SS3 BIOLOGY LESSON NOTE (FIRST TERM)

By Mr Godswill Chidiebere 09046124389

WEEK 1-2

SENSE ORGANS

Information about the mechanical, thermal, chemical and electromagnetic changes in the
internal and external environment are perceived by sensory receptors in our bodies. These
process the information and make us aware of changes.

There are four main groups of sensory receptors namely;

(i) Mechanoreceptors: these detect mechanical stimuli such as sound, gravity, pressure,
vibrations, etc.

(ii) Thermo receptors: These detect thermal stimuli such as heat and cold.

(iii) Chemo receptors: These detect chemical stimuli such as odour and taste.

(iv) Photo receptors: These detect electromagnetic visual stimuli such as light intensities and
colour.

The sensory receptors transform the stimulus received into a nerve impulse which is transmitted
to the central nervous system for interpretation and appropriate response.

FORMS OF SENSORY RECEPTORS

There are different types of sensory receptors;

(i) The primary sense cells: these exist as free nerve endings of nerve fibers e.g. the touch
receptors in the mammalian skin.

(ii) Secondary Sense cells: these consist of groups of cells that are not neurons but are
specialized to detect stimuli.

(iii) Sense Organs: These consist of secondary sense cells, sensory neurons and associated
structures e.g. the eyes and ears.

The human body has five sense organs namely;

(i) The eye for vision.

(ii) The ear for hearing and balancing

(iii) The nose for smelling

(iv) The tongue (taste buds) for tasting.

(v) The skin for receiving sensations of temperature, pain, touch and pressure.

THE ORGAN OF SIGHT

The two eyes are the organs of sight in every vertebrate. Each eye is a spherical organ located
in a bony socket in the skull.

It is held in position by six muscles;

(i) Two oblique muscles (lower and upper) and

(ii) Four rectus muscles (upper, lateral, median and lower ones).

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They control the free movement of the eye in many directions. To move the eyes the six
muscles work in opposing fashion against one another. Among these muscles towards the back
of the eye is a thick optic nerve which connects the eye to the brain.

A small portion of the front part is exposed. This part is protected by the upper and lower eyelids
and the eye lashes. Beneath the eyelids are tear glands. These secrete a saline fluid (tears)
which moisten the conjunctiva, washes away dust particles and insects, and destroys most
bacteria (it contains a chemical called lysozyme). This fluid drains through a tear duct into the
nasal cavity.

Muscles of the eye

Structure of the Eye

A vertical section of the eye shows the inside of the eyeball to be a fluid filled hollow structure.
Its wall consists of three layers;

(i) The sclera which is the outermost

(ii) The choroid which is the middle layer.

(iii) The retina which is the innermost layer.

(i) The Sclera or sclerotic layer is a white tough connective tissue which protects and maintains
the spherical shape of the eyeball. It bulges at the front to form a convex, transparent tissue
called cornea. At the, back it is perforated by the optic nerve. The front part of the eye is
covered by a tough, thin, transparent membrane called the conjunctiva. This covers and
protects the cornea.

(ii) The choroid is made up of pigmented cells. It is richly supplied with blood capillaries. These
supply nourishment and oxygen to the cells of the eye. The layer contains a black pigment
which absorbs light rays and prevents light reflection into the eye.

The choroid forms the iris in front of the eye. There is an aperture through the iris called the
pupil. Light enters the eyes through the pupil. The iris controls the amount of light passing
through the pupil. The iris has radial and circular muscle fibers. In bright light, the circular fibers

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contact while the radial fibers relax, hence, the pupil becomes smaller and less light enters the
eye. In dim light the reverse occurs.

(iii) The retina is the innermost layer of the eye. It is the light – sensitive layer and is restricted to
the back of the eyeball. It gets its nourishment from the capillaries of the choroids. It contains
two types of photoreceptors;

(a) The rods and

(b) The cones.

(a) The rods are the photoreceptors responsible for black and white vision, as well as night
vision.

(b) The cones are responsible for colour vision and are sensitive to high light intensities.

Images formed on the retina are always inverted and smaller than the real objects.

The part of the retina that contains the highest concentration of light sensitive cells is the yellow
spot (fovea centralis). It is the most sensitive part of the retina. A point with light insensitive cells
just below the fovea centralis is called the blind spot. The optic nerve leaves the eyeball to the
brain at the blind spot.

The optical apparatus of the eye is the cornea, the lens and the retina and they lie along the
optical axis of the eye. The lens is a crystalline, transparent, biconvex flexible disc, located just
behind the iris. It is held by the suspensory ligaments attached to the ciliary muscles which are
located on and originate from the choroid layer.

