S5 BILOGY NOTES CONTINUED… (MR.

LUBEGA)
ECOLOGY
Ecology refers to the study of the relationships of living organisms with each other and their non-
living/ physical surroundings.
Organisms live within a relatively narrow sphere (land, water and air) and the earth’s surface in
which life permanently exists, this is known as Biosphere/ecosphere. The biosphere is divided
into four major habitats namely;
 fresh water ( lakes and ponds, rivers and streams, wetlands),
 marine water(oceans) ,
 Estuaries.
 Terrestrial covering a few meters deep in the soil and a few kilometers into the
atmosphere.
On land, there are several biogeographical zones, in each of which there is characteristic plant and
animal life. These zones are called Biomes ( a large ecological area on earth’s surface with
distinctive plant and animal groups which are adapted to that particular environment)
Biomes include;
❖ Tropical rain forests,
❖ Tundra :where the ground is frozen much of the year and vegetation is sparse found in
arctic and Antarctic regions
❖ Desert including Hot and dry desert (where evaporation is high and there is too much
heat) and Cold deserts ( precipitation coming from colder water sources than rain, such as
snow or ice),
❖ Temperate deciduous forests.
❖ Temperate rain forests
❖ Boreal coniferous forest (taiga)
❖ Mediterranean scrub forest
❖ Grassland e.g. steppe and savanna
Within each biome only those organisms with the necessary adaptations for surviving the physical
conditions are found e.g.
(i) Tundra and coniferous forest biomes have organisms capable of withstanding long
periods of extreme cold

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 (ii) Desert organisms must be able to cope with intense heat and drought
(iii) Marine organisms must be able to thrive in salt water
Biomes are further divided into smaller units called zones, each with its unique properties e.g.
➢ A forest biome is divided into ground zone (consisting of millipedes & earthworms), and
canopy zone/aerial zone; (consisting of birds & monkeys); with each of these zones
supporting different animals that are adapted to the conditions within them.
➢ Aquatic biome divided into surface, intertidal, & benthic zones; with the organisms in the
intertidal zone withstanding wave action e.g snakes, snails, those in benthic zone not able to
withstand wave action e.g sponges, while organisms requiring much air supply e.g.
photosynthetic algae inhabiting surface zone.
➢ Desert biome divide into surface and subterranean zones; with those in surface zone adapted
to withstand extreme heat, while those in subterranean able to survive in low oxygen content.
ECOLOGICAL NICHE:
This is an organism’s entire way of life e.g. behavior, feeding habits and its role in the
community.
Types of ecological niche.
(a) Realized niche
Is a most restricted area of a habitat that an organism occupies as a result of presence of predators,
competitors, and parasites; limiting the habitat and roles performed by an organism, it’s smaller in
size.
(b) Fundamental niche
Is an entire area an organism can occupy in the absence of predators, competitors, and parasites;
allowing the organism experience a larger habitat and perform a variety of roles.
N.B: A species’ ecological niche is usually less extensive when competitors and predators are
present than when these are absent
COMPONENTS OF ORGANISM’S ENVIRONMENT
Organisms
An organism is a life form consisting of one or more cells. All organisms have properties of life,
including the ability to grow and reproduce. These properties of life require energy and materials
from the environment. Therefore, an organism is not a closed system. Individual organisms

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depend on and are influenced by the environment.

ENVIRONMENT
An environment refers to all conditions in which organisms live. This may be divided into the
abiotic and biotic components,
Abiotic components: these are non-living components of the environment, they may be physical
or chemical and include air, water, soil and components of climate such as light, temperature and
wind.
Biotic components: these are the living components of an environment including plants and
animals e.g. microbes, man, protozoans, arthropods.

FACTORS AFFECTING THE DISTRIBUTION OF ORGANISMS

The factors which determine where an organism lives are either biotic (living) or abiotic
(chemical and physical)
(a) ABIOTIC FACTORS
(i) Temperature
Environmental temperature is an important factor in the distribution of organisms because of its
effect on biological processes such as cell division, reproduction, excretion, photosynthesis etc.
Cells may rupture if the water they contain freezes (at temperatures below 0oC) by the formation of
ice crystals, and the proteins of most organisms denature at temperatures above 45oC. In addition,
few organisms can maintain an active metabolism at very low or very high temperatures, though
extraordinary adaptations enable some organisms, such as thermophilic prokaryotes and bacteria to
live outside the temperature range habitable by other life. Most organisms function best within a
specific range of environmental temperature. Temperatures outside that range may force some
animals to expend energy regulating their internal temperature, as mammals and birds do.
Fluctuations in temperature within aquatic environments are relatively small, since water has a
high specific heat capacity and therefore provide more stable habitats for many kinds of
ectothermic creatures than terrestrial environments.
Note:
Actual temperature of any habitat may differ in time according to the season and time of day, and
in space according to latitude, slope, degree of shading or exposure.

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Effect of temperature on the distribution of living organisms
Living organisms only survive in narrow ranges of temperature because enzymes in their bodies
work in narrow optimum ranges.
Most organisms are distributed in regions of moderate temperature such as tropics and temperate
regions where temperatures are favorable for activity of their enzymes,
Temperate plants and animals are distributed in relatively cool regions whose temperature rarely
exceeds 25oC while those that can withstand high temperature are distributed in the tropics or
even deserts.
In deserts, daytime temperature is very high and causes water to evaporate very quickly, hence the
major problem faced by living organisms being how to resist or tolerate desiccation therefore
inhabited by mostly xerophytic plants such as cacti, insects and a few mammals mostly camels
with such adaptations.
On the fringes of polar areas, the air temperature for much of the year is nearly always below
freezing point. Only lichens, mosses, a few flowering plants and endothermic animals such as
penguins and polar bears can tolerate such conditions, for similar reasons high mountain ranges
have a typical alpine flora and fauna.
Temperature also influences the distribution of plants by affecting their photosynthetic
physiology.
• C3 plants have enzymes which fix carbon dioxide better at relatively low temperatures and
are distributed in cool temperate regions.
• C4 plants have enzymes with higher optimum temperatures for carbon dioxide fixation and
are distributed in hotter tropical conditions
• CAM plants are distributed in regions with very high temperatures such as deserts,
because they can close their stomata during the day when it is very hot to minimize water
loss and open them at night when it is cool to take up carbon dioxide.
(ii) Light
Light absorbed by photosynthetic organisms such as green plants and bacteria provides the energy
that drives most ecosystems, thus light is a fundamental necessity and too little sunlight can limit
the distribution of photosynthetic species.

