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Biomes are very large ecological areas on the earth’s surface, with
fauna and flora (animals and plants) adapting to their environment.
Biomes are often defined by abiotic factors such as climate, relief,
A biome is NOT an ecosystem, although in a way it can look like a
massive ecosystem.
Plants or animals in any of the biomes have special adaptations that
make it possible for them to exist in that area.
Many units
are found
within one biome.
There are five major categories of biomes on earth.
In these five, there are many sub-biomes, under which are many more
well defined ecosystems.
The Desert Biomes:
They are the
 Hot and Dry Deserts,
 Semi-Arid Deserts,
 Coastal Deserts and
 Cold Deserts.
The Aquatic Biomes:
Aquatic biomes are grouped into two,
 Freshwater Biomes (lakes and ponds, rivers and streams,
wetlands) and
 Marine Biomes (oceans, coral reefs and estuaries).
The Forest Biomes:
There are three main biomes that make up Forest Biomes. These are;
 Tropical Rainforest,
 Temperate and
 Boreal Forests (also called the Taiga)
The Grassland Biomes:
There are two main types of grassland biomes:
 the Savanna Grasslands and the
 Temperate Grasslands.
The Tundra Biomes:
There are two major tundra biomes—
 The Arctic Tundra and
 The Alpine Tundra.
Biomes play a crucial role in sustaining life on earth.
For example,
 The Aquatic biome is home to millions of fish species and the
source of the water cycle.
 It also plays a very important role in climate formation.
 The terrestrial biomes provide foods, enrich the air with oxygen
and absorb carbon dioxide and other bad gases from the air.
 They also help regulate climate and so on.
Applying the idea of an ecosystem
The ecosystem concept can be applied to any scale from a small pool
of water to a whole ocean, or even to the biosphere.
Whatever its size, an ecosystem is relatively self-contained and tends
to maintain itself by recycling minerals.
Therefore in a pond, organisms die and decompose and the nutrients in
them are returned to the water.
In turn, the autotrophs remove these nutrients from the water and use
them in growth.
Ecosystems are often thought of as closed systems, but they are not
really closed.
No ecosystem is completely self-contained; they all need an outside
source of energy, usually the sun
Ecosystem goods and services
This is the extremely vital life-support services ecosystems provide to
human life, its well-being and future economic and social development.
For example: The benefits ecosystems provide include;
 food,
 water,
 timber,
 air purification,
 soil formation and
 Pollination.
Energy Flow of ecosystems
Ecosystems maintain themselves by cycling energy and nutrients
obtained from external sources.
The populations in an ecosystem interact through their feeding
relationships. A typical community has:
 Autotrophs (also known as primary producers) which produce
their own food.
 Herbivores (primary consumers) which eat the autotrophs.
 Secondary consumers which eat the herbivores
 Tertiary consumers which eat the secondary consumers
 Decomposers, mainly bacteria and fungi, which obtain nutrients
by breaking down the remains of dead organisms
At the first trophic level, primary producers (plants, algae, and some
bacteria) use solar energy to produce organic plant material through
Herbivores—animals that feed solely on plants—make up the second
trophic level.
Predators that eat herbivores comprise the third trophic level; if larger
predators are present, they represent still higher trophic levels.
Organisms that feed at several trophic levels (for example, grizzly bears
that eat berries and salmon) are classified at the highest of the trophic
levels at which they feed.
Decomposers, which include bacteria, fungi, molds, worms, and
insects, break down wastes and dead organisms and return nutrients to
the soil.
On average about 10 percent of net energy production at one trophic
level is passed on to the next level.
Processes that reduce the energy transferred between trophic levels
include respiration, growth and reproduction, defecation, and nonpredatory death (organisms that die but are not eaten by consumers).
The nutritional quality of material that is consumed also influences how
efficiently energy is transferred, because consumers can convert highquality food sources into new living tissue more efficiently than lowquality food sources.
The low rate of energy transfer between trophic levels makes
decomposers generally more important than producers in terms of
energy flow.
Decomposers process large amounts of organic material and return
nutrients to the ecosystem in inorganic form, which is then taken up
again by primary producers.
