and natality rates become equal to each
other. So, there is zero growth rate. A stable population is said to
be in equilibrium, or at saturation level.
This limit in population is a constant (K) and is imposed bythe
carrying capacity of the environment.

This type of population growth is
also
called Verhulst Pearl Logistic
growth.
8
Population Growth
(B) J-shaped Growth
curve
 It has only two phases :-
 Lag phase : It is period of adaptation of animals to
new environment so and thus, is characterized by
slow or no growth in population.
 Logarithmic or Exponential phase : It is
characterized by rapid growth in a population which
continues till enough food is available.
 The sudden increase in mortality is called population
crash. Lemming of tundra, some insect, algal blooms
and annual plants also show J-shaped curves.
 The population growth curve is S-shaped in
most of the organisms.
 Human population also shows S-shaped curve.

Change in population size during time interval
= (Birth + Immigration during time interval) – (Death +
Emigration during time interval)
The above expression in words may be represented in a
simple way by a mathematical model.
Suppose, N = population size and t = time. The Greek letter
delta, (Δ), indicates change. We can now represent change in
population as ΔN, and time interval as Δt.
The verbal equation can be written as
ΔN/ Δt = (B + I) – ( D + E ) in which, B = absolute number
of births in the population during the time interval, D = the
absolute number of deaths during that interval;
I = immigrants and E = emigrants.
I and E , being insignificant, may be ignored. Then the
equation simplifies to
ΔN/ Δt =B – D.
9
Population Growth

Organisms do not exist in absolute isolation but co-occur in a matrix of many species. In
order to survive, an organism must interact with its biotic and abiotic environment and thus
it becomes functional part of the ecosystem
An organisms interaction with its environment are fundamental to the survival of that
organism and the functioning of the ecosystem as a whole.
These interactions are referred to as population interactions and a particular species in a
particular area will be affected by presence or absence of other species because of these
interactions.

NEUTRAL INTERACTION
NEGATIVE INTERACTIONS
POSITIVE INTERACTIONS

NEUTRALISM
The two populations are not affected by each other whether they are coexisting or not. In
this interaction the fitness of one species has absolutely no effect on the other. However in
nature true neutralism is nonexistent and hence unlikely to find any suitable example.
Ecosystem itself characterize complex network of interactions so one cannot assert
positively that there is absolutely no interaction: positive or negative. Since true neutralism
is rare the term is often used to describe situations where interactions are merely
insignificant or negligible
An example of neutralism is interaction
between a rainbow trout and dandelion in a
mountain valley.

Allelopathy is a biological phenomenon by which
an organism produces one or more biochemicals that
influence the germination, growth, survival, and
reproduction of other organisms.
These biochemicals are known
as allelochemicals and can have beneficial (positive
allelopathy) or detrimental (negative allelopathy)
effects on the target organisms and the community.
Allelopathic plant Impact
Broccoli
Broccoli residue
interferes with
growth of other
cruciferous crops
that follow
Jungle rice
Inhibition of rice
crop
Black walnut
(juglone)
Inhibition of egg
plant, tomatoes,
potatoes.

INTERSPECIFIC INTERACTIONS.
A) POSITIVE INTERACTIONS
Mutualism
Commensalism
Protocooperation
B) NEGATIVE INTERACTIONS
Parasitism
Parasitoidism
Predation
Amensalism
Competition

A relationship between two species of
organisms in which both benefit from the
association.
Its an obligatory interaction i.e. the interacting
species are interdependent and cannot survive
without each other. The fungi and algae that
combine to form lichen are obligate mutualists.
 eg. mycorrhizae

Commensalism, in biology, is a relation between individuals of two species in which one species obtains food or
other benefits from the other without either harming or benefiting the latter. The commensal (the species that benefits
from the association) may obtain nutrients, shelter, support, or locomotion from the host species, which is
substantially unaffected. The commensal relation is often between a larger host and a smaller commensal; the host
organism is unmodified, whereas the commensal species may show great structural adaptation consonant with its
habits
Orchids - Some orchids grow on trees and that
does not harm the tree.
Shark and suckerfish: Sharks are larger in size and
suckerfish attaches itself to shark and feeds on the leftover
food of shark.

Protocooperation is where two species interact with each other beneficially; they have no need to interact
with each other - they interact purely for the gain that they receive from doing this. It is not at all necessary
i.e. not obligatory.
for protocooperation to occur; growth and survival is possible in the absence of the interaction.
a) Ants and aphids: The ants and aphids
interact to derive mutual benefit from each
other. The trees and shrubs are hosts to
honeydew secreting species such as aphids.
The ant gathers the honeydew produced by
aphids as their food. In turn, ants protect
them from their natural predators. Ants also
stimulate the honeydew secretion from aphid

Spider crab and algae - With the ocean as
its habitat, spider crabs often spend their
time in some of the most shallow areas of
the sea making them highly visible to
predators. However, living on their backs
are algae that act as camouflage.
Oxpeckers and zebras or rhinos - In
this relationship, the oxpecker (a bird)
lives on the zebra or rhino, sustaining
itself by eating all of the bugs and
parasites on the animal

 Competition is an interaction between organisms or species in which both the
species are harmed. Limited supply of at least one resource (such as food, water,
and territory) used by both can be a factor. Competition is one of many
interacting biotic and abiotic factors that affect community structure.
Competition among members of the same species is known as intraspecific
competition, while competition between individuals of different species is known
as interspecific competition. Competition is not always straightforward, and can
occur in both a direct and indirect fashion.
TYPES:
Resource Competition
Interference Competition

 PREDATION is a biological interaction where a predator (an organism
that is hunting) feeds on its prey (the organism that is attacked).Predators
may or may not kill their prey prior to feeding on them, but the act of
predation often results in the death of the prey and the eventual absorption
of the prey's tissue through consumption.

