Chapter 14: Ecosystems and Energy Flow [ Exercise,Solutions,Notes ]
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Multiple choice question
Exercise | Q 1.1 | Page 320
Which one of the following has the largest population in a food chain?
- Producers
- Primary consumers
- Secondary consumers
- Decomposers
Solution:
Decomposers
Exercise | Q 1.2 | Page 320
The second trophic level in a lake is ________________.
- Phytoplankton
- Zooplankton
- Benthos
- Fishes
Solution:
The second trophic level in a lake is Zooplankton.
Exercise | Q 1.3 | Page 320
Secondary consumers are __________.
- Herbivores
- Producers
- Carnivores
- Autotrophs
Solution:
Secondary consumers are Carnivores.
Exercise | Q 1.4 | Page 320
What is the % of photosynthetically active radiation in the incident solar radiation?
- 100%
- 50 %
- 1-5%
- 2-10%
Solution:
50 %
Exercise | Q 1.5 | Page 320
Give the term used to express a community in its final stage of succession?
- End community
- Final community
- Climax community
- Dark community
Solution:
Climax community
Exercise | Q 1.6 | Page 320
After landslide which of the following type of succession occurs?
- Primary
- Secondary
- Tertiary
- Climax
Solution:
Primary
Explanation: Landslides are considered to be an example of primary succession because the initial disturbance usually removes most of the original soil content and habitat; however, landslides can often retain remnants of the original soil.
Succession is generally regarded as primary when it occurs on a landslide where rock and subsoil, virtually free of organic matter are exposed, despite the fact that the landslide was formed the substratum just beneath a developed soil carrying vegetation.
Exercise | Q 1.7 | Page 320
Which of the following is most often a limiting factor of the primary productivity in any ecosystem.
- Carbon
- Nitrogen
- Phosphorus
- Sulphur
Solution:
Phosphorus
Very short answer question
Exercise | Q 2.1 | Page 320
Give an example of an ecosystem that shows an inverted pyramid of numbers.
Solution:
A tree ecosystem is an example of an inverted pyramid of numbers.
Exercise | Q 2.2 | Page 320
Give an example of an ecosystem that shows an inverted pyramid of biomass.
Solution:
The oceanic ecosystem is an example of an inverted pyramid of biomass.
Exercise | Q 2.3 | Page 320
Which mineral acts as a limiting factor for productivity in an aquatic ecosystem.
Solution:
Phosphorus is the mineral that acts as a limiting factor for productivity in an aquatic ecosystem.
Exercise | Q 2.4 | Page 320
Name the reservoir and sink of carbon in the carbon cycle.
Solution:
i. Reservoir of carbon in the carbon cycle in the atmosphere and ocean.
ii. Carbon which is present in rock and fossil fuels (like oil, coal, and natural gas) are the sink of the carbon cycle
Exercise | Q 3.1 | Page 320
Distinguish between the upright and inverted pyramid of biomass.
Solution:
| Upright Pyramid of Biomass | Inverted Pyramid of Biomass | |
i. | It is the type of ecological pyramid where the producers have maximum biomass and occupy a broad base and the consumers decrease in terms of biomass. | It is the type of ecological pyramid where the producers have less biomass and form a narrow base, while the consumers are more in terms of biomass. |
| E.g. | Upright pyramid of biomass in the grassland ecosystem. | The inverted pyramid of biomass in the oceanic ecosystem. |
Exercise | Q 3.2 | Page 320
Distinguish between Food chain and food web
Solution:
| Food Chain | Food Web | |
| i. | The food chain is a definite sequence of interactions between producers, consumers, and decomposers (saprophytes). | The Food web is a network of food chains that are interconnected at various levels forming an intricate web instead of a linear chain. |
| ii. | If any of the intermediate organisms are removed from the chain it affects the whole food chain. | In the food web, there is more than one alternative of food to most of the organisms; hence the removal of an organism does not affect the food web directly. |
Exercise | Q 4.1 | Page 320
Describe with examples of pyramids of number, and biomass.
Solution:
An ecological pyramid is a graphical representation of various environmental parameters such as the number of individuals present at each trophic level, the amount of energy, or the biomass present at each trophic level. Ecological pyramids represent producers at the base, while the apex represents the top-level consumers present in the ecosystem.
Exercise | Q 4.1 | Page 320
Define ecological pyramids.
