UNIT 2 CONCEPT OF ECOSYSTEM
UNIT 2: CONCEPT OF ECOSYSTEM
Key Unit Competence
Describe the different components of an ecosystem, biogeochemical cycles andhow energy flows in an ecosystem.
Learning objectivesBy the end of this unit, I should be able to:
– Describe an ecosystem
– State the types and properties of an ecosystem
– Describe the main components of an ecosystem
– Explain the ecological factors influencing the life of organisms in an ecosystem
– Define the terms: population, community, ecosystem, biome, niche and
biosphere
– Distinguish among; individuals, populations, communities, niche, habitat,
ecosystems, biomes, biosphere
– Describe feeding relationships in an ecosystem
– Describe a food chain and a food web
– Explain the relative merits of pyramids of numbers
– Analyse the relation between organisms (example: producers, consumers,
decomposers) and their trophic levels.
– Distinguish between abiotic and biotic factors
– Interpret energy flow diagrams
– Compare; gross primary, net primary production and secondary succession in
biotic communities
– Explain what is meant by trophic efficiency
– Explain energy flow and the recycling of nutrients in an ecosystem
– Describe biogeochemical cycles
– Identify processes, components, and roles of organisms in the hydrologic,
carbon and nitrogen cycles
– Distinguish between primary and secondary succession in biotic communities
– Appreciate the existence of different components of an ecosystem and their
roles in the life of organisms
– Beware of the effect of bioaccumulations at different trophic levels.– Recognise the source and transfer of energy in an ecosystem
Introductory activity
The following pictures indicate different types of ecosystems. Observecarefully the pictures A, B and C and answer the questions that follow.
1. What do you understand by the terms: ecosystem, biotic and abiotic factors?
2. Suggest the types of ecosystems illustrated by pictures A, B, and C.
3. Distinguish between abiotic and biotic factors illustrated on picture A, B and
C.
4. Describe how energy flows through ecosystem B and ecosystem C.
5. Explain how feeding relationships are expressed in food chains on picture B
and C.
6. Identify trophic levels in food chains and food webs on the picture B and
picture C.
7. What would happen if plant species are removed from an ecosystem ofpicture C?
Ecology is the study of how living things interact with each other and with their
environment. It is one of the major branches of biology with different areas that
overlap with geography, geology, climatology, mathematics, and chemistry cited
among other sciences. This lesson introduces fundamental concepts in ecology
with a particular focus on organisms and their environment. Organisms are
individual living things. Despite their tremendous diversity, all organisms have
the same basic needs such as energy and matter, obtained from the environment.
Therefore, organisms are not closed systems. They depend on and are influenced
by the environmental factors including abiotic (non-living factors such as water,
temperature, humidity…) and biotic (living factors such as animals, plants…). The
unit of nature consisting of all the biotic and abiotic factors in an area and theirinteractions is called an ecosystem.
2.1 Ecosystem
Activity 2.1Observe carefully the diagram below, and answer the questions that follow
1. Define an ecosystem and give its different types.
2. Distinguish among; individuals, populations, communities, niche,habitat, ecosystems, biomes and the biosphere.
Different concepts define levels in ecology. From the low to high level, the conceptsinclude:
a. Species
Species such as bees in figure 2.1 is defined as a group of organisms that can breedto produce fully fertile offspring.
Figure 2.1: Species of bees
b. Population
A population is defined as a group of organism of the same species which live in the
same habitat at the same time where they can freely interbreed. Elephants such asthose indicated in figure 2.2 constitute a population.
Figure 2.2: Population of elephants
c. Community
In ecology, a community consists of all populations of different species living and
interacting at a certain level in the same ecosystem. Animals indicated in the figure2.3 interact and share the same ecosystem
Figure 2.3: Ecological community
d. Niche
A niche refers to the role played by a species in its ecosystem. It includes all the ways
that the species interacts with the biotic and abiotic factors of the environment.
Two important aspects of a species’ niche are the food it eats and how the food is
obtained. Birds on the figure 2.4 live in the same ecosystem, but they have different
adaptations for food. For example, the longest slender beak of the nectarivore allows
it to sip the nectar from flowers, the short study beak of the granivore allows it tocrush hard and tough grains.