Types:
SOCIAL PREDATION
In social predation, a group of predators cooperates to kill
creatures larger than those they could overpower singly.
Social predators such as ions, hyenas,
and wolves collaborate to catch and kill large herbivores.
By hunting socially chimpanzees can catch colobus
monkeys that would readily escape an individual hunter.
SIZE SELECTIVE PREDATION
In size-selective predation, predators select prey of a certain
size. Large prey may prove troublesome for a predator,
while small prey might prove hard to find and in any case
provide less of a reward. This has led to a correlation
between the size of predators and their prey. Size may also
act as a refuge for large prey. For example, adult elephants
are relatively safe from predation by lions, but juveniles are
vulnerable.

ADAPTATIONS IN PREDATORS:
Vision: Unlike prey animals, whose eyes are placed on the sides of their head, predators’ eyes are
placed towards the front of their head, giving them three dimensional, binocular vision
Smell: Although predators rely heavily on their vision to find prey, they also have a well adapted
sense of smell. Some predatory animals such as the wolf, fox, and coyote rely more on their sense of
smell than their vision
Stalking: Animals that stalk their prey have developed special features which ensure that they can
sneak up on their prey undetected. Cats have soft padded paws with retractable claws which help
them walk silently.
Sit and Wait: The sit and wait strategy requires little energy; however, it does require lots of
patience. Animals who use this hunting strategy usually employ some form of camouflage. They
blend in with their surroundings and wait for their prey to come within striking distance.
Teeth and claws: Predatory animals are equipped with long, sharp claws and teeth specialised for
tearing, shearing, and cutting flesh.

ADAPTATIONS IN PREY:
Special weapons: When an animal is under attack, it will defend itself with all the weapons it has: arms,
legs, wings, hooves, teeth, claws, antlers, etc
Alarms: Prey animals sometimes make a lot of noise when they are attacked, hoping to scare the predator
away or at least warn others of the danger. Alarm calls are especially important for protecting young
Hearing: A well-developed sense of hearing is essential for the survival of prey animals. Many can hear a
predator approaching long before they can see it.
Disruptive Colouration: Markings that disguise the animal among its surroundings. It leads the eye away
from its shape and into the background. It makes the shape of the animal harder to pick out from the other
shapes around it
Sight: Prey animals need all-around vision to see advancing predators. Therefore, their eyes are located on
the sides of their head, which gives them a wide field of vision
Bluffing or Mimicry: is used by some animals to fool their predators into thinking they are dangerous,
already dead, or hurt. Some may copy the behaviour of a dangerous animal or have specialised body parts
that look dangerous. (Mullerian and Bateson Mimicry)

 A polar bear (Ursus maritimus) as the predator feeding on a bearded seal
Meat ants feeding on a cicada; some species can prey on individuals of far
greater size, particularly when working cooperatively.

Amensalism is any relationship between organisms of different species in which one
organism is inhibited or destroyed while the other organism remains unaffected.
EXAMPLE : Observed between the Spanish ibex and weevils of the
genus Timarcha which feed upon the same type of shrub. Whilst the presence of the
weevil has almost no influence on food availability, the presence of ibex has an enormous
detrimental effect on weevil numbers, as they consume significant quantities of plant
matter and incidentally ingest the weevils upon it.

Other examples:
Microcystis poduces toxic substance like hydroxylamine which
causes death of the fish and even of the cattle that drinks
contaminated water
Penicillium notatum releases an antibiotic called penicillin
which inhibits the growth of a variety of bacteria. Eg:
Staphylococcus.

Parasitism is a non-mutual relationship between species, where one species, the parasite, benefits at the
expense of the other, the host. Traditionally parasite (in biological usage) referred primarily to organisms
visible to the naked eye, or macroparasites (such as helminths). Parasites can be microparasites, which are
typically smaller, such as protozoa,viruses, and bacteria.
TYPES
OBLIGATE PARASITE
FACULTATIVE PARASITE
ECTOPARASITE
ENDOPARASITE
EPIPARASITE
HYPERPARASITE
KLEPTOPARASITE
BROOD PARASITE

Endoparasites in Humans
Entamoeba histolytica – Amoebiasis
Taenia saginata - Taeniasis
Fasciola hepatica- Fascioliasis
Ectoparasites
Hirudinea - Leeches
Candiru or vampire fish - Freshwater catfish in the
Amazon River
Head louse

A parasitoid is an organism that spends a significant portion of its life history attached
to or within a single host organism in a relationship where the host is ultimately killed.
Thus, parasitoidy is a similar evolutionary strategy to typical parasitism, except for the
more serious prognosis for the host.
A parasitoidal wasp ovipositing into the body of a spotted alfalfa aphid

Summary of Direct Pairwise Interactions Between Two Populations
Species
Type of Interaction A B Nature of Interaction
Competition – – Each population inhibits the other
Predation, parasitism, + – Population A, the predator, parasite,
and Batesian mimicry or mimic, kills or exploits members of population B, the prey, host, or
model
Mutualism, + + Interaction is favorable to both
Commensalism + 0 Population A, the commensal, benefits whereas B, the host, is not
affected
Amensalism – 0 Population A is inhibited, but B is unaffected
Neutralism 0 0 Neither party affects the other

Population ecology

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