ecosystem is a basic functioning unit in nature. It is made up of living
organisms and their non-living environment.
Heterotrophs and Decomposers
Autotrophs: Autotrophs are organisms that use
sunlight or chemical energy to manufacture or synthesize their own food from
inorganic substances by a process called photosynthesis. Autotrophs are mainly green plants and synthetic
protists and some bacteria. Since autotrophs are the only organisms that can
produce food in an ecosystem by using sunlight obtained from the sun, they are
known as producers.
Heterotrophs: Heterotrophs are organisms that
cannot manufacture food but depends and feed on ready-made food from the
producers or autotrophs. Heterotrophs include all animals, carnivorous or
insectivorous plants, fungi, and most protists and bacteria. Heterotrophs are called consumers. Animals that feed on plants only are
known as herbivores or primary consumers. Animals that feed on meat
(animals) only are known as carnivores or secondary consumers. Animals
that feed on both plants and animals are called omnivores or tertiary
Decomposers are organisms that feed and break down dead organisms or decaying
organic materials. In breaking down dead organisms, decomposers recycle
nutrients found in dead organisms as well as help with the eradication of waste
material. The two
most common examples are bacteria and fungi. Decomposition of dead organisms is
important because decomposers often releases useful substances that plants can
absorb for production of food again.
Food chains, food webs and trophic levels
Food chain is a feeding relationship that involves
the transfer of energy from producers to consumers through food. All food chain
usually starts or begins with a producer (green plant), the producer or
autotroph is fed on by an herbivore (primary consumer) which is in turn fed on
by a carnivore and so on. Each stage of the food chain or each
feeding level of the chain is called the trophic
level. The first level of the food
chain is known as the (primary) producer and is in most cases a plant e.g.
grass, cabbage etc. Animals (consumers) that eat plants (feed on the primary
producer) are called primary consumers. Organisms that feed on the primary
consumers are secondary consumers. Those
that feed on secondary consumers are called tertiary consumers. Quaternary
consumers then feed on tertiary consumers. Food chain
shows how food energy is transferred from one organism to another in a linear
pattern. Fig 9.1 is an example of a food chain in a terrestrial habitat. Also,
example of a food chain in an aquatic habitat is illustrated in fig 9.2.
Fig 9.1 An
example of food chain in a terrestrial habitat
Fig 9.2 An
example of food chain in an aquatic habitat
food web may be defined as a series of interconnecting food chains that shows
the feeding patterns amongst organisms in a habitat/ecosystem/community with
two or more inter-related food chains. Within an actual ecosystem the feeding
relations are more complex than simple food chains. This is because tertiary
consumers may also feed on primary consumers and secondary consumers. There is
also selective feeding among consumers. For example, secondary consumers may
not consume all primary consumers and primary consumers may not consume all
types of producers. In the food chains containing omnivores such as
humans that feed on consumers and producers, then this chain may become more
complex. As a result of this complexity, feeding relationships are represented
by food webs rather than food chains.
9.3 An example of food web
Trophic level is defined as the
feeding level or each stage in a food chain or food web. A trophic level refers to a level or a position in
a food chain or ecological pyramid. A food chain or ecological pyramid would start at 1st
trophic level. The 1st trophic level is occupied by the
primary producers (e.g. plants). The next trophic
level in a food chain or ecological pyramid is the 2nd trophic
level. In this level, the organisms occupying this level feed on the primary
producers and are referred to as primary consumers, i.e. herbivores. Organisms
feeding on the herbivores, called carnivores or secondary consumers occupy the next trophic level, i.e. 3rd
trophic level. The 4th trophic levels are occupied by tertiary
consumers. The food chain below shows three trophic levels.
Grass (1st trophic level) ? Goat (2nd trophic
level) ? Man (3rd trophic level)
pyramids of numbers, biomass and energy (Ecological pyramid)
A food or ecological pyramid shows
the relative sizes of different components of a food chain at the various
trophic levels. There are three types of ecological pyramid; these are pyramid of numbers, pyramid of biomass and pyramid of energy.
In a food pyramid, each trophic level is represented by a horizontal bar, with
the width of the bar representing the number of organisms, the
amount of biomass or the amount of energy available
at that level. The base of the pyramid represents the producer; the second
level is the primary consumer; the next is the secondary consumer and so on.
1. Pyramids of numbers
A pyramid of numbers
is the graphical method that shows the quantitative relationships between organisms
at each trophic level (i.e. each stage in a food chain or web). Pyramid of
number refers to the number of organisms at each trophic level which decreases
progressively from the first to the last trophic level in a food chain. A
pyramid of numbers shows the relative number of organisms at
each stage of a food chain.
Example 1: clover ? snail ? thrush ? hawk
Clover is a plant and it is the producer in this food chain. Its bar lies at
the bottom of the pyramid:
Fig 9.4 Pyramid of number
Energy is lost to the surroundings
as we go from one level to the next, so there are fewer organisms at each level
in this food chain. A lot of clover is needed to support the snail population.
A thrush eats lots of snails, and a hawk eats lots of thrushes, so the population
of hawks is very small.
Other pyramid shapes: Sometimes
the pyramid of numbers doesn’t look like a pyramid at all. This could happen if
the producer is a large plant such as a tree, or if one of the animals is very
small. Whatever the situation, the producer still goes at the bottom of the
Here are two examples like this:
2: Oak tree ? Insects
Fig 9.5 Another shape of pyramid of
An oak tree is very large so many
insects can feed on it.
Example 3: Grass ? Rabbit
Fig 9.6 Another shape of pyramid of
Fleas are very small and lots of
them can feed on a rabbit.