Figure 2.4: Adaptations of birds’ beak for food in an ecosystem
Another aspect of a species’ niche is its habitat. The habitat is the physical environment
in which a species lives and to which it is adapted. A habitat’s features are mainly
determined by abiotic factors such as temperature and rainfall, which in turn have
an influence on the traits of the organisms that live in that habitat. A habitat is also
influenced by biotic factors as it may contain many different species. However, in the
same habitat, two different species cannot occupy the same niche in the same place
for very long. This is known as the competitive exclusion principle. If two species
were to occupy the same niche, they would compete with one another for the same
food and other environmental resources leading to the extinction of the weakerspecies.
e. Ecosystem
An ecosystem consists of a natural unit consisting of all the living organisms in an
area functioning together with all the non-living physical factors of the environment.
The concept of an ecosystem can apply to units of different sizes. For example, a
large body of fresh water could be considered an ecosystem, and so could a small
piece of dead wood. Both contain a community of species that interact with oneanother and with the abiotic components of their environment.
Figure 2.5: Example of ecosystems
They are two major classification of ecosystems: natural ecosystem and artificial
ecosystem. Natural ecosystems are those ecosystems that are capable of operating
and maintaining themselves without any major interference by man. Natural
ecosystems are furthermore classified into terrestrial ecosystems including; forest,
grassland and desert, and in Aquatic ecosystems including fresh water ecosystem
such as; ponds, lakes, rivers and marine ecosystems such as ocean, sea or estuary.
Artificial Ecosystem are those ecosystems maintained by the intervention of humans.
They are manipulated by man for different purposes including; croplands, artificiallakes and reservoirs, townships and cities.
Figure 2.6: Artificial ecosystem
f. Biomes
A biome is a broad regional type of an ecosystem characterized by distinctive climate
and soil conditions and a distinctive kind of biological community adapted to those
conditions. Biomes are of various types including terrestrial and aquatic biomes.
Terrestrial biomes consist of all the land areas on Earth where organisms live. The
distinguishing features of terrestrial biomes are determined mainly by climate.
The dominant terrestrial biomes include; tundra, temperate forests, grasslands,temperate, tropical deserts, tropical forests and grasslands (Figure 2.7).
Figure 2.7: Different types of biomes
Aquatic biomes occupy the largest part of biosphere. These are divided into two,
i.e. marine and freshwater. The marine biomes e.g. oceans which is the biggest
of the two (Figure 2.8 below) have a very high salt concentration and have fauna
adapted to them. The fresh water biomes such as lakes and rivers have a low saltconcentration of less than 1%.
Figure 2.8: An example of aquatic biome
g. Biosphere
The biosphere is the portion of Earth inhabited by life and which represents the sumof all communities and ecosystems.
Application 2.1
1. Distinguish among; individuals, populations, communities, ecosystems,
biomes and biosphere.
2. Give an example of any three aquatic and three terrestrial ecosystems
found in Rwanda
3. Use the examples above and make a brief description of an ecosystem4. Discuss the competitive exclusion principle.
2.2 Properties of an ecosystem and ecological factorsinfluencing the life of organisms
Activity 2.2
1. Go to your school garden and collect 3 living things and 3 non living
things
2. Discuss differences and similarities between collected living and nonliving
things
3. Analyze carefully the diagram below and answer the questions that
follow:
Make a classification of living things by the letters A, B, C, D, E, F and G based
on the principle of being eaten by
2.2.1 Relationships in an ecosystem
In an ecosystem, living things have feeding relationships. In terms of sources of food,
organisms are classified as; producers, consumers, or decomposers.
– Producers are organisms that can manufacture their own food. They include;
green algae , green plants and other autotrophs that are able to make their own
food through photosynthesis or chemosynthesis
– Consumers are organisms that obtain food from other organisms because they
cannot make their own food. Based on their level of feeding, consumers are
classified as primary consumers when they feed directly on plants. Primary
consumers include herbivorous or omnivorous animals. Consumers are
also classified as secondary consumers, when they feed directly on primary
consumers. Secondary consumers include carnivorous animals. Tertiary
consumers are consumers that feed directly on secondary consumers and are
top carnivorous or omnivorous animals.
– Decomposers are organisms that break down the tissues of dead organisms
into simpler substances, for example bacteria and fungi that break down dead
plants and animals into compounds of carbon and nitrogen. These compounds
are released into the soil to be used by plants and animals for growth.