The shape of the lens changes when the muscles contract. This helps the eye to focus images
on the retina for far or near objects. The space in front of the lens is filled with watery aqueous
humour. The much bigger space behind the lens is filled with jelly –like vitreous humour, which
helps to maintain the spherical shape of the eyeball. Both are solutions of proteins, sugars and
salts in water. Apart from maintaining the spherical shape of the eye they assist in formation of
images by reflecting incoming light rays.

The Eye

IMAGE FORMATION

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To see an object, light rays from every point on the object enter the eye. The rays pass through
the cornea, aqueous humour, lens and vitreous humour along the optical axis.

The rays are refracted and brought to a focus on the retina where an image is formed. This
image is real, inverted and smaller than the object. The reflection of light within the walls of the
eyeball is prevented by the dark pigmentation of the choroid layer. The points of light which fall
on the retina stimulate the rods and cones (photoreceptors) in the retina. The electrical impulse
initiated is then transmitted to the brain which interprets it as upright with the correct size and
distance from the eyes. The object is most clearly seen when the image falls on the fovea
centralis.

ACCOMMODATION

This is a reflex action of the eyes. It is the ability of the eyes to focus properly, images of objects
from far and near on the retina.

When focusing on distant objects, the ring muscles of the ciliary body relax and move away
radially from the lens. This tightens the suspensory ligaments so that they pull radially on the
lens. These actions cause the lens to flatten and become thinner, so that light rays falling on it
are refracted only slightly and are able to converge on the retina.

When the eye focuses on a near object, the ring muscle of the ciliary body contracts so that it
moves radially towards the lens. This reduces the tension in the suspensory ligament and the
lens bulges out by virtue of its elasticity. This causes the light rays falling on the lens to be
refracted more towards the optical axis, thus bringing the rays to a focus on the retina.

EYE DEFECTS

Whenever an image cannot be formed properly on the retina, we say the eye has a defect.
Some of the major eye defects are;

(i) Short – sightedness (myopia).

(ii) Long – sightedness (hypermetropia)

(iii) Presbyopia

(iv) Astigmatism

(v) Colour blindness.

(vi) Cataracts

(vii) Night blindness

(i) Short – sightedness (myopia)

People who are short sighted can see near objects clearly but distant objects appear blurred.
This may be due to;

(a) The eyeball being longer than normal from back to front.

(b) The lens may be too strong

(c) The refractive power of the eye may be too great

(d) The distance between the lens and the retina is increased.

The result is that the light rays are focused in front of the retina because parallel light rays are
refracted too much. The image that falls on the retina is therefore out of focus and blurred. For
near objects however, rays of light falling on the lens converge on the retina and a clear image
is seen.
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Short –sightedness can be corrected by using spectacles with suitable concave lenses. The
lenses will diverge the rays from a distant object to the correct extent before they are focused on
the retina by the eye lenses.

(ii) Long- sightedness

People who are long – sighted can see distant objects clearly but near objects appear blurred.
This may be due to;

(a) The eyeball being too short

(b) The lens being too weak and the refractive power too little.

(c) The eye lens is not sufficiently convex.

(d) The diverging light rays are not sufficiently refracted.

As a result of one or the other of these conditions, rays of light from a near object are not bent
sufficiently enough to come to a focus on the retina but forms outside the retina.

Long – sightedness can be corrected by using spectacles with suitable convex lenses. The
lenses will converge the rays from a near object towards the eye lenses which bend them
further to converge on the retina.

(iii) Astigmatism

People who suffer from astigmatism have a cornea with an uneven curvature.

Light rays that arrive at the surface of the cornea are refracted unevenly so that distorted
images are formed on the retina.

In some cases, vertical parts of a figure can be seen clearly while the horizontal components
appear blurred.

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Astigmatism may be corrected by using lenses of uneven surfaces that functionally compensate
the cornea.

(iv) Presbyopia: The eye lens or ciliary muscles of many old people may become inelastic or
hardened so that accommodation is reduced. The defect may correct with the use of weak
convex lenses or bifocal lenses with upper and lower halves.

(v) Colour – blindness: In a colour blind person, one or more types of cones are absent or
defective. It is an inherited condition and can be detected by using special charts. It cannot be
corrected.

(vi) Cataract: This condition usually occurs in old people. The lens become cloudy and sufficient
light cannot pass through it. The affected person cannot see objects clearly. It can be corrected
by removing the affected lens and replacing it with a plastic one or by using spectacles with
suitable lens.