Energy is not recycled during decomposition, but rather is released,
mostly as heat
(This is what makes compost piles and fresh garden mulch warm).
Figure shows the flow of energy (dark arrows) and nutrients (light
arrows) through ecosystems.
An ecosystem is made up of organisms, which have established
themselves in the given area and have continued to survive and have
not become extinct.
The species hence possess genes, which fit the environment and are
tolerant to disturbances like flood, fire, drought; and a reproductive rate
that balances the natural catastrophes.
The birth rate of organisms will have to be optimum to avoid
overpopulation and hence starvation.
The human population is a good example.
As technological evolution brings down our normal death rate, social
evolution lowers the birth rate to strike a balance.
Biological evolution is however much slower than social or
In ecosystems, organisms constantly adjust themselves to geologic or
climatic changes and to each other.
As an example, the bats developed sonar to find the moths and the
moths developed ears sensitive to the bat’s frequency.
The behavioral adaptations are also reflected in the anatomy or the
body structure of the organisms.
This evolutionary pattern is very common and is called character
The process of life evolution started from lower plants and progressing
to higher plants, lower animals, higher animals and finally to man.
Biogeochemical cycle
A biogeochemical cycle or substance turnover is a pathway by which
a chemical substance moves through both the biotic (biosphere) and
abiotic (lithosphere, atmosphere, and hydrosphere) components
of Earth.
A cycle is a series of change which comes back to the starting point and
which can be repeated.
Water, for example, is always recycled through the water cycle, as
shown in the diagram.
The water undergoes evaporation, condensation, and precipitation,
falling back to Earth.
Elements, chemical compounds, and other forms of matter are passed
from one organism to another and from one part of the biosphere to
another through biogeochemical cycles.
The term "biogeochemical" tell us about the biological, geological and
chemical factors.
The circulation of chemical nutrients like carbon, oxygen, nitrogen,
phosphorus, calcium, and water etc. through the biological and physical
world are known as "biogeochemical cycles".
The element is recycled, although in some cycles there may be places
(called reservoirs) where the element is accumulated or held for a long
period of time (such as an ocean or lake for water).
Important biogeochemical cycles
The most well-known and important biogeochemical cycles, for
 the carbon cycle,
 the nitrogen cycle,
 oxygen cycle
What is The Carbon Cycle?
The carbon cycle is very important to all ecosystems, and ultimately
life on earth.
The carbon cycle is critical to the food chain.
Living tissue contains carbon, because they contain proteins, fats and
The carbon in these (living or dead) tissues is recycled in various
Let's see how this cycle works using the simple sketch below:
Human activities like heating homes and cars burning fuels
(combustion) give off carbon into the atmosphere.
During respiration, animals also introduce carbon into the atmosphere
in the form of carbon dioxide.
The Carbon dioxide in the atmosphere is absorbed by green plants
(producers) to make food in photosynthesis.
When animals feed on green plants, they pass on carbon compounds
unto other animals in the upper levels of their food chains.
Animals give off carbon dioxide into the atmosphere during
Carbon dioxide is also given off when plants and animals die.
This occurs when decomposers (bacteria and fungi) break down dead
plants and animals (decomposition) and release the carbon compounds
stored in them.
Very often, energy trapped in the dead materials becomes fossil fuels
which are used as combustion again at a later time.
The nitrogen cycle
Nitrogen is also key in the existence of ecosystems and food chains.
Nitrogen forms about 78% of the air on earth.
But plants do not use nitrogen directly from the air.
This is because nitrogen itself is unreactive, and cannot be used by
green plants to make protein.
Nitrogen gas therefore, needs to be converted into nitrate compound in
the soil by nitrogen-fixing bacteria in soil, root nodules or lightning.
To understand the cycle better, let us consider the diagram below:
1. Nitrogen is introduced into the soil by precipitation (rain, lightning).
2. Nitrates don’t only come from Nitrogen in the air.
They can also be obtained by the conversion of ammonia, commonly
used in fertilizers by nitrifying bacteria in the soil.
Some root nodules can also convert nitrogen in the soil into nitrates.
3. Plants build up proteins using nitrates absorbed from the soil.
4. When animals like cows, eat these plants, they, in turn, use it to build
animal protein.