In a food chain, producers such as plants produce their own energy without
consuming other life forms. They gain their energy from conducting photosynthesis
via sunlight. Consumers exist on the next level of the food chain and they are three
main types of consumers namely herbivores, carnivores and omnivores. Consumersget the energy by feeding on plants or by eating other carnivores or herbivores.
2.2.2 The ecological factors influencing the life of organisms in an
ecosystem
In an ecosystem, life is influenced by biotic and abiotic factors.a. Abiotic factors
Light: Light plays an important role in the species composition and development
of vegetation. Light is abundantly received on the surface of the earth from solar
energy and it is used by primary producers to do photosynthesis. Light intensity
shows special variations due to the factors like atmospheric water layer, particles
dispersed in the air, etc. Furthermore, the vegetation of an area may also affect the
light intensity. In deep shade under trees, or under water, light becomes limitingfactor below which photosynthesis is not sufficient for effective growth.
Temperature: Temperature is a measurement of the degree of heat. Like light,
heat is a form of energy. The radiant energy received from the sun is converted into
heat energy. Heat is measured in calories. The temperature at which physiologicalprocesses are at their maximum efficiency is called optimum temperature.
The minimum, optimum and maximum temperatures are called cardinal
temperatures. The cardinal temperature varies from species to species and in the
same individual from part to part. The distributions of plants, animals are alsoinfluenced by temperature.
Water: Water is an indispensable part of land contributing to soil productivity, and
the well beings of organisms. All physiological processes take place in the medium
of water. For example, cellular protoplasm is made up mostly of water contributingto the maintenance of cells and hence the entire living organism survives.
Rainfall: The rainfall provides water to plants and animals, and determines the
types of vegetation in a given region. For example, the evergreen forests are found
in tropical regions. Changes in rainfall influence the vegetation types in different
parts of the earth, and in turn, vegetation causes changes in the types of forests,
animals and birds. The quantity of water that a soil holds or that infiltrates into the
soil depends upon the properties of soil and type and density of vegetation covering
it. In a bare area, the rain drops beat the compact surface of the soil and loosen thesoil particles which are washed away.
Wind: Air in motion is called wind. It modifies the water relation and light conditions
of a particular region, and brings about a number of physical, anatomical and
physiological changes of plants. Such changes are breakage and uprooting of
plants, deformation, erosion and deposition of different organic particles. The wind
accelerates transpiration, removes solid moisture and at high velocities causes soil
erosion, which contributes to the removal of the surface soil, rich in organic matterand fine mineral particles.
Humidity: Humidity is greatly influenced by intensity of solar radiation, temperature,
altitude, wind, and water status of soil. Low temperature causes higher relative
humidity by decreasing the capacity of air for moisture. Processes as transpiration,absorption of water are influenced by atmospheric humidity.
Atmospheric Gases: Some principal gases like nitrogen, oxygen, carbon-dioxide,
helium, hydrogen, methane, and ozone are found in atmosphere. In addition to
these gases, there are water vapor. Industrial gases, dust, smoke particles, microorganisms
are present in the atmosphere. These gases have different influences onthe environment and hence on the living things.
Biotic Factors
The biotic factors constitute the living organisms of the environment and their
direct or indirect interactions. The population occurring together in an area interacts
with each other in several ways including predation, competition for mating and for
different natural resources including; food, water and oxygen.
b. Edaphic Factors
Edaphic factors deal with different aspects of soil, such as the structure and
composition of soil, its physical and chemical features. A galaxy of complex factor
constitutes the soil. Soil is usually defined as any part of earth’s crust in which plants
root. The soil is constituted as a result of long-term process of complex interaction
leading to the production of a mineral matrix in close contact with interstitial
organic matter both living and dead organisms. Soil is composed of; mineral matter,
soil organic matter or humus, soil water and soil solutions, and biological systemsincluding bacteria, fungi, algae, protozoans and arthropods.
Application 2.2
1. Discuss the ecological factors driving the biodiversity of Akagera National
Park.
2. Discuss the relationship between plant diversity and soil composition.2.3. Energy flow in an ecosystem
Activity 2.3Observe carefully the diagram below and answer the questions that follow.