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In forests, shading by leaves in the treetops makes competition for light especially intense,
particularly for seedlings growing on the forest floor.
In aquatic environments, every meter of water depth selectively absorbs about 45% of the red
light and about 2% of the blue light passing through it. As a result, most photosynthesis in aquatic
environments occurs relatively near the surface.
There are three basic aspects of light that influence activities of living organisms
❖ Light intensity: the amount of light energy per unit area reaching a place per second. It
relates to brightness of the light. This influences activities such as photosynthesis in plants
and vision in animals
❖ Light quality: the relative wave length or colours present in light. Only light at certain
wavelengths can be used by different photosynthetic pigments affecting distribution of
organisms mostly in aquatic environments e.g. some sea weeds such as red algae, with
different light intercepting pigments can survive in locations where green algae would find
light quality limiting.
❖ Light duration (photoperiod): relating to relative length of day and night, plants and
animals show photoperiodic responses that synchronise their activities with seasons such
as flowering and germination in plants, migration, hibernation and reproduction in
animals.
Effects of light on activity of organisms
(a) Positive effects
 Provide energy required in synthesis of organic compounds by photosynthetic organisms
 Required in synthesis of chlorophyll
 Triggers conversion of etioplasts to chloroplasts
 Far red light can cause elongation of internodes resulting in etiolation
 Light causes leaf expansion in plants
 It provides vision which enables organisms to see around their environments in order to
search for food, escape predation.
 Stimulus for timing of diurnal / circadian rhythmic behavior and seasonal rhythmic
behavior e.g seasonal migration in birds and breeding seasons in many animals
 Stimulates germination of some seeds i.e. photoblastic seeds
 Stimulates flowering of many plants

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  Ultra-violet light stimulates synthesis of vitamin D by the skin of mammals
 Provides most of the heat necessary in temperature regulation of many organisms e.g.
ectotherms
 Phototaxic movements of animals and unicellular plants are important for locating suitable
habitat
 Light breaks dormancy of seeds
 It enables the mechanisms photoreceptions in eyes
 Stomatal opening and closure; with most plant species opening their stomata during
day(when there is light) to allow excretion, gaseous exchange and closing during night (in
absence of light/darkness).
 Courtship; with some animals preferring light so as to carry out courtship while others
prefer darkness
 Phototropism, by redistributing auxins on the darker sides of shoots and roots, with cells
on darker side elongating more than those on illuminated side.
(b) Negative effects
 Ultra-violet light can cause skin cancer
 The sun's rays having more ultraviolet radiation are more likely to damage DNA and
proteins in alpine environments limiting the survival of organisms.
 In other ecosystems, such as deserts, high light levels can increase temperature stress if
animals are unable to avoid the light or to cool themselves through evaporation.
 Excessive can cause bleaching of chlorophyll reducing photosynthetic productivity of the
ecosystem
 Infra-red absorbed by water increase water temperature as well reducing amount of
dissolved oxygen in water, making water unfavorable for life of some aquatic organisms.
(iii) Humidity
This is often expressed as relative humidity, it is a measure of the moisture content of the
atmosphere and is expressed as a percentage. Relative humidity of air is influenced by its
temperature.It influences the rate of evaporation of water from surfaces of living organisms and
rate of transpiration in plants in turn affecting the ability of organisms to withstand drought
A decrease in humidity increases rate of evaporation or transpiration putting living organisms at a
risk of desiccation
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Living organisms are generally favoured by relatively high humidity as this reduces the rate of
waterloss from their bodies.
Organisms with large moist surface area are in particular very sensitive to humidity changes.
These organisms are distributed and restricted to humid areas or become active only under humid
conditions e.g. toads, slugs, mosses and liverworts
Controls other activities of animals like feeding, hunting, and movements e.g earth worms
experience a larger ecological niche when the environment is humid.
Controls opening and closure of stomata; therefore affecting rate of photosynthesis and
transpiration.
(iv) Rainfall
This influences lives of organisms both directly and indirectly as it is main source of water
utilized by organisms. Amount of rainfall in a given area determines the abundance, distribution
and types of organisms in the area
Where temperatures are relatively high all year around with a hot drought period favors survival
of deciduous plants capable of dropping their leaves in dry seasons and growing them again in
cooler, wetter seasons
Excessive water in soils which aren’t well drained may result into water logging which may not
favour most plants. However, some plants such as rice, sugar cane etc. are highly distributed in
waterlogged soils.
Most organisms are distributed in areas where there is abundant water supply and animals in
particular are abundant in areas near water sources
However, the actual distribution of organisms depends on the extent to which an organism is
dependent on water for its activities and on its ability to conserve it.
Where rainfall is quite low like in hot deserts only organisms with adaptations to prevent
excessive water loss for plants (xerophytes) and animals that can conserve water using diverse
methods can survive hence their higher abundance.
Ecological significances of water
➢ Habitat for many aquatic organisms e.g. frogs, fish etc
➢ Raw material for photosynthesis
➢ High thermal capacities leads to cooling of terrestrial organisms upon evaporation e.g.
plants during transpiration, some animals during sweating.

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 ➢ Agent for fruit, seed, spore, larva and gamete dispersal
➢ Condition for germination
➢ Highly transparent; therefore allowing light to reach acquatic organisms, for
photosynthesis; and aquatic predators to locate their prey
➢ Important factor in decay and decomposition; therefore increases in recycling of
nutrients in an ecosystem.
➢ Is an agent for soil formation through its effects like frost action during weathering of
rocks.
➢ Offers support to aquatic animals enabling their locomotion
➢ A medium for chemical reactions in cells of living organisms
➢ A medium for transport of dissolved substances e.g. mineral salts
➢ Source of dissolved oxygen upon which aquatic organisms dependent for their
respiration
Attention: Clegg fig. 3.23 page 59
(v) Atmospheric pressure
On the surface of the earth, atmospheric pressure varies with altitude. Variations in atmospheric
pressure affects the amount of oxygen available for respiration and of carbon dioxide for
photosynthesis. These gases in turn affect the distribution of organisms
Availability of oxygen
Most living organisms are aerobes and require oxygen gas for the release of energy in respiration
At high altitudes there is the same proportion of oxygen present as sea level, but its partial pressure
is less so it is not readily absorbed by living organisms together with low temperature, it explains
why there is very little mountain fauna above a height of about 4.5 km, where the partial pressure
of oxygen is roughly half that at sea level
In water there is less oxygen about less than 1% by volume of the gas and is still less accessible to
aquatic organisms, near the surface of water bodies where the water is in contact with air the water
may be saturated with oxygen. Photosynthetic plants help to maintain high concentrations of
dissolved oxygen. Deeper down, where there are no photo-autotrophs and because oxygen diffuses
very slowly through water, much less dissolved oxygen is present, thermal convection currents
carry some oxygen downwards from the upper layers.

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In the deepest waters there may be little or no oxygen gas available, here only invertebrates whose
oxygen-carrying pigments become saturated at very low oxygen tensions and anaerobic bacteria
can survive.
Factors determining amount of oxygen in aquatic environments
Its solubility,
The temperature,
Partial pressure of oxygen in the air,
Amount of solutes dissolved in water and on the rate at which it is consumed by aquatic
organisms.
Availability of carbon dioxide
Carbon dioxide is one of the raw materials essential for photosynthesis.
At high altitudes the partial pressure of carbon dioxide gas is low together with the low
temperatures, growth of plants at a height of more than 6km in mountainous areas.
However, carbon dioxide has a high solubility in water but a very low partial pressure and thus
volume of carbon dioxide dissolved in water is generally small. Never the less aquatic plants do not
get short of carbon dioxide for photosynthesis as water contains large quantities of carbonate and
hydrogen carbonate ions formed when carbon dioxide reacts with water, thus growth of plants is
not limited.
EDAPHIC/ SOIL FACTORS
Soil is the upper, weathered layer of the earth’s crust. It consists of disintegrated rock, organic
matter, air, water, dissolved minerals and various living organisms. The study of soil is known as
pedology.
Importance of soil
➢ Provides habitat for some soil living organisms e.g. rats, moles
➢ Provides plants with anchorage for roots
➢ Supplies water and inorganic nutrients to plants
➢ Supplies plants with essential air for root growth
(i) Soil Ph
Influences physical properties of soil and availability of certain minerals e.g. phosphates, calcium
ions magnesium and iron to plants, thus affecting their distribution in soil; i.e. tea and coffee
plants thrive well in acidic soils

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Affects activity of decomposers e.g. in acidic medium, the rate of decomposition is reduced,
subsequently recycling of matter in an ecosystem reduced. This influences the distribution of
vegetation.
(ii) Water content;
Water in the soil probably exerts the greatest influence on plants.
Types of soil water
(i) Gravitational water: is water that drains away under the influence of gravity when
soil is over supplied with water, it temporarily displaces air from spaces between soil
particles.
(ii) Capillary water: this is the bulk of soil water after gravitational water has drained
away. It is the main source of water to plant life.
(iii) Hygroscopic water: is a thin film around the surface of mineral particles, it is
unavailable to plants as it is held by strong surface forces such as hydrogen bonds.
(iv) Chemically combined : this is retained within mineral substances as part of chemical
structure e.g. water in hydrated iron (iii) oxide
Note:
Field capacity is when soil holds maximum possible quantity of capillary water.