(vii) Night – Blindness: people suffering from this defect cannot see clearly in dim light. This is
usually due to a deficiency of vitamin A which is required to manufacture rhodopsin in the
photoreceptors.

Other eye defects include pterygium, glaucoma, conjunctivitis, river blindness. etc.

CARE OF THE EYES

(i) Do not clean the eyes or rub them with dirty handkerchiefs or dirty fingers.

(ii) Do not read books in dim light

(iii) See a doctor in case of eye infections.

(iv) Avoid looking into very bright light/lights of high intensity.

WEEK 3
In vertebrates, the two ears perform the functions of hearing and balancing. The ear of man is
located in the temporal bone on the skull.

The ear has three distinct parts namely;

(i) The outer ear.

(ii) The middle ear.

(iii) The inner ear.

(i) THE OUTER EAR

This consists of the pinna; an outer ear tube or auditory canal called the external auditory
meatus and the ear drum / tympanum is made of cartilage covered by skin. It collects sound
waves and directs them along the external auditory meatus to the tympanic membrane
(eardrum). It also detects the direction of sound waves.

The auditory meatus is a narrow passage which contains fine hairs and wax- producing glands.
The wax produced prevents entry of insects, germs and dust into the ear.

The tympanum is a thin tissue which separates the outer ear from the middle ear. It pulsates
when sound waves strike it.

(ii) THE MIDDLE EAR

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This is a small, air-filled chamber buried in the skull. It contains three tiny bones called ossicles
that articulate with one another and form a bridge across the middle ear held in place by
muscles.

The ossicles include:

(a) The malleus (hammer), which is attached to the typanum (ear drum).

(b) The incus (anvil) which connects the malleus at one end by a hinge joint to the stapes at
the other end by a ball and socket joint.

(c) The stapes (stirrup) which fits into the inner ear at a membrane called the oval window
(fenestra ovalis).

The ossicles transmit vibrations across the tympanic membrane to the oval window. They also
magnify the pressure on the oval window about thirty times.

A second opening covered by a membrane, the round window, connects the middle ear with the
inner ear. The middle ear is air filled and connected to the nasopharynx by a narrow tube called
the Eustachian tube. The tube allows air from the mouth to enter and leave the middle ear. It
opens during yawning and helps to equalize air pressure on both sides of the eardrum. This
helps prevent the bursting of the eardrum.

However, infections may travel from the throat and nose to the middle ear through this tube.

(iii) THE INNER EAR:

The organs of hearing and balance are found in the inner ear. It consists of a system of bony
canals (the bony labyrinth) filled with a fluid called the perilymph. Within this system of canals is
another system of membranous channels (the membranous labyrinth) filled with fluid called the
endolymph.

The two systems contain;

(i) The semicircular canals together with the utricle and saccule, which are concerned with
balance and

(ii) The cochlea, which is the organ of hearing.

The ear

HEARING
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Hearing occurs as follows:

(i) The pinna in each ear collects sound waves.

(ii) The sound waves pass through the external auditory meatus, strike the tympanum and make
it vibrate.

(iii) The vibrations are transmitted across the three ossicles of the middle ear.

(iv) In the middle ear, the vibrations are magnified about thirty times.

(v) Vibrations of the oval window cause vibration in the round window and the perilymph of the
inner ear.

(vi) The perilymph in turn causes the endolymph to vibrate.

(vii) Vibrations in the endolymph of the cochlea stimulate its sensory cells to generate nervous
impulses.

(viii) The nervous impulses are transmitted by the auditory nerve to the brain which interprets
the pitch, quality and loudness of sounds and thus, hearing takes places.

ORGANS OF BALANCE

The organs of balance are the semi- circular canals. These are connected to the sac-like utricle
and saccule. Two of the three semi-circular canals are vertical while the third is horizontal. The
planes of the canals are at right angles.

Each semicircular canal has a swollen base called the ampulla which contains sensory cells.
Within each ampulla is the cupula which is a gelatinous structure that can sway from side to
side by the movements of the endolymph. At the base of the cupula, there are sensory cells
which have hair – like projections stretching into the cupulae. These sensory cells
(mechanoreceptors) synapse with sensory neurones that form part of the auditory nerve.

The utricle and saccule have gelatinous plates with chalky granules called maculae in their
basal walls. At the base of each plate are mechanoreceptors which synapse with the sensory
neurones that form part of the auditory nerve.

The ear; note the cochlea, the three semicircular canals, the utricle and saccule

The cochlea is a spirally coiled tube partitioned into three membranous, longitudinal canals.
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The outer canals are filled with perilymph while the middle canal is filled with endolymph. The
middle canal has thousands of mechanoreceptors along its length which synapse with sensory
neurones that are part of the auditory nerve. The mechanoreceptors are grouped into organs of
corti.