1. Discuss how the energy flows in the above food chain of living things.
2. Indicate which living organisms above are consumers, decomposers in
the figure.
3. Discuss the role played by organism represented by the letter C.4. What would happen if A is removed from the food chain?
Energy enters in an ecosystem in the form of sunlight or chemical compounds. Some
organisms including plants and green algae use sunlight energy to make their own
food. Other organisms get energy through food by eating producers or consumersor by decomposing producers and consumers.
2.3.1 Food chains and food webs
Food chains and food webs are diagrams that represent feeding relationships. They
show who eats who. In this way, they model how energy and matter move through
ecosystems.
a. Food chains
A food chain represents a single pathway through which energy and matter flow
through an ecosystem. Food chains are generally simpler than what really happensin nature. Most organisms consume and are consumed by more than one species.
Figure 2.9: Illustration of a food chain (Source shutterstock.com)
b. Food Webs
A food web represents multiple pathways through which energy and matter flow
through an ecosystem. It includes many intersecting food chains. It demonstratesthat most organisms eat, and are eaten, by more than one species.
Figure 2.10: Illustration of the Food Web
c. Trophic levels
The feeding positions in a food chain or web are called trophic levels. The different
trophic levels are defined in the table below (Table 2.1). All food chains and food
webs have at least two or three trophic levels, the maximum being of four trophic
levels. Many consumers feed at more than one trophic levels. Humans, for example,
are primary consumers when they eat plants, secondary consumers when they eat
meat from primary consumers, and are tertiary consumers when they eat meat of
secondary consumers.
Table: 2.1. Description of producers, primary, secondary and tertiary trophiclevels
2.3.2 Ecological pyramids
Ecological pyramid is a graphical representation in the form of a pyramid showing
the feeding relationships of groups of organisms. It is often represented in a waythat the producers are at the bottom level and then proceeds through the various
trophic levels in which the highest is on top. There are 3 types of ecological
pyramids: pyramid of numbers, pyramid of biomass and pyramid of energy.Pyramid of numbers
Pyramid of numbers is a graph representing the total number of individuals present
at each trophic level. This type of pyramid can have two different forms depending
on the number of organisms: upright and inverted. In an upright pyramid of numbers,
the number of organisms generally decreases from the bottom to top. This generally
occurs in grassland and pond ecosystems where plants occupy the base of the
pyramid. An inverted pyramid of numbers, on the other hand, is just the opposite
of the upright one. It is usually observed in tree ecosystems with the trees as theproducers and the insects as consumers.
Figure 2.11: Figure 2.11: illustration of the upright pyramid of numbers
d. Pyramid of biomass
Biomass is defined as the amount of biomass per unit area product of the living
material present in an organism and the total number of organisms present in a
specific trophic level. In less complicated terms, it refers to the food available for
the succeeding trophic level. A pyramid of biomass is a depiction of the amount of
food available and how much energy is being passed on at each trophic level. Most
the biomass that animals consume is used to provide the energy, converted to newtissues, or just remain undigested.
Most of the time, pyramids of biomass are in a true pyramidal shape with biomass
in the lower trophic levels are greater than the trophic levels above them. Like the
pyramid of numbers, the pyramid of biomass can either have two forms: upright and
inverted. Usually, terrestrial ecosystems are characterized by an upright pyramid of
biomass having larger base for primary producers with the smaller trophic levels for
consumers located at the top (figure 2.17). Aquatic ecosystems are the complete
opposite as they will assume the inverted structure of the pyramid. This is because
the phytoplankton producers with generally smaller biomass are located at the base
while the consumers having larger biomass are located at the top of the pyramid(figure 2.18)
Figure 2.12: Illustration of upright pyramid of biomass(left) and the inverted pyramid of biomass(right).
In other words, the phytoplankton has a short turnover time, which means they have
a small standing crop compared to their production. The turnover time is calculatedby the following formula:
2.3.3 Pyramid of energy
The pyramid of energy shows the overall energy in the ecosystem and how much
energy is required by organisms as it flows up the higher trophic levels. This pyramid
shows that energy is transferred from lower trophic levels with more amount of energy
(producers) to higher ones (consumers) and converted in the biomass. Therefore, it
can be concluded that organisms found at the highest trophic levels of shorter food
chains bear greater amount of energy than the ones found in longer ones. Unlike
the first two ecological pyramids, the pyramid of energy is always illustrated in an
upright position, with the largest energy carriers at the base. The pyramid shows thetotal energy stored in organisms at each trophic level in an ecosystem.