Solution:
Pyramid of number: The number of individuals at each trophic level is shown in the pyramid. The pyramid of numbers (for example of a grassland) is upright. In this, there is a decrease in the number of organisms starting from primary producers (plants) to top consumers (carnivores).
Pyramid of biomass : The pyramid of biomass is a graphic representation of the amount of biomass per unit area sequence-wise in rising trophic levels with producers at the base and top carnivores at the apex. Pyramids of the biomass of a tree or. grassland ecosystem is upright and the pyramid of a pond ecosystem is inverted.
Exercise | Q 4.2 | Page 320
What is primary productivity?
Solution:
Primary productivity:
Primary productivity is the rate of generation of biomass in an ecosystem that is expressed in units of mass per unit surface (or volume) per unit time i.e. g/m2/day.
The mass unit may relate to dry matter or to the mass of carbon generated.
Exercise | Q 4.2 | Page 320
Give a brief description of the factors that affect primary productivity.
Solution:
Factors that affect primary productivity are as follows:
- It depends on the plant species inhabiting a particular area.
- It depends upon environmental factors such as light, temperature, water, precipitation, etc.
- It depends upon the availability of nutrients.
- It also depends upon the photosynthetic capacity of plants. The greater the photosynthetic activity, the higher will be the primary productivity.
Exercise | Q 4.3 | Page 320
Define decomposition.
Solution:
i. Decomposition is the process of the breakdown of complex organic matter into inorganic substances like carbon dioxide, water, and nutrients by decomposers.
ii. Raw materials for decomposition are dead remains of plants and animals, fecal matter, detritus.
iii. This process requires oxygen. Temperature and soil moisture are important factors that indirectly help soil microbes for decomposition.
iv. Warm and moist environment favors decomposition whereas, low temperature and anaerobic conditions inhibit the process.
Exercise | Q 4.3 | Page 320
Describe the processes and products of decomposition.
Solution:
The steps of decomposition are fragmentation, leaching, catabolism, humification, and mineralization.
a. Fragmentation: Detritivores like earthworms break down detritus into smaller fragments or particles.
b. Leaching: In this process, water-soluble inorganic nutrients percolate into the soil horizon and get precipitated as unavailable salts.
c. Catabolism: The bacterial and fungal enzymes degrade detritus into simpler inorganic substances. All of the above steps occur simultaneously.
d. Humification: It leads to the accumulation of a particularly decomposed, dark-coloured, amorphous, a colloidal organic substance called humus. Humus serves as a reservoir of nutrients. It is resistant to microbial action and undergoes decomposition at an extremely slow rate. Humus changes the soil texture and increases the capacity of water holding in the soil.
e. Mineralization: Some microorganisms degrade humus and release inorganic nutrients by the process of mineralization.
Exercise | Q 4.4 | Page 320
Write important features of a sedimentary cycle in an ecosystem.
Solution:
- Sedimentary cycles have their reservoirs in the Earth’s crust or rocks. Nutrient elements are found in the sediments of the Earth. Elements such as sulphur, phosphorus, potassium, and calcium have sedimentary cycles.
- Features of a sedimentary cycle in an ecosystem are as follows:
- Earth’s crust is the main reservoir of phosphorus and other minerals, such as calcium and potassium that undergo sedimentary cycles.
- The rate of release of minerals that take part in the sedimentary cycle is regulated by various environmental factors temperature, moisture, and nature of the soil.
- Sedimentary cycles are slower than the gaseous cycles therefore they take more time to complete. They take a long time to complete their circulation and are considered as less perfect cycles. This is because, during recycling, nutrient elements may get locked in the reservoir pool, thereby taking a very long time to come out and continue circulation. Thus, it usually goes out of circulation for a long time.
- Sedimentary cycles are considered as less perfect cycles as, during recycling, nutrient elements may get locked in the reservoir pool, thereby taking a very long time to come out and continue circulation
Exercise | Q 4.5 | Page 320
Describe the carbon cycle and add a note on the impact of human activities on the carbon cycle.
Solution:
i. Reservoir of carbon:
- All life forms on earth are carbon-based because carbon is the main component of all the organic compounds of protoplasm. It constitutes 49% of the dry weight of organisms.
- 71% of carbon is found dissolved in oceans. The oceanic reservoir regulates the amount of carbon dioxide in the atmosphere.
- Carbon present in the rock and fossil fuels like oil, coal, and natural gas has been away from the rest of the carbon cycle for a long time. These long-term storage places are known as the sink.