Addition of more water which cannot be drained away leads to water logging; and anaerobic
conditions, affecting mineral ion uptake by active transport, subsequently affecting osmotic
uptake of water, due to decreased osmotic potential gradient, causing plants to dry out.
Plants like rice, marshes, and sedges have developed air spaces among root tissues, allowing
some diffusion of oxygen from aerial parts to help supply the roots.
(iii) Biotic content;
Microorganisms like bacteria and fungi carry out decomposition of dead organic material,
therefore recycling nutrients back to the soil. b
Burrowing organisms (macro organisms) e.g. earthworms, termites and rodents
➢ Improve drainage and aeration by forming tunnels in the soil.
➢ Earthworms also improve soil fertility by mixing of soil, as they bring leached minerals
from lower layers within reach of plant roots.
➢ They also improve humus content, by pulling leaves into their burrows

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 ➢ Also press soil through their bodies making its texture fine.
Interest: fig page 63 clegg and mackean
(iv) Air content;
Volume of air in soil largely depends on the shape and size of the mineral particles.
Spaces between soil particles is filled with air from which plant roots obtain oxygen by diffusion
for aerobic respiration, Also essential for aerobic respiration by microorganisms in the soil that
decompose the humus.
In waterlogged soils, with soil air displaced by water, plant roots are deprived of oxygen and
plants in such conditions may die if the conditions persists.
(v) Mineral salts
A wide variety of minerals is necessary to support healthy plant growth. Different species make
different mineral demands and therefore the distribution of plants depends to some extent on the
mineral balance of a particular soil.
Ways used by plants growing in mineral salt deficient soils obtain certain mineral salts
❖ Insectivorous plants such as pitchers, Venus fly trap etc. trap insects either using their leaves
or flowers and release enzymes that digest the insect so that the soluble products of digestion
rich in minerals are absorbed.
❖ Leguminous plants form association with nitrogen fixing bacteria of the rhizobium genus
which are found in root nodules, these fix nitrogen into ammonium and nitrates which can
be utilized by the plant
❖ Most plants enter mychorrhizal relationships where a fungus grows on the surface of the
roots (ectotrophic mychorrhiza) or sends out its hyphae to penetrate into the root
(endotrophic mychorrhiza). The fungus absorbs mineral salts especially phosphates which
are availed to the plants while the fungus in return obtains support from the plant root as
wellas organic nutrients e.g. carbohydrates
❖ Parasitic plants e.g. striga (witch weed) and cuscuta (dodder) obtain nutrients from their
host plants.
❖ Direct fixation of nutrients into the soil e.g. by lightening which causes nitrogen in air to
react with oxygen to form nitrogen dioxide which subsequently reacts with oxygen and
water to form nitrates that are deposited, dissolved in rain water into the soil.

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 (vi) Soil temperature
▪ This affects physical e.g. soil formation, chemical e.g. decay and biological e.g. respiration
and nitrogen fixation processes in the soil.
▪ Influences the rate of water absorption which decreases with temperature. High soil
temperatures in particular reduce the amount of water in the soil.
▪ Influences seed germination. Germination is higher when soils are warm than when they are
cold
▪ Influences root growth and growth of underground plants parts like tubers
▪ Influences the activity of soil microorganisms. Among the other factors affecting soil
temperature are colour, texture, structure, water content, humus content and the presence
and absence of vegetation.
HOW BIOTIC FACTORS AFFECT THE DISTRIBUTION AND ABUNDANCY OF
ORGANISMS
NB. Biotic factors are those that relate to how living organisms influence each other. And how
relationships between them influence their distribution and population. These include:
✓ competition
✓ predation
✓ antibiosis and allelopathy
✓ mimicry
✓ symbiotic relationships such as parasitism, commensalism and mutualism
✓ pollination &dispersal,
✓ human influence
(i) Competition
This is a relationship whereby two individuals of the same species or different species struggle to
obtain same environmental resources which are in limited/short supply within the same ecological
habitat. This can be classified as
(i) Intraspecific competition: is the competition between organisms of the same species for the
same resources. This may be for nutritional needs, for mates, or for breeding sites.
Intraspecific competition tends to have a stabilizing influence on population size. If the
population gets too big, intraspecific population increases, so the population falls again. If the
population gets too small, intraspecific population decreases, so the population increases again.

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Note: Intraspecific competition for limited resources can lead to evolution of new special forms
of behavior such as territoriality, ritualized fighting, and dominance hierarchies.
Application
A graph showing the effect of sowing density on (a) yield (dry crop) of clover and (b) seed
production by shepherd’s purse

Total dry mass produced per unit area of soil is the same over a wide range of sowing density (a).
This may come about because many seedlings die when seeds germinate close to each other. The
few plants that survive then have an adequate share of resources so they grow quite well or most
of the seedlings may survive and grow into mature plants, but each plant has a lesser share of
resources so is smaller than normal
Seed production (b) is reduced when plants are overcrowded, because many fail to flower and set
seeds
(ii) Interspecific competition
Is an interaction that occurs when individuals of different species compete for a resource that
limits their growth and survival. E.g. weeds growing in a garden compete with garden plants for
soil nutrients and water, grasshoppers and cows in fields compete for grass they both eat.

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Interspecific competition is very intense when competitors occupy the same niche or have
significantly overlapping niches, in this case one of the competing species must;
❖ migrate to another area if possible
❖ shift its feeding habits or behaviour through natural selection and evolution
❖ suffer a sharp population decline to the point of extinction
❖ reach an equilibrium situation in which neither species succeeds as well as it would in
the absence of the competitor.
To explore this type of competitive interaction, G. F. Gause conducted a series of experiments on
two closely related species of ciliated protists, Paramecium aurelia and Paramecium caudatum.
He cultured the species under stable conditions, adding a constant amount of food every day.
When Gause grew the two species in separate cultures, each population grew rapidly and then
remained constant at what was apparently the carrying capacity of the culture

But when Gause cultured the two species together, P caudatum was driven to extinction in the
culture. Gause inferred that P. aurelia had a competitive edge in obtaining food, and he
concluded that two species competing for the same limiting resources cannot coexist in the same
place.
In the absence of disturbance, one species will use the resources more efficiently and thus
reproduce more rapidly than the other. Even a slight reproductive advantage will eventually lead
to local elimination of the inferior competitor, an outcome called competitive exclusion.