Starting with primary consumers, each trophic level in the food chain has only 10
percent of the energy of the level below it (Figure 2.18). The energy available at agiven trophic level is measured in Kilojoules per square metre per year (kJm-2Y-1).
Figure 2.13: Illustration of the Pyramid of energy
2.3.4 Limitations of ecological pyramids
While the three ecological pyramids are highly specific to the aspect of ecosystem
they want to describe, all of them still tend to overlook important aspects. Some of
these limitations are the following:
– These types of pyramids only are applicable in simple food chains and not for
the food webs and they also do not consider the possible presence of the same
species at different trophic levels.
– None of the three ecological pyramids provide any idea related to variations in
seasons and climates.
– Other organisms like microorganisms and fungi are not given specific role inthe pyramids despite their vital roles in ecosystems.
Application 2.3
1. All scientists agree that the activities of living organisms play an important
role in driving biogeochemical cycles, and that organism shape their
environment to a considerable extent.
a. Explain how, herbivores affect their grassland environment.
b. What would happen if herbivores were removed from Akagera National
Park?
c. What would happen to Akagera National Park if overgrazing occurs?
2. Explain why is only small portion of the solar energy that strikes Earth’s
atmosphere stored by primary producers.
3. The diagrams A, B, C and D indicate different cases of pyramid of numbers.Using your knowledge on pyramids, analyses and interpret each diagram
4. Discuss the reasons why the transfer of energy in an ecosystem is referred to
as energy flow, not as energy cycling.
2.4 Ecological succession
Activity 2.4
In pair discuss the following:
1. What happen to a but a month after bush fire?
2. What would happen to your school basketball playground after 1, 5, 50,500 years if it was completely abandoned?
Communities are not usually static, and the numbers and types of species that live in
them generally change through time. This is called ecological succession. Important
cases of succession are primary and secondary succession.
a. Primary succession
Primary succession occurs in an area that has never been colonized such as bare
rock. This type of environment may come about when lava flows from a volcano and
hardens into rock, a glacier retreats and leaves behind bare rock or when a landslide
uncovers an area of bare rock.
The first species to colonize a disturbed area are called pioneer species including
bacteria and lichens that can live on bare rock. These species change the environment
and make the way for other species to come into the area. Along with wind and
water, they help weather the rock and form soil. Once soil begins to form, plants can
move in from pioneer species to intermediate stages and to climax communities
(Figure 2.14). At first, the plants include herbs, grasses and other species that can
grow in thin, poor soil. As more plants grow and die, organic matter is added to the
soil. Soil is improved and get the capacity to hold water. The improved soil allowsshrubs and trees to move into the area.
Figure 2.14: Primary succession
b. Secondary succession
Secondary succession occurs in a formerly inhabited area that was disturbed. The
disturbance could be a fire, flood, or human action such as farming. This type of
succession is faster because the soil is already in place. In this case, the pioneer
species are plants such as grasses, birch trees, and fireweed. Organic matter fromthe pioneer species improves the soil and lets other plants move into the area.
Figure 2.15: Secondary succession
Similarities and differences between primary and secondary succession are
summarized in the following table:Table: 2.2 Comparison between primary succession and secondary succession
Application 2.4
Differentiate between primary and secondary succession
2.5 Bioaccumulation and Bio magnification
Activity 2.5
Use the school library and search additional information on the internet.Discuss between bioaccumulation and bio magnifications
2.5.1 Bioaccumulation
Bioaccumulation refers to the accumulation of toxic chemical substances such as
pesticides, or other chemicals in the tissue of a particular organism. Bioaccumulation
occurs when an organism absorbs a substance at a rate faster than that at which thesubstance is lost by catabolism and excretion
2.5.2 Bio magnification
Bio magnification is a process by which chemical substances become more
concentrated at each trophic level. Bioaccumulors of toxic substances such as heavy
metals and polychlorinated biphenyls that slowly increases up in concentration in
living organisms including bacteria, algae, fungi, and plants.Bioaccumulants enter
a body through contaminated air, water, and/or food, and keep on accumulating
because they are very either slowly metabolized, not all metabolized, or are excretedvery slowly
2.5.3 Example of the causes of bio magnification
Some toxic chemicals were deliberately put in the environment to kill insect pests.