- The element carbon is a part of seawater, the atmosphere, rocks such as limestone and coal, soils, as well as all living things.
ii. Cyclic pathway of carbon:
- Carbon as CO2 moves from the atmosphere to plants during the process of photosynthesis to produce food.
- Carbon moves from plants to animals, through food chains.
- At the time of exhalation, the CO2 gas is released into the atmosphere. Thus, carbon moves from living things to the atmosphere.
- Decomposers also contribute substantially to CO2 in the atmosphere, by their processing of waste materials and dead organic matter of land and oceans.
- When fossil fuels burn to power factories, power plants, motor vehicles, most of the carbon quickly enters the atmosphere as carbon dioxide gas.
- Most of the remainder is dissolved in seawater and deposited as calcium or magnesium carbonate compounds that make up shells of marine animals.
- The additional sources for releasing CO2 in the atmosphere are the burning of wood, forest fire and combustion of organic matter, fossil fuel, and volcanic activity.
- The ocean absorbs some carbon in the form of CO2 from the atmosphere. This carbon gets dissolved in the ocean water. Some amount of the carbon which is fixed is lost to sediments and removed from circulation.
iii. The impact of human activities on the carbon cycle
- The carbon cycle is significantly influenced by human activities.
- Rapid deforestation and the massive burning of fossil fuels for energy and transport have significantly increased the rate of release of carbon dioxide into the atmosphere.
Ecosystem and energy flow solutions and notes
An ecosystem is a self regulatory and
self sustaining structural and functional unit of
nature (biosphere). It contains both biotic and
abiotic components. Biotic components interact
with each other and also with the surrounding
environment. Tansley (1935) coined the
term ecosystem.
Ecosystems vary greatly
in size from a small pond to large oceans or
small farmland to village. Entire biosphere
can be considered as one global ecosystem,
made up of many local ecosystems. Since the
earth ecosystem is too big and too complex
to be studied, it is divided into two basic
categories, viz. terrestrial and aquatic.
Forest,
grassland and desert are the types of terrestrial
ecosystems while lakes, wetlands, rivers and
estuaries are the types of aquatic ecosystems.
The ecosystems can also be classified as
Natural ecosystems and Artificial ecosystems.
Natural ecosystems do not require any human
inputs, in other words they are self-sustainable.
Artificial ecosystems e.g. a farm land, a fish
tank or even a large pond used for rearing fish,
require constant input in terms of energy or
materials
In this chapter, we will study and
analyse the structure of the ecosystem, in order
to appreciate the input (productivity), transfer
of energy (food chain/web, nutrient cycling)
and the output (degradation and energy loss).
We will also look at the relationships, chains
and webs that are created because of the energy
flows within the system.
Ecosystem :
Structure and Function :
We have already studied the various
biotic and abiotic components of the
environment. We know that all these biotic
and abiotic components influence each other.
Let us now look at these components with an
integrated approach and see how the flow of
energy takes place in ecosystem. Interaction
of biotic and abiotic components, results in
a physical structure that is characteristic for
each type of ecosystem. Identification and
enumeration of plant and animal species of an
ecosystem gives its species composition.
Biotic and abiotic components differ as the
locations vary in space and time. The variation
due to space results in spatial pattern. There are
two types of spatial patterns. viz. Stratification
and Zonation.
Vertical distribution of different species of
plants and animals occupying different levels,
is known as stratification.
For example, trees
occupy top vertical strata or layer of a forest,
shrubs the second and herbs and grasses occupy
the bottom layer. Similar stratification is also
observed in the open seas as epipelagic, mesopelagic, bathy-pelagic and benthic zones Horizontal distribution of plants and
animals on land or in water, is called zonation.
Zonation is observed in aquatic (wetlands) as
well as terrestrial ecosystems, but it is easily
seen at the junction of the two. Edges of a large
lake or beach show pronounced zonation in the
form of Inter-tidal, Littoral, Sub-littoral zones.
The biotic and abiotic components of an
ecosystem are all linked together to function as
an ‘ecosystem unit’ through various processes
like, Productivity, Decomposition, Nutrient
cycling and Energy flow. In fact, these are
functional aspects of ecosystem.
Any ecosystem must perform these
four processes for its sustainance (to be self–
sustaining).