Each organ of corti, has basilar and tectorial membranes which vibrate by the movement of fluid
in the canals.

When the head is moved horizontally or vertically such as when a person twists around in an
upright position or bends the head forward, backward or sideways, the movements push the
cupula in the ampulla of the affected semi – circular canal i.e. horizontal movements affect
horizontal canals while vertical movements affect vertical canals.

The endolymph moves in opposite direction to the movement of the head, the cupula therefore
moves in the same direction as the endolymph. The movement of the cupula stimulates the
mechanoreceptors to send off nervous impulses to the brain through the auditory nerve. In
response, the brain sends out motor impulses to the appropriate muscles to react in a way as to
maintain the balance of the body in the new position.

The sensory hairs of the maculae within the utricle and saccule respond to gravity. When the
head is moved vertically or horizontally the displacement of the chalky granules bend the hairs
under the gelatinous plates which initiates impulses to the brain through the vestibular nerve
(part of the auditory nerve.)

CARE OF THE MAMMALIAN EAR

(i) Avoid the use of sharp or pointed objects in cleaning or scratching the ear.

(ii) Seek prompt and appropriate medical attention if there is an ache, or a discharge from the
ear.

(iii) Avoid inserting dirty fingers into the ear or else the ear may become infected by bacteria.

(iv) Workers in noisy environments should wear ear muffs to prevent excessive and prolonged
exposure to noise which may cause deafness.

(v) Remove the wax in the ear canal regularly with soft cotton wool.

(vi) Avoid receiving blows to the side of the ear, else the eardrum may burst.

EAR DEFECTS

The major ear defect is deafness which may be partial or temporary, or permanent. People with
defects in the outer or middle ear may be aided with earphones. Those with defects in the inner
ear may be helped with an implanted receiver. However, some have total deafness and may
need to learn sign language and how to read lips.

EVALUATION:

1. Describe the structure of the ear.

2. Briefly discuss how the ear carries out its function of (a) hearing (b) balance.
3. State three ways of caring for the ear.

ASSIGNMENT

Make a large well labelled diagram of the mammalian ear.

Week 4
SENSE ORGAN – THE NOSE

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THE ORGAN OF SMELL

The nose (olfactory organ) is the organ of smell. It consists of groups of chemo receptors found
in the epithelial lining of the upper portion of the nasal cavity. They are primary sense cells
which form fine processes that extend into the surface film of mucus over the epithelium.
Chemicals in the surrounding air dissolve in the mucus film and stimulate the chemo-receptors
which sendoff sensory impulse through the olfactory nerves to the brain.

The sense of smell in dogs is acute. This makes dogs very efficient hunters so they can track
and catch their prey easily and also sense the presence of predators and escape quickly.

In other organisms’ chemo-receptors may occur in other parts of the body for example the
butterfly has chemo-receptors on its antennae for sensing its environment.

The Nose

EVALUATION

1. Enumerate four groups of sensory receptors stating their functions.

2. Discuss briefly the sense of smell in mammals.

Week 5
THE ORGAN OF TASTE

In mammals the organ of taste is the tongue. Taste chemoreceptors are grouped on the upper
surface of the tongue, on the soft palate and in the epithelium at the back of the mouth. These
chemoreceptors are specialized sense cells that synapse with sensory neurones. On the tongue
they are found in the taste buds. Taste buds lie in grooves on the surface of the tongue.
Substances have to be in dissolved form to stimulate the taste receptors.

The tongue is sensitive to four primary tastes – sweat, sour, salty and bitter. The back of the
tongue is sensitive to bitter taste; the sides to salty and sour stimuli while the tip is sensitive to
sweet sensations. The tongue can also detect alkaline taste, texture of food as well as its
temperature.

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 Taste Receptors

Taste buds in the tongue

EVALUATION

1. State the 4 types of sensation the tongue is sensitive to and the sections of the tongue
that respond to these sensations.
2. Briefly discuss the organ of taste.

ASSIGNMENT
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Make a large well labelled diagram of the tongue showing the four primary taste areas.

Week 6
THE SKIN

The sensations of touch, pressure, pain, cold and heat are detected by the mammalian skin
which is richly supplied with sensory receptors.

The sensory receptors in the skin are mainly primary sense cells. Some of the dendrites are
finely branched with free ends while others are closed within capsules. The sensory receptors
are not evenly distributed on the epidermis but are concentrated in the certain areas of the skin
which makes such areas more sensitive to specific stimuli. For instance, the touch receptors
are sensitive to small amounts of pressure and are located close to the surface of the skin,
attached to hair follicles and concentrated in the finger tips, face and neck.