One of these pesticides is Dichloro Diphenyl Trichloroethane (DDT), which was
used to control mosquitoes and other insect pests. It was commonly sprayed on
plants and eventually entered water supplies. There it was absorbed by microscopic
organisms, which in turn were eaten by small fish and the small fish eaten by larger
fish from where it could have transferred to other animals, where it accumulates in
the fat tissue of animals at the top of the food chain. This food chain shows typicalconcentrations of DDT found in a food chain (in parts per million, ppm):
Another biological magnification of Polychlorinated Biphenols (PCBs) was found in
the food web of great lakes, where the concentration of PCBs in herring gull eggs, at
the top of the food web, is nearly 5,000 times that in phytoplankton at the base ofthe food web.
Figure 2.16: Biological magnification of PCBs in a Great Lakes food web.
2.5.4 Consequences of bio magnification
The first sign of the problem was a decline in the number of predator birds. Studies
showed that the eggs of these birds were easily cracked. In fact, the weight of the
mother sitting on the eggs cracked them. It was finally discovered that DDT was
building up in the tissue of the birds and interfering with the calcium needed for theshell to be hard.
Figure 2.17: Biomagnification of pesticides in food chain
2.5.5 Relationship between bioaccumulation and bio magnification
Figure 2.18: Differences and similarities between bioaccumulation and bio magnification
2.5.6 Prevention and reduction of bioaccumulation of toxic substances
The following are some of the ways to prevent and to reduce bioaccumulation of
toxic substances:
– Do not put harmful substances into water system or storm drains.
– Reduce the use of toxic chemical pesticides.
– Eat certified organic foods when possible.– Avoid fishing or spending time in contaminated areas.
Application 2.5
1. Discuss how the addition of excess nutrients to a lake threatens the
population of fishes.
2. In the face of biological magnification of toxins such as DDT, discuss thelevels of food chains where it is healthier to feed on
2.6 Efficiency of ecological production
Activity 2.6
Use the books from the school library and search further information from
the internet. Discuss the roles of efficiency of ecological production and
make a brief description of the ecosystem primary production, total primaryproduction, and net primary production.
2.6.1 Efficiency of primary production
The amount of light energy converted to chemical energy in the form of organic
compounds by autotrophs during a given period of time is called ecosystem
primary production (R). Most primary producers use light energy to synthesize
energy rich-organic molecules, which are subsequently broken down to generate
adenosine triphosphate (ATP). The total primary production in an ecosystem’s gross
production (GPP) is the amount of light energy that is converted to chemical energy
by photosynthesis per unit time.
Note that not all of this production is stored as organic material in the primary
producers because they use some of the molecules as fuel in their own cellular
respiration. The net primary production (NPP) equals the gross primary production
minus the energy used by the primary producers for respiration(R), as it is summarizedin the following formula, i.e
NPP = GPP – R.
In many ecosystems, NPP is about one-half of GPP.
To an ecologist, net primary production is the key measurement because it representsthe storage of chemical energy that will be available to consumers in the ecosyste
Figure 2.19: Illustration of the net primary productivity
2.6.2 Efficiency of secondary production
The amount of chemical energy in consumer’s food that is converted to their own
biomass during a given period of time is called the secondary production of the
ecosystem. Consider the transfer of organic matter from primary producers to
herbivores, the primary consumers. In most ecosystems, herbivores eat only a small
fraction materials produced by plants. Moreover, they cannot digest all the eaten
plant materials. Thus, much of primary production is not used for consumers. In thiscase, the secondary production is calculated by:
Net Secondary Production (NSP) = Gross Secondary Production (GSP) – Respiration(R)
Figure 2.20: Net secondary production
2.6.3 Ecological production efficiency
Production efficiency is the percentage of energy stored in assimilated food that isnot used for respiration. It is calculated as follows:
Production efficiency is expressed in percentage (%)
As an example, when a caterpillar feeds on a plant leaf, only about 33 J of out 200 J, or one-sixth
of the energy in the leaf is used for secondary production or growth. The caterpillar uses some of
the remaining energy for cellular respiration and passes the rest in faeces. The energy contained in
faeces remains in the ecosystem temporarily, but most of it is lost as heat after the faeces are
consumed by detritivores. The energy used for caterpillar’s respiration is also lost from theecosystem as heat.