2. Pyramids of biomass
Pyramid of numbers sometimes is not the best way to represent a food
In this case a pyramid of biomass (the dry mass of an
organism) is a better diagram to use. It shows the total mass of
organisms at each stage of a food chain.
In general, all producers have a higher biomass than the primary consumer, so a
pyramid will always be produced.
The total energy (and biomass) present at a lower tier of the pyramid, must be
greater than the higher tiers in order to support the energy requirements of
the subsequent organisms.
Fig 9.7 Pyramid of numbers and
pyramid of biomass.
3. Pyramid of Energy
Pyramid of energy is defined as
the amount of energy present in the living organisms at the different trophic
levels of a food chain. It represents a progressive decrease in energy from the
first trophic level to the last trophic level in a food chain or web. The producers at the first trophic
level contain most of the energy. The primary consumers have lesser energy
while the secondary or tertiary consumers have the least energy. The producers,
e.g. grasses form the base of the pyramid while the tertiary consumers e.g.
hawk from the apex. Energy, thus, decreases from the base of the pyramid to the
Fig 9.8 Pyramid of energy
A normal-shaped pyramid is always
produced because there is a reduced amount of energy at each successive
TRANSFORMATION IN NATURE
exists in different forms. One form
of energy can be transformed into another form of energy. In nature,
transformation of energy is carried out by living organisms. Such energy
transformations are governed by the law of thermodynamics. The sun is the
ultimate and external source of energy for ecosystems on earth.
Energy Losses in the Ecosystem
about 1% of the solar energy that reaches the earth’s surface is available for
use by the primary producers (i.e. green plants) in the ecosystem. 20% of the
solar energy from the sun is reflected by the vegetation, 40% warms the
vegetation, soil and air, while the remaining 39% of the solar energy
evaporates water. The primary producers manufacture food using light energy
from the sun, carbon dioxide and water by a process called photosynthesis. The
primary producers transform the light energy into chemical energy. Only about
half of the solar energy (i.e. ½ of 1% = 0.5%) that falls on the leaf of a
plant can be absorbed by chlorophyll. Let’s say if 100 units of energy are
absorbed by the chlorophyll in the chloroplast (i.e. 0.5% of the solar energy),
about ¼ of the absorbed solar energy (1/4 of 0.5% = 0.125% of solar energy
which is 25 units) ends up as chemical energy in food such as starch. The rest
is lost as heat energy. The
plant uses about 30% of chemical or food energy for its own metabolic
activities. The remaining 70% of the chemical or food energy is stored in the
plant. The remainder that is stored is called the net primary productivity
(NPP) while the overall food produced per unit area per unit time is called
gross primary productivity (GPP). Primary consumers feed on the producers.
However, they do not consume all the food that is manufactured by the producers
e.g. on a grassland, about 40% of the grass may be eaten by herbivores
remaining about 60%. This
means that only part of the NPP of the ecosystem is passed on to the primary
consumers. The rest is broken down when the producers die. Only a small portion
of the solar energy gets into the bodies of the final consumers. The rest of
the energy is lost as heat along the food chain. Energy is lost in an ecosystem
through vegetation, soil, air, heat, evaporation of water and effects of wind.
The first law of thermodynamics: The first law of thermodynamics
states that energy can neither be created nor destroyed but may be transformed
from one form into another.
The second law of thermodynamics:
states that no transformation of energy from one state to another is 100%
efficient, since some are lost in form of heat.
OF THE TWO LAWS ON FOOD CHAIN, PYRAMID OF ENERGY AND ENERGY FLOW
The first law: Energy
is generated through the sun and is transferred from the producer to the final
consumers. The green plants transfer the energy to the primary consumers which
again transfer the energy to the secondary consumers. In all, the sum total of
the energy remains constant.
2. The second law: As energy is being transferred from producers to
primary consumers, to secondary consumers and finally to the tertiary
consumers, energy is lost in each trophic level as heat. It is clear that there
is no transfer of energy from one level to another that is 100% efficient. This
means that energy is lost to the environment in form of heat.
The first law: Even
though energy is transformed into various forms in the successive trophic
levels, the sun’s total energy is constant.
2. The second law: During
energy transformation in the successive trophic levels, a part of it is
converted into heat which is lost causing a progressive drop in energy in
successive trophic levels and also the pyramid shape of feeding relationship.
The first law: As
the producers convert the solar energy to useful energy, the energy flows from
the producers to primary consumers, then to secondary consumers and finally to
the tertiary consumers in a food chain.
2. The second law: The transfer of energy between trophic levels is
not 100% or it is not completely transferred. Successive levels have less of
useful energy and support fewer organisms or individual lives. Primary
producers or plant have the highest amount of energy. When herbivores feed on
plants, the energy level is reduced. When carnivores consume the herbivores,
the energy level is further reduced.
Decomposition in Nature
organisms that breakdown dead organic materials. During decomposition, a lot of
the chemical energy in the organic compounds is lost as unstable heat energy.
The decomposers only absorb a small amount of nutrients and energy for their
use. The rest is released into the soil, air and water. The nutrients released
are used by plants to manufacture their food.
Process of Decomposition
The decomposers secrete enzymes
onto their food source. These enzymes break down complex organic compounds into
simple soluble inorganic compounds. The decomposers only absorb a small amount of
nutrients and energy for their use. The rest is released into the soil, air and
water. Products released during decomposition are gases (e.g. carbon dioxide, hydrogen
sulphide, ammonia, and water vapour), heat energy and nutrients.