Pressure receptors (pacinian corpuscles) are sensitive to large amounts of pressure. Each
receptor consists of a single nerve ending surrounded by connective tissue and are found
mainly in the dermis, joints and the muscles.

Thermo receptors are sensitive to cold and heat sensations and are widely distributed in the
epidermis and dermis. They are free nerve endings some of which are enclosed in capsules.

They play an important role in the regulation of body temperature in warm blooded animals.

Overstimulation of the thermo receptors and pressure receptors cause sensations of pain.
Special pain receptors called nociceptors are also present in the epidermis and many internal
organs. Sensations of pain prevent us from hurting ourselves.

A pain stimulus is converted into an electrical impulse that travels along a neurone fibre that
ends at the spinal cord. At the nerve cord, neurochemicals are released at the synapse which
fires the next neurone sending the impulse to the brain. If the pain is persistent analgesics
become necessary. Analgesics work at the synapses in the spinal cord to prevent the firing of a
second neurone.

Stretch receptors (mechanoreceptors) are found strapped around muscle fibres situated deep
within skeletal muscles. The stretching of the skeletal muscles causes the stretched receptors to
initiate impulse to the spinal cord and finally to the brain. Analysing impulses from various
mechanoreceptors enable the brain monitor the position of the body and so work to maintain the
balance of the entire body.

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 The Skin

EVALUATION

1. Mention four sensory neurones found in the skin.

2. Discuss the functions of the sensory neurones mentioned above.

Week 7
CONTENT:

1. Succession

(i) Meaning of succession

(ii) Structural changes in species composition, variety and increase in number.

(iii) Primary succession (a) Aquatic and (b) Terrestrial habitat

(iv) Secondary Succession-Meaning and examples

(v) Characteristics of a stable community.

2. Overcrowding

(i) Population density

(ii) Important factors affecting population

MEANING AND DEFINITION OF SUCCESSION

Succession can be defined as the series of progressive changes at different times in plants and
animal’s communities in a given habitat until a stable community is established.

This succession is a result of changes in form, structure, composition, variety and numbers of
plants and animals’ communities in a particular habitat over a period of time.

It occurs in newly formed natural habitats such as bare rocks, seashore or pond. It can also take
place in artificial habitats such as abandoned farmlands, cultivated forests or grassland.

A stable or climax community takes a long time to form as it is usually gradual. The organism of
a stable community has structural appearance that is now definite and may not change. The
species would have gone through several changes, would have displaced several species that
are less advanced in number. A climax habitat is also an equilibrium environment.

ECOLOGICAL SUCCESSION

This is an orderly and gradual change of different species of plants and animal communities in a
given area over a period of time.

The condition of the area becomes gradually modified by their activities so that a climax
balanced community is established.

Process of ecological succession on a bare piece of land-Terrestrial habitat

(i) The first living organism on a bare plot of land are the pioneers which are algae and mosses

(ii) As the pioneers live and die, they add nutrients to the soil.

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(iii) This creates conditions that are more favorable to other plants like ferns, herbs, lower
animals like insects, worms which could not live there before.

(iv) As they live and die, their remains further improve the soil conditions.

(v) Thus, perennial herbs, grasses and other animals such as lizards, snakes, toads, and rats
are able to live there.

(vi) Subsequently, a point is reached at which the soil can support the growth of taller species
such as shrubs, trees and higher animals.

(vii) Higher animals, birds, various mammals may be found in the habitat at this stage.

(viii) Thus, a climax community is established which is in equilibrium with the environment.

Community; A community is a group of different living population of living organisms living

together in a habitat and interacting with each other.

STRUCTURAL CHANGES IN SPECIES COMPOSITION, VARIETY AND INCREASE IN

NUMBER

In any given habitat, available communities of plants through their activities modify the
environment. This leads to an unstable state to the existing community of plants and animals.
Structural modifications in new group of plants and animals make the habitat suitable for their
newcomers. After sometimes, the activities of these more suitable organisms again modify the
environment. With this modification, groups are again incapable of surviving as new colonizers
appear that are more suited to the environment. This results in competition among the
organisms such that the stronger ones survive and the weak ones die off with their decay
forming food as well as adding nutrients to the habitat.

The changes occur in species structure, composition, different varieties of plants and animals
with increase in numbers.

Ecological succession can be studied in abandoned farmland, pond, lawn, and building sites.