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Competitive advantages of P.aurelia are;
High rate of reproduction.
High growth rate.
Good nutrient absorptive capacity/greater efficiency in obtaining food.
Being small in size, it requires less food hence can easily survive when food is scarce.
Producing chemical substances which inhibit or affect growth of p.caudatum without
affecting itself
Survivorship, long life span.
A graph showing the population growth of two species of paramecium grown
separately and together

Gause’s (Russian biologist) competive exclusion principle states that “onmly one
species (population) in a given community can occupy a given ecological niche at any
one time”
Other experiments
(i). Two species of flour beetles, Tribolium castenum and T. confusum were kept in the laboratory
in bottles of flour acting as a habitat and providing food for them, under variable temperature
conditions(24-34) and humid condtions (very humid , 70%RH& 30% RH).

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Observation. At high temperatures and in very humid conditions, Tribolium castenum succeded
better, while at low temperatures and very dry conditions T. confusum did better. Whatever the
conditions, only one of the species eventually survived.
(ii) in dark weeds where lemma gibba effectively outcompetes lemma polyrrhiza

MECHANISMS OF COEXISTENCE: NICHE DIFFERENTIATION

Two species cannot coexist permanently in a community if their niches are identical. However,
ecologically similar species can coexist in a community if there are one or more significant
differences in their niches. When competition between species with identical niches does not lead
to local extinction of either species, it is generally because one species' niche becomes modified.
In other words, evolution by natural selection can result in one of the species using a different set
of resources. The differentiation of niches that enables similar species to coexist in a community
is called resource partitioning. This occurs living organisms potentially occupying the same
niche divide resources e.g. space among themselves so that each utilizes a limited amount of
resources within the environment. Resource partitioning results into niche differentiation
Niche differentiation refers an evolutionary change in resource use, caused by competition over
generations
Forms of resource partitioning/ niche differentiation
❖ Specialization of morphology and behavior for different foods, such as beaks of birds
which may be modified for picking up insects, drilling holes, cracking nuts, tearing flesh
e.g. When three species of ground finches of Galapagos Islands occur on separate islands,
their bills tend to be the same intermediate size, enabling each to feed on a wider range of

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 seeds, but where they co-occur, there is divergence in beak size to suit each finch species
to feeding on seeds of either small, medium or large size, but not all sizes.
❖ Spatial separation e.g. stratification such as canopy dwellers and forest floor dwellers
Examples:
(i) Each of the five species of common warblers (insect-eating birds) minimises
competition with the others by (i) spending atleast half its feeding time in a
different part of spruce tree branches e.g. some hunt at the extreme top, others at
the lower portion, some mid way etc (ii) Consuming somewhat different insect
species.
(ii) micro-organisms cultured in long cylindrical vessels can partition space where one
occupies lower layers of the vessel while the other occupies upper layers of the
container
❖ Temporal separation e.g. different species of eagles in a forest feed at different times of
the day e.g. bald headed eagles are most active early mornings and evenings while the
white-breasted eagles feed vigorously towards noon, Hawks and owls feed on similar prey,
but hawks hunt during the day and owls hunt at night..
The tendency for characteristics to be more divergent when populations belong to the same
community than when they are isolated is termed character displacement e.g. variation in beak
size between different populations of the Galapagos finches e.g. Geospiza fulginosa and
Geospiza fortis.
Ecological significance of competition
❖ Has an evolutionary significance of increasing biological fitness of species by weeding
out organisms with a competitive disadvantage leaving only well adapted individuals to
survive
❖ Regulate population size of living organisms especially intraspecific competition
❖ Influences distribution of living organisms, where living organisms usually become
distributed in areas where competition is minimum.
❖ Leads to colonization of wide range of habitats
❖ Leads to polymorphism; the existence of the same species of organism in two or more
genetically discontinuous forms or morphs living within the same habitat, resulting in
maximum utilization of resources in a wide range of environment

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 ❖ Leads to adaptive radiation
Application
Question.
1.(a). Explain the role of competition in regulating the size of population.
(b). Duck weed grows on or near the surface of ponds. Its growth can be measured by
counting the number of fronds. Two species of duckweed , Lemna trisulca and Lemna minor
were grown separately, and together , in identical beakers in the laboratory
Total number of fronds.
Species grown separately Species grown together
Days L. trisulca L. minor L. trisulca L. minor
0 30 30 30 30
16 63 78 48 105
36 126 142 84 234
46 177 225 84 324
54 165 276 48 360
60 129 219 45 354
(i).Draw graphs to compare the rates of growth of the two species when grown separately
and when grown together.
(ii) What do the graphs suggest about the growth rate of the two species grown separately?
(iii)Account for this difference.
(iv)Offer an explanation for the interaction of the two species when grown together.
(v).Account for the changes in the growth rate between 46 and 60 days for Lemna trisulca
when grown together.
(ii) Predation. This is an interspecific relationship whereby one organism, the predator attacks, kills
and feeds on another organism of different species, the prey. Predation influences distribution of
living organisms where predators are found mostly where there is prey e.g. herbivores are found
where there is suitable plant material whereas prey tend to avoid areas occupied by their
predators.
A predator is an organism which hunts, attacks and feeds on another organism.
A prey is an organism that is fed on by the predator.

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PREDATOR- PREY INTERACTIONS IN ECOSYSTEMS

Initially, the population of the prey is higher that the population of the predator.
Populations of prey increase rapidly, prey provide an abundant food supply to predators
which therefore reproduces rapidly and their population increases rapidly.
The increase in predator population results into over consumption of prey, decreasing prey
population rapidly; few prey are remain, thus predators start to compete for the few
remaining prey those predator species lacking competitive advantage fail to get prey and die
due to starvation, this results in rapid decrease in the population of predators which allows
the prey population to recover and fluctuations go on.
Predator prey populations are therefore regulated by a negative feedback mechanism that keeps
the population at levels that the environment can support.
The cyclic fluctuations are out of phase because the predator population attains a peak after the
prey population has started declining
The fluctuation above occur in a natural environment without influences of the other species.
Differences in the fluctuations may be caused by
(i) Human influence e.g.
(ii) Where the predator has alternative sources of food , it may cause the population of of
a preferred prey to decrease to extinction without its numbers declining since it feeds
on alternatives.
CO-EVOLUTION OF PREDATOR AND PREY
Co-evolution is a process in which two organisms of different species associate overtime and
eventually develop a relationship of mutual dependence and benefit over time as a result of
natural selection.