The ecosystem understudy may
be as small as a pond or entire biosphere as
a whole. The process of productivity involves
conversion of inorganic chemicals into organic
material with the help of the radiant energy
of the sun by the autotrophs and consumption
of the autotrophs by heterotrophs. The
Decomposition is the break down of dead
organic material and mineralization of the
dead matter. The nutrient cycling is the storage
and transport of nutrients. (minerals released
in decomposition process are used again by
autotrophs). The energy flow is unidirectional
flow of energy from producers to consumers
and finally dissipation and loss as heat.
Example- Think of a small pond ecosystem.
It is fairly a self-sustainable unit that explains
the complex interactions which exist in any
aquatic ecosystem.
A pond is a shallow water
body in which all the above four aforesaid
basic processes of an ecosystem are observed.
The abiotic component is water with all the
dissolved inorganic and organic substances and
also the rich soil deposit at the bottom of the
pond. The solar input, the cycle of temperature,
day-length and other climatic conditions
regulate the rate of function of the entire pond.
The producers include the phytoplankton,
algae and other aquatic plants. The consumers
are represented by the zooplankton, aquatic
insects and fish. The decomposers are the fungi,
bacteria located at the bottom of the pond.
a. Productivity :
A constant input of solar energy is the basic
requirement for any ecosystem to function
and sustain. Productivity refers to the rate of
generation of biomass in an ecosystem. It is
expressed in units of mass per unit surface (or
volume) per unit time, for instance grams per
square metre per day (g/ m2
/ day). The mass
unit may relate to dry matter or to the mass
of carbon generated.
It can be divided into gross primary
productivity (GPP) and net primary productivity
(NPP). Gross primary productivity of
an ecosystem is the rate of production of
organic matter during photosynthesis. Plants
themselves use a considerable proportion of
this GPP for their respiration. Hence, gross
primary productivity minus respiratory losses
(R) constitute the net primary productivity
(NPP).
Net primary productivity is the
available biomass for the consumption, to
heterotrophs (herbivores, carnivores and
decomposers). The annual net primary
productivity of the whole biosphere is
approximately 170 billion tons (dry weight) of
organic matter. Of this, the productivity of the
oceans is only 55 billion tons. Rest of course, is
from land based ecosystems Primary productivity (GPP) depends on the
plant species inhabiting a particular area.
It also
depends on a variety of environmental factors,
availability of nutrients and photosynthetic
capacity of plants. Therefore, it varies in
different types of ecosystems. Secondary
productivity is defined as the rate of
formation of new organic matter by consumers.
Alternatively, it is the rate of assimilation of
food energy at the level of consumers. It is the
amount of energy available to consumer for
transfer to the next trophic level.
b. Decomposition :
Decomposers break down complex organic
matter into inorganic substances like carbon
dioxide, water and nutrients, and the process
is called decomposition. Dead remains of
plants and animals, including fecal matter,
constitute detritus, which is the raw material
for decomposition. The important steps in the
process of decomposition are fragmentation,
leaching, catabolism, humification and
mineralization.
Detritivores (e.g. earthworm) break down
detritus into smaller fragments or particles.
This process is called fragmentation.
By the process of leaching, water soluble
inorganic nutrients go down (percolate) into the
soil horizon and get precipitated as unavailable
salts. Bacterial and fungal enzymes degrade
detritus into simpler inorganic substances. This
process is called as catabolism. It is important
to note that all the above steps in decomposition
operate simultaneously on the detritus.
Humification and mineralization occur
during decomposition in the soil. Humification
leads to accumulation of partially decomposed,
a dark coloured, amorphous, colloidal organic
substance called humus that is resistant to
microbial action and undergoes decomposition
at an extremely slow rate. Humus formation
changes soil texture and increases water
holding capacity of soil.
Being colloidal in nature humus serves as
a reservoir of nutrients.
The humus is further
degraded by some microbes and release of
inorganic nutrients occurs by the process
known as mineralisation.
Decomposition as a process requires
oxygen. Temperature and soil moisture are
the most important factors that regulate
decomposition indirectly to help soil microbes.
Warm and moist environment favours
decomposition whereas low temperature and
anaerobic conditions inhibit decomposition.
14.2 Energy Flow :
Sun is the only source of energy for all
ecosystems on the earth except for the deep-sea
ecosystems. Of the total incident solar radiation,
less than 50 % of it is photosynthetically active
radiation (PAR). Plants and photosynthetic
bacteria (autotrophs) fix energy to prepare food
from simple inorganic materials. Plants capture
only 2-10 % of the PAR and this small amount
of energy sustains the entire living world.