The initial organisms are usually fewer hardy species of plants and animals and the starting
point of succession is the formation of soil or land.

In aquatic habitat, the environment is often a newly formed pond that has shallow water with
microscopic plants and animals like bacteria, algae and protozoa. These are sample in
structure, exhibit simple live processes and easy to make complex populations

CONDITIONS NECESSARY FOR ECOLOGICAL SUCCESSION.

For ecological succession to take place, the following must be present:

1. Colonizing materials

2. Colonizers

3. Soil formation-terrestrial habitat

4. Pond or seashore-Aquatic habitat

 Followed by rooted and flowing aquatic plants

 Followed by marsh with partially submerged plants like sedges and reeds.
 Followed by dry soil with shrubby growth
 Shrubby growth is finally succeeded by forest which is the stable or climax community.

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Initial succession once started occurs at a fast rate which gradually slows down. The colonizers
have short life cycle and low nutrient requirements. Their remains decay and add mineral salts
and other organic materials that improve the quality of the soil.

At the beginning of succession, the rate of changes occurs at a very fast rate and gradually
slows down to a climax community when no more natural changes are taking place.

Initial biotic colonizers such as Lichens, Mosses, Algae and Liverworts have short life cycles
and low nutrient requirements. Their remains decay and add mineral salts and other organic
materials to enrich the soil quality.

TYPES OF SUCCESSION

Two main types of succession are recognized by ecologists.

(i) Primary Succession

(ii) Secondary Succession

(i) Primary Succession: This takes place in an area where biotic communities are absent, i.e.
bare rocks or ground and involves the formation of soil in a terrestrial habitat. In aquatic habitat,
it occurs in lakes and ponds filled with sediments and vegetation that will become swamps and
at last, forests.

Primary succession takes a long time to reach climax. Certain natural phenomenon like land
slide, extreme drought, sand dunes, earthquake, thunder storm and hurricanes also give rise to
primary succession.

(ii) Secondary Succession

Secondary Succession occurs in an area that was previously existing on a terrestrial habitat that
had disturbances leaving soil and certain organisms behind. Disturbances like fire disasters,
droughts or floods.

Secondary Succession is much faster than primary Succession. This is as a result of the
existing soil and suppressed plants that can easily start the process.

CHARACTERISTICS OF A STABLE COMMUNITY

A climax or stable community has peculiar characteristics such as:

(i) A stable community is not totally homogenous. It changes with age

(ii) It depends on the climate of the region and it is a state of dynamic equilibrium with the
climate and it is self-sustaining.

(iii) Its numerous micro-habitats are inhabited by the different organisms.

(iv) A climax community has the biggest, tallest and most highly developed vegetation the
habitat can support.

OVERCROWDING

Overcrowding occurs when the number of organisms in a habitat are more than the resources
such food, nutrients in case of plants, light, space available to the organisms.

The phenomenon implies that the population has increased beyond the carrying capacity of the
habitat.

POPULATION AND POPULATION DENSITY.

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Population is the total number of organisms of the same species living in a particular area at a
particular time e.g, population of students in a school, population of snakes in a farm land.

Population density is the average number of organisms of a species per unit area of a habitat.

For example,

The population of a state is the total number of individuals in the state.

Population density of the state is Total number of people Area of the state Total number of
people Area of the state

Population density can be determined mathematically using the following formulae

Population density =Population size Area of habitat=Population size Area of habitat

For example;

(a) If the area of a classroom is 40m2 and the number of students in the class is 80. What is the
population density?

Population density =Population size40m2=8040=Population size40m2=8040

Population density =2=2 students per square meter

(b) If 80 students are in a class room whose area is 40m2. What is the space available to each
student?

Space per organism can be determined by using the formula:

Space =Area of classroom No. of students=4080=12m2=Area of classroom No. of

students=4080=12m2

Population size is determined by multiplying density with total area of the habitat.

Population size = Population density × Area of habitat

Increase in population density reduces available resources per organism. This automatically
results in competition and struggle between the organism for the available space and resources.
In this struggle, organisms that compete better will survive while the weak ones will die off.

Lizards in the Winter

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Reptiles cannot produce their own body heat. During the cold winter months, some reptiles
enter a state of torpor, which resembles the hibernation of mammals. This dormant, sleeplike
state can last several months. These sand lizards survive the winter in this fashion, huddled
together to share each other’s body heat. When spring arrives, they emerge from their
underground burrows and bask in the warmth of the sun.

FACTORS AFFECTING POPULATION

The following are the factors that could make the population of a given place to increase or
decrease:

(i) Natality: This factor brings about increase in population as a result of production of young
ones.