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During co-evolution species become so adjusted to each other’s presence that overtime, through
natural selection, may develop a relationship of mutual dependence and benefit.
In predation, prey with better defense strategies such as being able to run fast, ability to
camouflage, possess defense weapons such as horns are the ones most likely to escape predation,
on the other hand, predators with better hunting strategies e.g. fast locomotion, ability to
camouflage so that they aren’t seen and having weapons like horns etc. are more likely to capture
prey. This has led to co-evolution of predators and prey through natural selection i.e. as predators
evolve better hunting strategies, prey evolves better defensive strategies to combat them.
Overtime, this has led to evolution of better adapted predator and prey
Evolutionary significance of predator –prey
Predation usually eliminates the unfit (aged, sick, weak). This gives the remaining prey access to
the available food supply and also improves their genetic stock hence, enhances the chances of
reproductive success and longtime survival, thus pass on their good traits to their off springs
increasing their survival.
Ecological significance of predation
❖ Source of food to predator
❖ Provides a selection pressure that eliminates poorly adapted predators or prey resulting in
evolution of better adapted predators and prey
❖ Regulates the population of both predator and prey by negative feedback
❖ Minimizes interspecific competition among predators and prey through regulation of
their population size
❖ Minimizes intraspecific aggression among prey species
❖ Utilized by man in biological control
❖ Influences population distribution of predator and prey
How are the predation suited for capturing prey?
➢ Ability to camouflage in their environment therefore not easily seen by the prey
➢ High locomotory speed to out run, out swim or out fly the prey
➢ Have long and sharp canine teeth which pierce and kill the prey
➢ Group hunting increasing chances of capturing prey
➢ Strong curved claws for capturing and holding the prey
➢ Some have soft pads in the soles to minimize noise when stalking prey

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 ➢ Being nocturnal, they hunt, capture and feed at night when the prey are resting or when
their sense of vision is poor
➢ Formation of traps which they use to capture prey e.g. cobwebs of spiders
➢ High intelligence associated with well-developed brains so as to use foot marks, sounds,
droppings to locate their prey
➢ Well developed olfactory organs that give a predator an excellent sense of smell in order
to detect prey at a distance and those in hiding by the smell
➢ Well developed sense of sight to see the prey at a distance
➢ Some have stinging cells for paralyzing prey
How are prey species suited to avoid predation?
➢ Warning calls: this is sound made by some animals to warn others about approaching
predators. Such calls are made by birds and mammals which exhibit parental care
➢ Protective resemblances where the animal resembles objects in its environment to confuse
the predators e.g. most insects resemble plant parts e.g. twigs and flowers
➢ Group defense where animals live in groups like herds so that each gains collective
protection from members e.g. schools of fish, herd of antelope, flocks of birds, deers
➢ Some preys secrete poisonous or repellant substances e.g. scorpions, caterpillars, some
grasshoppers, culex mosquito eggs
➢ Defensive weapons e.g. horns, spines which they use to physically defend themselves against
the predators
➢ Possession of protective body covers e.g. scale in snakes and fish, shells in tortoises.
➢ Production of mucus on the body making their body slippery so as to easily escape from
predators e.g. fish
➢ Production of nasty smell which irritate the predators for example hedgehogs, cockroaches
➢ High speed of locomotion by some preys so that they can out swim, out run, out fly their
predators
➢ Some prey camouflage by changing colour e.g. chameleon and cuttlefish, or having
deceptive colours that blend with the background e.g. arctic hare in its winter fur blends into
snow.

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 ➢ Some prey species discourage predators with chemicals that are poisonous (e.g. oleander
plants), irritating (e.g. bombardier beetles), foul smelling (e.g. stinkbugs and skunk cabbages)
or bad tasting (e.g. monarch butterflies and buttercups)
➢ Some species gain protection to avoid predation by mimicking (looking and acting like) other
species that are distasteful to the predator e.g. the non-poisonous viceroy butterfly mimics the
poisonous monarch butterfly.
➢ The electric fish Malapterurus (a cat fish) produces high voltage discharge of up to 350v that
shocks any predator that makes contact with it.
➢ Some prey scare predators by puffing up e.g. blowfish, or spreading wings e.g. peacock.
Browsing (grazing)
This is a form of predation in which animals feed on plant materials. Such animals are called
browsers if they feed on branches of shrubs e.g. elephants, giraffes and they are called grazers if
they feed surface plant materials such as grasses e.g. cows, goats
Adaptations of plants to deter (resist) browsers and grazers
➢ Have thorns, hairs that reduce palatability and also used as defence devices
➢ Some are brightly coloured palatabl parts which may not be attractive to some herbivores
➢ Secretion of bitter sap / foul smelling chemicals
➢ Some plants like Mimosa pudica show positive haptonasty upon touch by herbivores
➢ Some open their leaves at night and close them during day to avoid them being fed on by
herbivores
➢ Maintaining palatable parts e.g. leaves at greater heights where they are inaccessible by
browsers or grazers
➢ Possession of very tough covering e.g. barks of trees or shells of nuts
➢ Secretion of chemicals substances that induce moulting in the effensive stage of the life
cycle of the animal e.g. buttercap produces chemicals that induce moulting of the larvae
of butterflies into a non leaf eater butterfly
➢ Some secrete toxic/ poisonous chemicals e.g. alkaloids
Adaptations of herbivores to obtain plant materials
➢ Possession of a highly flexible tongue that can move in all directions to easily pluck off
the vegetation

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 ➢ Some possess horny pad which is very hard for plucking plant materials and minimizes
the piercing effect of the thorns
➢ Some have long snouts to probe through the thorns in order to get leaves
➢ Some have long necks to enable them graze on leaves high up on the canopy
➢ Wide molars to provide a large surface area for crashing plant materials
Ecological significance of plant-grazer relationship
➢ It determines the distribution of grazers since they tend to be more abundant where there
are suitable pastures \
➢ Man uses grazers to biologically control plant species that may compete with the crops
e.g. the cactus moths control the cactus plants
➢ It may lead to dispersal of fruits, spores, seeds, parasites since grazers move from place to
place in search of pastures.
(iii)Antibiosis; is the secretion by organisms chemical substances into their surrounding that may be
repellant to members of the same species or different species
Two types of antibiosis
Intraspecific antibiosis secretion of chemical substances by organisms into their
surrounding that may be repellant to members of the same species e.g. male rabbits
secrete pheromones from their submandibular salivary glands that are used to mark
territory as a warning to other bucks that the territory is occupied.
Interspecific antibiosis secretion by organisms chemical substances into their
surrounding that may be repellant to members of the different species e.g. penicillium
(a fungus) secretes antibiotics that inhibit bacterial growth, ants release pheromones to
warn off other members of a species in case of danger, sunflower releases chemicals
from its roots and fallen leaves which inhibit germination of seeds of other species
giving sunflower a competitive advantage. This is also referred to as allelopathy.
Ecological significance of antibiosis
❖ Some organisms which produce antibiotics are used to biologically control pests,
pathogens and weeds
❖ It influences the distribution of organisms in a habitat where pheromones are produced
❖ Interspecific antibiosis reduces competition by inhibiting growth and survival of
competitors

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(iv) Human influence.
Humans are one single species with most influence on the distribution of other organisms,
through man’s activities such as hunting, fishing, farming, bush burning, reclamation of swamps,
urban development, and pollution can dictate which organisms grow where. Man can change
habitats and create new ones.
(v) Pollination
Flowering plants utilize insects such as bees, moths to transfer their pollen from one member of a
species to another, this results in the development of a highly complex form of interdependence
between the two groups. Therefore such insects are always most abundant in areas where
flowering plants grow.
(vi) dispersal
Plants depend on a wide range of animal species for the dispersal of their seeds, such animals
greatly influence the distribution of those particular plants.
(vii) Mimicry
This is where an organism resembles another organism which is either harmful, distasteful or
unpalatable to predators in order to avoid predation. E.g. the African swallow tail butterfly
(Papilio dardinus) resembles another unpalatable butterfly Amaurus albimaculata so that
predators avoid attacking it.
Forms of mimicry
(i) Batesian mimicry occurs when the palatable species mimics other distasteful species
e.g. viceroy butterfly mimics the poisonous monarch butterfly, the harmless hoverfly
mimics the painful stinging wasp.
(ii) Mullerian mimicry occurs when both the mimic and mimicked are unpalatable and
dangerous e.g. the five spot Burnet and related moths.
(viii) Camouflage
This is where an organism possesses body colorations that resemble closely the color patterns of
their environment/ background so that they are not easily spotted by their predators therefore
escaping predation. Most organisms prefer to stay in habitats with a background resembling the
colour pattern of their bodies
Exists in various forms;

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 (i) warning colouration , conspicuous colouring that warns a predator that an animal is
unplalable or poisonous e.g. poisonous frogs, some snakes, monarch butterflies, and
some grasshoppers
(ii) disruptive colouration/patterning , works by breaking up the outlines of an animal
with a strongly contrasting pattern, thus decreasing detectability e.g. group of zebras
(iii) cryptic colouration allows an organism to match its background and hence become
less vulnerable to predation e.g. chameleon.