Therefore, it is very important to know
how the solar energy captured by plants flows
through different organisms of an ecosystem.
Directly or indirectly, all organisms are
dependent for their food on producers. Hence
there is unidirectional flow of energy from
sun to producers and then to consumers. The
direction can not be reversed. Energy can be
used only once in the ecosystem.
The autotrophs need a constant supply
of energy to synthesize the molecules they
require. The autotrophs are called producers.
In a terrestrial ecosystem, major producers
are herbaceous and woody plants. Likewise,
producers in an aquatic ecosystem are
phytoplankton and algae.
called consumers (heterotrophs). If they feed
directly on the plants, they are called primary
consumers, and if the animals eat other animals
which eat plants, they are called secondary
consumers. Likewise, you could have tertiary
consumers too.
The primary consumers are also known
as herbivores. Some common herbivores are
insects (grasshopper, aphids), birds (parrot)
and some mammals (sheep, cattles, goat,
donkey) in terrestrial ecosystem and molluscs
in aquatic ecosystem. The consumers that feed
on these herbivores are carnivores, (secondary
consumers). Those animals that depend on
the primary carnivores for food are called
secondary carnivores.
You have studied several food chains
and food webs that exist in nature. Food chains
are always straight and usually have four or
five trophic levels. There are three types of food
chains viz. grazing, detritus and parasitic.
Starting from the plants (or producers) food
chains and food webs are formed such that an
animal feeds on a plant or on another animal
and in turn is food for another. The energy
trapped by the producer, is either passed on to
a consumer or remains trapped till the producer
organism dies. Death of organism is the
beginning of the detritus food chain/web.
The detritus food chain (DFC) begins
with dead organic matter. It is composed
of decomposers which are heterotrophic
organisms, mainly fungi and bacteria. They
meet their energy and nutrient requirements
by degrading the detritus. These are known as
saprotrophs. Decomposers secrete enzymes
that breakdown dead organic materials
into simple, inorganic materials, which are
absorbed by them. Detritus food chain may be
connected with the grazing food chain at some
levels. In a natural ecosystem, some animals
like cockroaches, crows, bears, man, etc. are
omnivores. Omnivores eat producers as well
as consumers. These natural interconnection of
food chains make it a food web.
ade depend on the condition of the soil,
availability of water, the environment as also
the seeds or other propagules present. Since
soil is already there, the rate of succession is
much faster and hence, climax is also reached
more quickly. Figure 14.14 shows the sequence
of stages 1 to 8 in a forest ecosystem after fire.
It is important to understand that succession,
particularly primary succession, is a very slow
process, maybe requiring thousands of years
for the climax to be reached.
Another important fact to understand is
that all the successions whether taking place in
water or on land, proceed to a similar climax
community – the mesic.
Ecosystem Services :
Healthy ecosystems are the base for a wide
range of economic, environmental and aesthetic
goods and services. The products of ecosystem
processes are named as ecosystem services,
for example, healthy forest ecosystems purify
air and water, mitigate droughts and floods.
The Millennium Ecosystem Assessment
report 2005 defines Ecosystem services as
benefits people obtain from ecosystems and
identifies four categories of ecosystem services
as follows.
• Supporting services include services such
as nutrient cycling, primary production, soil
formation, habitat provision and pollination
maintaining balance of ecosystem.
• Provisioning services include food
(including seafood), raw materials
(including timber, skins, fuel wood), genetic
resources (including crop improvement
genes, and health care), water, medicinal
resources (including test and assay
organisms) and ornamental resources (furs,
feathers, ivory, orchids, butterflies, etc.)
• Regulating services include Carbon
sequestration, Predation regulates prey
populations, Waste decomposition and
detoxification, Purification of water and air,
and pest control.
• Cultural services include cultural, spiritual
and historical, recreational experiences,
science and education, and Therapeutics
(including animal assisted therapy)
Following are the main ecological
services :
Fixation of atmospheric CO2 and release
of O2
are the most important services provided
by an ecosystem. Photosynthetic activity of
photoautotrophs sequesters carbon (in CO2
form) from the atmosphere and releases O2 as a
byproduct. O2
not only purifies air but is also
used for respiration by all aerobes.
Pollination of plants brought about by
wind, water or other biotic agencies, is also an
important ecosystem service, without which
there would be no crops and no fruits.
Though the value of all such services of
biodiversity is difficult to determine, it seems
reasonable to think that biodiversity should
carry a hefty price tag.
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