(ii) Mortality: This brings about decrease in population as a result of death.

(iii) Immigration: Increase in population as a result of individual moving into an area from
another area.

(iv) Migration: decrease in population as a result of individuals moving away from an area.

(v) Abiotic factors such as temperature, water, light, oxygen, food, soil and availability of space
affects population either positively or negatively.

(vi) Biotic factors such as predation, competition, parasitism, diseases, natality, mortality,
terrestrial behavior and dispersal of seeds also affects population.

IMPORTANCE OF FACTORS AFFECTING POPULATION

A number of factors affect the population of a habitat. These factors are known as
environmental resistance. They include biotic and abiotic factors.

Examples of biotic factors are: water, light, heat, space, soil and its content, temperature,
rainfall, food (animals) and essential nutrients for plants and plant growth.

Excess or scarcity of the above factors usually affects the population. In animal population,
abundance of food will result in the animal feeding well, increased rapid and healthy growth and
increased reproduction which will in turn increase the population

Likewise, scarcity shortage in food supply will result in animals struggling and competing for the
available food thus poor feeding and starvation will set in. these in turn will result in poor growth,
poor and delay in reproduction which will end up in reduction in population.

Birth Rate: This is the rate at which mammals produce young ones. Increase in birth rate will
increase the population while decrease in birth rate will decrease the population.

Death Rate: This is the rate at which animals die. Increase in the numbers of individuals that die
reduces the population while decrease in the number of individuals that die increases the
population.

Abiotic factors like water and light when insufficient for plants will affect plant growth and yield.
Shortage of water could result in wilting or even the plant drying up, poor yield and food
shortage. Food shortage will trigger off several ecological behaviors among communities

EVALUATION:

1. Define ecological succession

2. Clearly identify the difference between succession and climax community
3. Briefly explain the process of succession.
4. List habitats on which studies on succession could be carried out.
5. List the pioneer biotic colonizers on land and in water habitat.
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 6. Itemize characteristics of a stable or climax community
7. What is overcrowding?

Week 8

Ecology of Population II

CONTENT:

1. Relationship between succession and competition.

2. Factors that may cause overcrowding.

3. Measures adapted by nature to avoid overcrowding e.g.

(i) Dispersal of seeds and fruits

(ii) Migration of organisms during unfavourable periods.

(iii) Terrestrial behaviours in some organisms e.g lizards

4. Effects of food shortage.

RELATIONSHIP BETWEEN SUCCESSION AND COMPETITION

Succession refers to the series of changes that takes place in animals and plants that inhabit an
environment at different times until a stable community emerge.

Competition is the interaction that occurs between members of the same population, two or
more population in a community in the process of obtaining needed resources such as food,
light, water, space and mates that are in short supply in a given habitat at a given time.

The pioneer colonizers are usually species with simple life processes and simple structures and
are not species adapted to withstand harsh environmental conditions. Yet, their activities
improve the conditions of the environment making it possible for new species to grow. Activities
of the new species further usher in improved conditions. As a result of the improved conditions,
the pioneer colonizers and some of the new ones, no longer fit into the changed environment.

Both old and new inhabitants now compete for limited resources. New species eventually
succeed in becoming dominant. The pioneers are subsequently pushed out from the habitat in
the sense that they gradually die out as the environmental changes become too complex for
them to survive.

The dominant species again further improves the environment making it to be less suitable for
them to survive in while new species are again enabled to survive and compete more
successfully with them.

Note that successive species have improved structural features. In general, through
competition, communities keep changing their competitions to cope with improved
environmental conditions until a stable community is attained.

EVALUATION

1. Define the following terms:

(i) Succession

(ii) Competition

(iii) Pioneer inhabitants with examples

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2. Briefly explain how competition brings about succession

FACTORS THAT CAUSE OVER-CROWDING.

Factors that can result in overcrowding are:

1. Increased natality– increase in birth rate

2. Reduced mortality– reduction in death due to favourable factors.
3. Reduced death rate– abundant food, favourable conditions.
4. Absence of predators– few or no predators to prey on organisms.
5. Inadequate space– available space is small thus there will be congestion
6. Social habits in social insects such as bees’ wasps and ants.
7. Absence of dispersal– when seeds and fruits fall directly under parent plant and
germinate.
8. Immigration– more individuals coming into a place
9. Favorable food supply– increase in food supply to a particular habitat will attract more
individuals to increase the population which eventually will lead to overcrowding.

MEASURES USED BY NATURE TO AVOID OVERCROWDING

Naturally, different organisms have developed different means of overcoming overcrowding.