N.B: For symbiotic relationships refer to nutrition

II- CONCEPT OF ECOSYSTEM
An ecosystem is a natural unit of living components together with the non-living components
through which energy flow and nutrients cycle, influencing each other and interacting to form a
relatively stable, self-perpetuating system.
Stable in a sense that it can adjust to changes within itself through a feedback process thus
capable of self-regulation (homeostasis), it is self-perpetuating in that it can continue on its own
without the necessity for humans or other influence, this they do through maintaining a balance
between their input and output environments
Two major inputs in ecosystem include energy and nutrients.
Energy basically comes from the sun and flows through an ecosystem linearly and energy lost
from an ecosystem can’t be recovered
Nutrients these continuously flow between abiotic and biotic components within an ecosystem
STRUCTURE OF AN ECOSYSTEM
An ecosystem consists of biotic components and abiotic components
Abiotic components of an ecosystem include soil, water and climate. Soil and water contain a
mixture of organic and inorganic nutrients
Climate include environmental variables such as light, temperature, humidity and rain or snow
which influences the population size, types of organisms and distribution of organisms in an
ecosystem
Biotic components consist of autotrophs, heterotrophs and decomposers
Autotrophs are producers while heterotrophs are consumers within an ecosystem

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1. Producers:
These are autotrophs capable of synthesizing complex organic food materials from simple
inorganic food raw materials e.g. carbon dioxide and water. Examples include; large green
terrestrial plants e.g. trees, shrubs, grass.
For aquatic ecosystem, the producers are microscopic algae, blue green bacteria. Others are
flagellates like euglena, volvox, chlamydomonas etc. These are collectively called
Phytoplanktons (microscopic marine producers)
NB:
Some producers use chemical energy derived from breakdown of chemical compounds like
sulphur to convert carbon dioxide and water into high energy compounds like carbohydrates e.g.
sulphur bacteria i.e. they are chemosynthetic.
2. Consumers:
These are organisms that get energy and nutrients by feeding on other organisms or their remains.
They are classified as;
(i) Primary consumers (Herbivores):
Are consumers that eat plants e.g. insects, birds, most mammals (grazers), some consumers do not
eat the producer but live as plant parasites e.g. aphids, some fungi and even other plants e.g.
broomrape, orobanche, mistletoe
In aquatic ecosystem, they include; water fleas, fish, crabs larvae, barnacles, molluscs, and
protozoans, collectively known as zooplanktons (microscopic marine consumers).
(ii) Secondary consumers, tertiary and other top consumers:
Secondary consumers feed on herbivores and are therefore referred to as carnivores the tertiary
and other top consumers feed on secondary consumers or tertiary as appropriate. Tertiary and
other top consumers can be classified as
❖ Predators: that hunt and kill others for food
❖ Scavengers/ carrion feeders: that feed on dead organisms but do not kill them e.g. vultures,
hyenas, marabou stocks etc.
❖ Parasites: which do not eat their prey but feed off the host organism while it continues to
live

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Detrivores and decomposers:
Decomposers are microorganisms, mainly fungi and bacteria which live as saprotrophs on dead
organic matter
Detritivores
Are organisms that feed on small fragments of decomposing or dead material termed as detritus
e.g. rag worms in estuarine environments, earth worms, mites, maggots, wood lice, termites and
springtails in terrestrial ecosystems, sludge worms in fresh waters etc.
Significance of detritivores in decomposition process
Dead plant material is often deficient in nitrogen which limits microbial activity, the faeces of
detritivores contain more nitrogen and moisture than the dead vegetation they eat, upon action of
detritivores plant material is used much more readily by decomposer bacteria and fungi.

The role of detritivores in decomposition can be demonstrated through studying the effect of
detritivores on the disappearance of oak leaf discs buried in nylon bags of mesh size 7mm and
those less than 0.5mm as shown on the figure below.

The mesh size determined which decomposers and detritivores had access to the leaf discs, in
7mm mesh size bags, where earthworms could get into, the discs were broken down much faster
than in those with mesh size less than 0.5mm, where only microbes could enter
Relationship between producers, consumers and decomposers
Organic materials synthesized by producers are eaten and assimilated by consumers. With the aid
of decomposers, all the organic materials incorporated into the bodies of the consumers are

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eventually broken down into inorganic materials, these are then rebuilt into organic compounds
by the synthetic activities of the producers

Properties or characteristics of an ecosystem

❖ Feeding relationships
❖ Energy flow / energy transfer
❖ Cycling of materials
❖ Succession and climax ( changes in an ecosystem)
❖ Homeostasis of an ecosystem/ balance of nature
❖ Productivity in an ecosystem
ENERGY FLOW THROUGH AN ECOSYSTEM
The energy flow in an ecosystem is basically non-cyclic. It is passed along a feeding hierarchy in
a chain called food chain. Each feeding stage (feeding level) in a food chain is called trophic level
The source of energy in an ecosystem is the sun. Sun energy is received inform of
electromagnetic radiations. Most of the sun’s radiation is reflected or absorbed immediately from
the clouds, dust, soil, water or vegetation in the atmosphere and the earth’s surface.
Only about 1% of the sun’s radiation is trapped by the photosynthetic organisms such as green
plants which are primary producers (autotrophic organisms) and converted into chemical energy
in a process of photosynthesis. It is then transferred from one feeding level to another through
feeding relationships like food chains or food webs

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Along the food chain, only a small proportion of the available energy is transferred from one
feeding level to another; much energy is lost as heat during sweating and evaporation, excretion ,
respiration, egestion, and some remains locked up in indigestible parts of the plant like cellulose,
or bones, hooves, hair, skin etc. of animals.
The number of organisms decrease at each successive feeding level because of the great energy
losses, so the energy left in organisms is little to support large numbers of top consumers; limiting
the length of food chain( not exceeding five trophic levels( feeding level in a food chain
containing given amount of energy).
Energy flows out of the trophic levels in the following ways
❖ Lost as heat
❖ In structures synthesized by the plant but not contributing to increase in biomass e.g.
shedding leaves, bark, flower and production of seeds for dispersal
❖ Death in plants and animals
❖ Plants eaten by herbivores
TROPHIC EFFICIENCY/ ECOLOGICAL EFFICIENCY
Is the percentage of energy at one trophic level that is converted into organic substances at the
next trophic level.
The rate at which this chemical energy is formed and stored by the primary producers (green
plants) per unit time is known as gross primary productivity (GPP). Some of this chemical energy
is utilized by green plants and in the process lost as heat through respiration and photorespiration.
The remaining energy stored in form of carbohydrates per unit area per unit time after respiration
and photorespiration is known as Net primary productivity (NPP). The NPP becomes available to
the primary consumers which are the herbivores at the next trophic level. The herbivores feed on
the primary consumer to acquire this chemical energy. The rate at which primary consumer (the
herbivores) accumulate and store this chemical energy per unit area per unit time is called
secondary productivity (SPP). Some energy is lost in the primary consumers as heat through
respiration, excretion, growth, repair, reproduction, inedible plant parts, egestion, etc. the
remaining energy is passed into the secondary and tertiary consumers in the next trophic levels
when they feed on the herbivores. In the process energy is again lost as heat through the same
processes. Decomposers which include bacteria and fungi utilize energy from every trophic level
when the organisms in each stage of feeding relations die and begin to decompose.