5. When these animals (cows) poop, pee or die, the urea, excreta or
carcass are broken down by decomposers and the nitrogen is reintroduced into the soil in the form of ammonia.
6. Nitrates in the soil can also be broken down by denitrifying bacteria
(in specific conditions) and sent into the air as nitrogen.
This process can help make the soil infertile because it will lack the
nitrates needed for plant use.
Once nitrogen gets back into the air, the cycle continues.
Food chains
All living things need to feed to get energy to grow, move and
But what do these living things feed on?
Smaller insects feed on green plants, and bigger animals feed on
smaller ones and so on.
This feeding relationship in an ecosystem is called a food chain.
Food chains are usually in a sequence, with an arrow used to show the
flow of energy.
Below are some living things that can fit into a food chain.
A food chain is not the same as a food web.
A food web is a network of many food chains and is more complex.
See the food web illustration below you can pick out a basic food chain
from the web:
Green plants  Grasshopper  Frog  Bird  Hawk
In the diagram above, the arrows show the direction of energy flow. It
points to the animal doing the eating.
Energy transfer
Energy is transferred along food chains from one level to the next.
Some of the energy is used up in growth, reproduction repair,
movement and other ways, and not made available to the next level.
Shorter food chains retain more energy than longer chains.
Used up energy is absorbed by the environment.
Ecosystem boundaries are not marked (separated) by rigid lines.
They are often separated by geographical barriers such as deserts,
mountains, oceans, lakes and rivers.
As these borders are never rigid, ecosystems tend to blend into each
This is why a lake can have many small ecosystems with their own
unique characteristics.
Scientists call this blending “ecotone”.
This refers to a variety of living organisms in an area
Areas with a high biodiversity are those with lots of different species.
Biodiversity can be considered at the following levels;
 habitat diversity- the number of different habitats in an area
 Species diversity- the number of different species and the
abundance of each species in an area. E.g a woodland having
many different species of plants, insects, birds and mammals.
 Genetic diversity- the variation of alleles within a specie
1. Sampling
A sample of the population is taken and estimates about the whole
habitat is based on the sample.
Sampling can be random or non-random.
 To avoid bias in the results, the sample should be random
 Sometimes it’s necessary to take a non-random sample. For
example, when there’s a lot of variety in the distribution of
species in the habitat and you want to make sure all the different
areas are sampled.
 There are three types of non-random sampling
 Systematic-this is when samples are taken at fixed intervals
 Opportunistic- this is when samples are chosen by the
 Stratified- this is when different areas in a habitat are
identified and sampled separately in proportion to their part
of the habitat as a whole.
Biodiversity is affected by species richness and species evenness.
The greater the species richness and species evenness in an area, the
higher the biodiversity.
 Species richness is the number of different species in an area.
 It’s measured by taking random samples of a habitat and
counting the number of different species.
 Species evenness is a measure of the relative abundance of each
species in an area.
 The more similar the population size of each species, the
greater the species evenness.
 Its measured by taking random samples of a habitat, and
counting the number of individuals of each different species
Habitat x and y both contain two different species and 30 individual
Habitat x
Habitat y
Species 1
Species 2
 Species richness in the two habitats is the same -2
 In habitat y the individual organisms are more evenly distributed
between the different species-it has greater species
2. Index of diversity
 Species richness is a very simple measure of biodiversity. But
species present in a habitat in very small numbers shouldn’t be
treated the same as those with bigger populations.
 An index of diversity is another way of measuring biodiversity.
It is calculated using an equation that takes into account both
species richness and species evenness.
 Calculate an index of diversity (d) using this formula
d = N (N-1)
⅀ n (n-1)
N = total number of organisms of all species
n = total number of organisms of one species
⅀= sum of
 When calculating an index of diversity using this formula, the
higher the number you end up with, the more diverse the area is.
If all the individuals are of the same species (i.e. no biodiversity)
this index will have a value of 1.
There are 3 different species of flower in this field-a red species, a white
and a blue. There are 11 organisms altogether, so N=11. There are 3 of
the red species, 5 of the white and 3 of the blue. So the species diversity
index of this field is:
3(3-1) + 5(5-1) + 3(3-1)
= 3.44