Application 2.6
1. As part of a new reality show on television, a group of overweight people are
trying to safely lose in one month as much weight as possible. In addition to
eating less, what could they do to decrease their production efficiency forthe food they eat?
2. Tobacco leaves contain nicotine, a poisonous compound that is energetically
expensive for the plant to make. What advantage might the plant gain by
using some of its resources to produce nicotine?
3. If an insect eats plant seeds containing 100J of energy, energy from which 30
J is used for respiration while 50J remains in faeces.
4. a. Calculate the net secondary production.b. Estimate the production efficiency.
2.7 Biogeochemical Cycles
Activity 2.7Observe carefully the diagrams below and answer the questions that follow.
1. Name the biogeochemical cycles represented by X, Y and Z.
2. For the biogeochemical cycles denoted X, Y and Z, make a description of
steps represented by the letters A, B and C.
3. What do you understand by the term biogeochemical cycle?4. Discuss the importance of biogeochemical cycles to living things e.g. man.
A biogeochemical cycle is a closed loop through which a chemical element or
water moves through ecosystems. In the term biogeochemical, bio- refers to
biotic components and geo- to geological and other abiotic components. During
biogeochemical cycle, chemicals cycle through both biotic and abiotic components
of ecosystems. For example, an element might move from the atmosphere to the
water of the ocean, goes to ocean organisms, and then back to the atmosphere torepeat the cycle.
Elements or water may be held for various lengths of time by different components
of a biogeochemical cycle. Components that hold elements or water for a relatively
short period of time are called exchange pools. For example, the atmosphere is
an exchange pool for water. It holds water for several days. This is a very short time
compared with the thousands of years the deep ocean can hold water. The ocean
is an example of a reservoir for water. A reservoir is a component of a geochemicalcycle that hold elements or water for a relatively longer period of time.
2.7.1 Water Cycle
Earth’s water is constantly in motion. Although the water on Earth is billions of years
old, individual water molecules are always moving through the water cycle. The
water cycle describes the continuous movement of water molecules on above and
below Earth’s surface. Like other biogeochemical cycles, there is no beginning or
end to the water cycle. It just keeps repeating. During the cycle, water occurs in its
three different states: gas (water vapour), liquid (water), and solid (ice). Processes
involved in changes of state in the water cycle include; evaporation, sublimation,and transpiration.
Figure 2.21: Illustration of the water cycle
2.7.2 Carbon Cycle
Carbon is essential to all life as it is the main constituent of living organisms. It serves
as the backbone component for all organic polymers, including; carbohydrates,
proteins, and lipids. Carbon compounds such as carbon dioxide (CO2) and methane(CH4) circulate in the atmosphere and influence global climates. Carbon circulates
between living and non-living components of the ecosystem primarily through
the processes of photosynthesis and respiration. Plants and other photosynthetic
organisms obtain CO2 from their environment and use it to build biological
materials. Plants, animals, and decomposers (bacteria and fungi) return CO2 to the
atmosphere through respiration. CO2 trapped in rock or fossil fuels can be returned
to the atmosphere via volcanic eruptions, or fossil fuel combustion. The movement
of carbon through biotic components of the environment is known as the fast
carbon cycle.
Figure 2.22: The carbon cycle
2.7.3 Nitrogen Cycle
The atmosphere is the largest reservoir of nitrogen on Earth. It consists of 78%
nitrogen gas (N2). Similar to carbon, nitrogen is a necessary component of biological
molecules. Atmospheric nitrogen (N2) is converted to ammonia (NH3) by nitrogenfixing
bacteria in aquatic and soil environments. These organisms use nitrogen to
synthesize the biological molecules they need to survive. Some nitrogen-fixing
bacteria live in soil, others live in the root nodules of legumes such as; peas andbeans. In aquatic ecosystems, some cyanobacteria are nitrogen fixing.