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PRODUCTIVITY IN AN ECOSYSTEM.
Productivity refers to the rate at which energy and biomass or organic matter is produced per unit
time in a unit area by organisms in an ecosystem.
Can be measured using several methods i.e
• Harvest crop
• Through oxygen production of the given area of the ecosystem.
• Amount of carbon dioxide consumed during photosynthesis.
• Rate of consumption or use of raw materials
Can be divided into;
1. Gross productivity; is the total amount of energy and organic matter stored in an organism
over a period of time.
2. Net productivity; is the amount of energy and organic matter stored in an organism and
passed onto the next trophic level.
3. Primary productivity. Is the rate at which certain amount of energy and Biomass is produced
and stored by primary producers per unit time in a unit area. It is further divided into,
(i) Gross primary productivity (GPP).
Is the rate at which certain amount of energy and Biomass is produced and stored by primary
producers per unit time in a unit area before any loss of energy due to respiration and
photorespiration.
(ii) Net primary productivity (NPP).
Is the amount energy and biomass available per unit time in a unit area after loss of some energy
from the Gross primary productivity due to respiration and photorespiration

i.e NPP = GPP - Respiration.

In C3 plants, NPP = GPP - Respiration + Photorespiration.
Note: C3 plants have a lower NPP than C4 plants.
4. Secondary productivity; Is the amount of energy incorporated into the body of consumers.
Gross secondary productivity (GSP).
Is the rate at which certain amount of energy and Biomass is produced and stored by primary
consumers per unit time in a unit area.

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The amount of the energy and biomass that remains in primary consumer per unit time in a unit
area after some energy have been lost from the GSP due to excretion, egestion and respiration is
referred to as the Net Secondary Productivity (NSP).
Carnivores have higher Net secondary productivity (NSP) than herbivores because of the following
reasons,
• The diet of carnivores is rich in proteins which is easily digested and soluble products
efficiently absorbed. In this case, very little energy is lost, while herbivores feed on diet rich
in carbohydrate cellulose (plant material) which is not easily digested or only partially
digested, a lot of energy is lost in the undigested parts and hence herbivores have lower
NSP.
• Carnivores do not have symbiotic microbes to consume part of the energy from their diet,
while herbivores have Cellulase secreting bacteria in their guts, these bacteria utilize some
of the energy from the cellulose.
• Their faeces contain much less undigested matter.
Note
Net secondary productivity is higher in exotherms than in endotherms, because;
Energy from absorbed food, is used in replace the lost heat to their surrounding, inorder to maintain
a constant body temperature, unlike exotherms that depend mostly on behavioral means to maintain
their body temperature.

FACTORS THAT INFLUENCE PRODUCTIVITY OF AN ECOSYSTEM.

Level of nutrients especially phosphate and nitrates. The higher the level the greater the
productivity as they are used in protein synthesis hence more dry matter is formed.
Temperature. The higher the temperature the higher the productivity since temperature
activate enzymes involved in photosynthesis.
Carbon dioxide concentration. The higher its concentration in air, the greater the
productivity since carbon dioxide is a raw material for photosynthesis. The lower the
concentration of carbon dioxide the, the lower the productivity.
Amount of light available. The higher the light intensity, the more energy fixed and the
greater the productivity. Lower light intensity results into decreased productivity.

31
 Availability of water. Plenty of water increases rate of photosynthesis since water is a raw
material for photosynthesis, hence more energy is fixed raising the productivity.
Length of growing season. The greater the length, the higher the productivity. The shorter
the length, the lower the productivity.
Relative population size of primary producers. The greater size of green plants, the more
energy fixed and the higher the productivity.
Nature and type of tree species. Certain tree species tend to be more productive than others,
therefore the more abundant these tree species are, the greater the productivity. The fewer
such tree species, the lower the productivity.
Concentration of pollutants in air. The higher the concentration, the lower the amount of
photosynthesis and less energy fixation, hence the lower productivity.
Chlorophyll concentration in case of aquatic ecosystem, upper zones of lake have higher
chlorophyll content than deeper zones, hence the greater the concentration of chlorophyll,
the higher the productivity.
Soil fertility, humus, mineral content. Productivity increases with increase in Soil fertility,
humus, mineral content. They are utilized in protein synthesis and formation of new organic
matter. Decrease in Soil fertility, humus, mineral content, decreases productivity.
Abundance of decomposers which enable nutrient recycling making the nutrients available
for synthesis of new organic matter. This will increase productivity. Few decomposers
reduce productivity.
FEEDING INTER-RELATIONSHIPS IN AN ECOSYSTEM.
This is biotic factor which influence an environment, they include,
• Food chains.
• Food webs.
• And ecological pyramids.
The feeding inter-relationships arise due to grazing, predator-prey relationship, parasitism,
mutualism, symbiosis, etc.
FOOD CHAINS AND FOOD WEBS.
Food chain is a linear sequence or series of organisms existing in an ecosystem through which
chemical energy formed and stored (Carbon compounds produced) by the green plants and other
photosynthetic organisms are systematically transferred.

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Each organism in the series feeds on and derives energy from preceding one, it is also consumed
by another organisms following it and provides energy for that organism. The energy in the food
chain is passed along the hierarchy in a chain called food chain. Each feeding level in a food chain
is called trophic level. Some energy is lost when it passed from one level to another; this is why
food chains are short.

TYPES OF FOOD CHAINS.

There are two types of food chains,
(i) Grazing food chains.
(ii) Detritus food chains.
GRAZING FOOD CHAINS
Is the linear nutritional sequence of organisms in an ecosystem where the chemical energy is passed
in which the first trophic level is occupied b a green plant or green algae and the second trophic
level is a grazing animal (Herbivore) and the subsequent levels by the carnivores.
EXAMPLES.

DETRITUS FOOD CHAINS

Is the linear nutritional sequence of organisms in an ecosystem through which a chemical energy is
passed and in this case the first trophic level is occupied by the detritus, second by detritivores and
the subsequent levels by the carnivores.
EXAMPLES.

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Fragments of decomposing materials are called detritus and many small animals feed on them,
contributing to the process of breakdown (decomposition), these animals are called detritivores.
Examples of detritivores include,
(i) Wood land detritivores.
Earth worms, wood lice, blow fly maggot,
(ii) Sea shore detritivores.
Ragworm, dog whelk, sea cucumber.
(iii) Terrestrial detritivores.
Earth worms, wood lice, millipedes, mites, nematodes, termites, springtails etc.