Some of these methods are as follows:

(i) Dispersal of seeds and fruits: animals such as birds, insects, and man help in spreading
seeds and fruits from parent plants to distant places where they germinate and grow thereby
avoiding overcrowding. Explosive mechanism is also used by plants to dispose seeds and fruits
far away from the parent plant. Otherwise, matured seed and fruits would fall directly under the
parent plant and germinate and cause serious overcrowding.

(ii) Migration of organisms: with adverse conditions, organisms tend to move from the congested
area to another. Usually, the movement is to areas where the conditions are more favourable.
The organisms move from the overcrowded area to areas where there are fewer organisms.
This is called emigration as the organisms move outside the habitat to reduce overcrowding.

(iii) Territorial behaviours: to overcome overcrowding, some organisms claim some areas to
themselves guard it and ensure no other organisms of the same specie do not come in. Lizards,
some birds Tilapia fish are examples of organisms that exhibit territorial behaviour.

(iv) Irruption: This refers to the large-scale emigration of animals at irregular intervals to reduce
overcrowding e.g Locust.

EVALUATION:

List and explain four ways organisms naturally reduce overcrowding with examples.

FOOD SHORTAGE AND ITS EFFECT

One of the very essential resources in a habitat for which organisms compete is food.

Food is essential for growth, repairs and reproduction. As earlier said, abundance of food in a
habitat can lead to increased population which may result in overcrowding.

However, scarcity of food in any habitat or among any population of organisms has adverse
effect such as reducing growth and reproduction.

Causes of food shortage

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Some causes of food shortage are:

(i) Overpopulation: Overpopulation occurs when increase in population is not matched with
corresponding increase in food production.

(ii) Drought: Severe drought which is absence of rainfall for a prolonged period of time will
adversely affect agricultural activities that will lead to poor harvest eventually food shortage.

(iii) Natural disasters such as war, floods and earthquakes can destroy farmlands, result in poor
or no harvest and automatically food shortage.

(iv) Poor storage facilities: In developing countries like ours, a lot of harvests of agricultural
products are wasted due to inadequate processing and storage of farm products which results
in food shortage.

(v) Diseases of plants and animals: Certain diseases such as mealy bug diseases of cassava,
mosaic virus of tomato, pepper, and avian flu in birds cause sicknesses in these organisms
giving rise to poor production.

EFFECTS OF FOOD SHORTAGE

Food shortages have adverse effects on organisms in any population. Some effects of food
shortage are as follows:

(i) Competition for available food occurs. This result in aggression, cannibalism, predation and
survival of the ‘‘fittest’’ results. There are two types of competition:

(a) Intra-specific competition

(b) Inter-specific competition

(ii) Food shortage results in poor feeding which can result in poor growth, delayed maturity and
reduction in the rate of reproduction. Increase mortality and increase maternity rates are
possible consequences.

(iii) It can result in under functioning of body organs and systems, lower resistance to
diseases with resultant death from sicknesses and diseases.

(iv) Food shortage results in emigration that is movement of individuals i.e., organisms out of the
habitat to other habitat or community in search of food thereby reducing the population.

GENERAL EVALUATION:

1. (a) Define ecological succession

(b) State two differences between primary and secondary successions.

(c) Name and describe three major communities in a plant succession.

(d) List four factors that can give rise to primary bare surfaces.

2. (a) What do you understand by climax community?

(b) Explain the process of secondary succession in abandoned farmland.

(c) State the difference between primary and secondary succession

3. (a) Define the following: (i) Succession (ii) Over-crowding (iii) Population

(b) Itemize the characteristics of the following: i. Stable community ii. Succession

4. (a) State the importance of factors that affect population.

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(b) write short notes on the (i) Food (ii) Causes of food shortage (iii) Effects of food shortage in a
habitat/community (iv) Write your suggestions on how wastages can be avoided in harvested
farm products.

5. (a) Briefly describe the term overcrowding.

(b) Describe the sequence of events that occurs in primary and secondary succession of a fresh
water habitat.

(c) A number of birds were introduced into a new habitat with limited resources. Explain how the
population of the birds will change with time.

SUGGESTED PRACTICALS

1. Students should be grouped between five to ten maximums

2. Each group should be given portion/different habitats such as the school farm, a palm
tree, any fruit tree in the campus to carry out specific ecological studies.
3. Students should be taught and made to write reports as they collect both plants and
animal sample for specific studies such as changes in vegetation, wind direction,
samples of population of different types and more.
4. Visit abandoned farmland
5. Possibly visit a nearby pond or stream for sample of both plants and animals

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