Figure 2.23: Illustration of the nitrogen cycle (Adapted from Pearson Education, 2003)
2.7.5 The Greenhouse Effect
The greenhouse effect is a natural process that warms the Earth’s surface. When the
sun’s energy reaches the Earth’s atmosphere, some of it is reflected back to space
and the rest is absorbed and re-radiated by greenhouse gases. Greenhouse gases
include water vapor, carbon dioxide, methane, nitrous oxide, ozone and some
artificial chemicals such as chlorofluorocarbons (CFCs). The absorbed energy warms
the atmosphere and the surface of the Earth. This process maintains the Earth’s
temperature at around 330C warmer than it would otherwise be, allowing life on
Earth to exist. The problem we now face is that human activities particularly burning
fossil fuels (coal, oil and natural gas), agriculture and land clearing are increasing the
concentrations of greenhouse gases. This is the enhanced greenhouse effect, whichis contributing to the global warming.
Application 2.7The diagram below shows the carbon cycle.
Identify processes labelled ①, ② and ③.
b. Describe two ways by which carbon can be removed from the cycle for
long period of time.
c. Describe two activities of humans that are disrupting the naturalcarbon cycle.
End of unit assessment 2
Section A: Multiple choice questions
Choose the letter that best answers the question or completes the statement
1. All of life on Earth exists in a region known as
a. Ecosystem
b. Biome
c. Biosphere
d. Ecology
2. Groups of different species that live together in a defined area make up
a. Population
b. Community
c. Ecosystem
d. Biosphere
3. The series of steps in which a large fish eats a small fish that has eaten algae
is a) Food web b) Food chain c) Pyramid of numbers d) Biomass pyramid
4. The total mass of living tissue at each trophic level can be shown in
a. Energy pyramid
b. Pyramid of numbers
c. Biomass pyramid
d. Biogeochemical cycle
5. An ecosystem is not considered to be self-sustaining if
a. There is interaction between biotic and abiotic factors
b. Some of its living organisms incorporate energy into organic compounds
c. Cycling of materials occurs between organisms and their environment
d. It lacks a constant supply of energy
Section B: Questions with short answers
6. What is the meaning of the term ecology?
7. Name the different levels of organization within the biosphere, from smallest
to largest
8. How is sunlight important to most ecosystems?
9. By what process do:
a. Decomposers convert organic matter into ammonia
b. Bacteria convert gaseous nitrogen into ammonia
c. Nitrosomonas convert ammonia into nitritesd. Pseudomonas convert nitrates into gaseous nitrogen
10. Why is the transfer of energy and matter in a food chain only about 10
percent efficient?
Section C: Essay questions
11. Describe the three different types of ecological pyramids.
12. Why do the rectangles in a pyramid of energy get smaller at each higher
trophic level?
13. Discuss the reasons why the secondary succession is usually much faster
than primary succession?
14. The diagram below shows part of the nitrogen cycle
a. Name a genus of bacteria which is responsible for each of the reactions
A, B, C and D.
b. Describe the conditions in which the bacteria responsible for reaction
D will thrive.
15. The table below shows mean values for primary productivity for four
ecosystems: temperate deciduous forest, tropical forest, temperate grassland,and intensively cultivated land in a temperate region
a. Suggest two reasons to account for the higher primary productivity of
a tropical forest compared with a temperate forest.
b. Suggest explanations for the difference in primary productivity
between temperate grassland and intensively cultivated land.
c. Describe how you would estimate the fresh biomass of the producersin a grassland ecosystem.
16. The diagram shows a number of stages in an ecological succession in a lake.
a. Use information from this diagram above and explain what is meant by
an ecological succession.
b. Give two general features this succession has in common with other
ecological successions.
c. A number of small rivers normally flow into the lake. These rivers flow
through forested areas. Explain how deforestation may affect the process
of succession in the lake.
17. Use the skills learnt in classroom and give answers to the following questions:
a. What is an ecosystem?
b. What is the required information to fully describe the make-up of an
ecosystem?
c. Discuss the flow of energy through ecosystems and make a description of
the various ways in which human activity can influence the energy flow at
all levels in terrestrial ecosystems
18. As part of a science project, Abingondo Diane is trying to estimate total primary
production of plants in a prairie ecosystem for a period of one year. Once per
quarter, Abingondo cuts a plot of grass with a lawnmower, do a collection and
weighs the cuttings with the main purpose to estimate plant production. Whatis missing for Abingondo to estimate the total primary production?