HOW SOME DETRITIVORES INFLUENCE THE COMPONENTS OF TERRESTRIAL

ECOSYSTEM.
Termites live in nests called termitaria, they build galleries below the soil surface with cemented
walls, and this prevents free aeration in the soil and reduces proper drainage of the soil. Termites
also carry plant materials into the soil and break them into smaller fragments, increasing their
surface area for the process of decomposition by aerobic bacteria. This releases nutrients to the soil.
Termites eat other organisms, lowering organic matter content of the soil. Their faecal materials
contain uric acid which is incorporated into the termitarion but also adds into the humus content of
the soil.
Earth worms burrow into the soil, burrowing promotes and improves aeration of the soil, oxygen
is available for activities of aerobic bacteria which increases the rate of decomposition of organic
matter, more nutrients are released into the soil. The nitrogenous wastes of the earth worms add
more ammonia and nitrates in the soil. Dead individuals are decomposed to add humus into the soil.
Earth worms ingest the soil particles and mix the plant materials within the soil. Earth worms
contain mucus and bacterial polysaccharides which hold together the fine particles, improving the
crumb structure of the soil. The crumbs are not easily dispersed by water and promote the granular
texture of the soil.
FOOD WEB.
Is an interconnected food chains in which an animal or organism at one trophic level has several
alternative animals that it can feed on at different trophic levels, and also has many other animals
that can feed on it at different trophic levels.

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EXAMPLE OF A FOOD WEB IN A TYPICAL FRESH WATER POND.

ECOLOGICAL PYRAMIDS.
Feeding relationships and energy transfers through the biotic component can be quantified and
shown diagrammatically as ecological pyramids. Ecological pyramids provides basis for

35
comparing, different ecosystems, seasonal variation within an ecosystem, changes in an ecosystem.
There three types of ecological pyramids, these include the following,
❖ Pyramid of numbers.
❖ Pyramid of Biomass.
❖ Pyramid of Energy.
Short comings of the Ecological pyramids include the following,
✓ It is quite not easy to identify the trophic levels of an organism as many organisms feed at
several trophic levels.
✓ It omits the detritivores organic matter content, yet much of the energy fixed may be passed
into the detritivores.
✓ It only considers energy stored by the green parts of the plant and consumed by the
herbivores, yet some herbivores cannot digest chlorophyll, others eat only seeds or fruits or
nectar.
TYPES OF ECOLOGICAL PYRAMIDS.
PYRAMID OF NUMBERS.
Is a bar diagram indicating the relative numbers of individuals at each trophic levels in a food chain.
Example.

The length of each bar indicates the relative number of organisms at each trophic level. It can be
noticed from the pyramid of numbers that there is progressive decline in the number of individuals
at each trophic level. This is because, a lot of energy is lost each time it passed from one trophic
level to another in a food chain. This places a natural limit or reduces the biomass and this loses of
energy causes the food chain be short, i.e. not more than six levels exist. Therefore, to support
individuals at one trophic level more energy from the individuals at the levels below is required
and is achieved by having more individuals at the lower trophic levels.

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ADVANTAGES OF PYRAMID OF NUMBERS.
(i) It is easy to carry out.
(ii) It is relatively cheaper to conduct, does not require many equipment to do measurements.
DISADVANTAGES OF PYRAMID OF NUMBERS.
(i) All individuals are counted as the same, yet not all individuals have the same sizes. For
example, an oak tree is counted as one individual in the same way as an aphid.
(ii) No account is made for juveniles and other immature forms of species whose diet and energy
requirements may differ from that of the adults.
(iii) The numbers of some individuals is so large that it is difficult to represent them accurately on
the same scales other species in food chain with considerable numbers. For example, Millions
of black flies may feed on a single rose-bush and this relationship cannot be effectively drawn
to scale on a pyramid of numbers. Such conditions give rise to unusual pyramids of numbers.
(iv) Does not indicate the source of energy in an ecosystem.
EXAMPLES OF UN USUAL PYRAMIDS OF NUMBERS.

(a) In a normal pyramid of numbers for comparison.

(b) The producer is a single plant such as a single tree.
(c) Producer is a single plant, infested with parasites (primary consumers) and
the later are parasitized by further parasites.
(d) A large number of producers are eaten by a single primary consumer which
is infested with parasites (Secondary consumers)
Some of the disadvantages are overcome by use of a pyramid of Biomass.
PYRAMID OF BIOMASS.
Is a bar diagram of proportionate length indicating dry mass of all organisms at each trophic level.
Biomass is the weight of living material per unit volume or area.

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 ADVANTAGES OF PYRAMID OF BIOMASS.
(i) Biomass provides a relatively accurate measure of the amount of energy in each trophic level.
(ii) It gives the measure of total productivity in each trophic level.
DISADVANTAGES OF THE PYRAMID OF BIOMASS.
(i) It is impossible to measure exactly the biomass of all individuals in a population. A sample is
usually taken and measured and this sample may not be a representative of all organisms at a
particular trophic level.
(ii) The biomass of individuals varies from one season to another. For example, Biomass of
deciduous tree in summer may be different from those in winter, so, the sample only measures
the amount material present at a particular time. This is called standing crop and gives no
indication of total productivity.
(iii) It involves destroying or killing of the living organisms in order to obtain the dry weight.
(iv) No source of energy is indicated.
(v) It is more laborious and expensive to conduct.
(vi) It is very much time consuming because it involves many steps.
(vii) The standing biomass or standing crop biomass which is the biomass at the time of sampling
does not indicate exactly the productivity.
PYRAMID OF ENERGY.
Is a bar diagram in proportion to indicate the total energy utilized at each trophic level. The total
productivity of primary producers of a given area can be measured for a given period. But obtaining
the necessary data can be a complex and difficult affair.
ADVANTAGES OF PYRAMID OF ENERGY.
(i) It represents the amount of energy per unit area or volume passed from one trophic level to
another. So, it is more accurate.
(ii) It represents total productivity at each trophic level.
(iii) It takes into account the energy from the sun which is the source of energy in an ecosystem.
(iv) It enables comparison of different ecosystems, so, the importance of one ecosystem to another
can be determined.
(v) Unusual and inverted pyramids are not obtained.
(vi) The energy content of each individual is determined independently of the others; it shows that
no two individual species can have the same energy content.

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DISADVANTAGES.
(i) It is most difficult to obtain data for pyramid of energy since it requires a lot of technical
know-how.
(ii) It is expensive to carry out because it requires some sophisticated equipment to do
measurements.
ECOLOGICAL TECHNIQUES OF STUDYING FEEDING RELATIONS OF
ORGANISMS.
(i) Direct observations of what the organism eats.
This method has some disadvantages,
✓ It can not be applied to aggressive animals.
✓ It can not be applied to organisms living in concealed and hidden habitat.
✓ It can not be applied on organisms which refuse to eat under observation. For example Rodents
and some birds.
(ii) Faecal analysis.
This involves studying the content of faeces of a given animal.
This method has some disadvantages that include,
✓ Some food can not be seen in the faeces because they are already digested and absorbed.
✓ It can not be applied on animals which eat their faeces like Rabbits a practice known as
ecopathy. The faeces are eaten because they still contain more nutrients following little
digestion and absorption which occurred within the gut.
(iii) Examination of the stomach content.
This is to find out what they feed on and it requires identification skills after enzyme action and
mastification.
(iv) Use of radio-active tracers.
It involves labeling available foods using radio-active substances and their trace their presence or
absence in the animals gut. It can be used in all animals no matter their nature.
NOTE: To come up with more reliable information, more than one method is used.

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