Ecology

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Chapter 21

• Ecology

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Objectives

• Students should be able to
• Briefly describe the non-cyclical nature of
  energy flow
• Explain the terms producer, consumer and trophic
  level in the context of food chains and food webs
  

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• Explain how energy losses occur along food
  chains, and discuss the efficiency of energy
  transfer between trophic levels
• Describe and interpret pyramids of numbers and
  biomass
• Describe how carbon is cycled within an ecosystem
  

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The Ecosystem

• An ecosystem is an ecological system formed by
  the interaction of living organisms and their
  non-living environment
• In any ecosystem, the living organisms are
  identified as producers, consumers or decomposers

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• Producers
• Mainly green plants, some algae and bacteria
• They convert light energy to chemical energy and
  store it as food during photosynthesis
• Provide food substances, energy and oxygen to
  other organisms
• Consumers
• Obtain energy from other organisms
• Herbivores feed directly on plants (primary
  consumers)
• Carnivores feed on herbivores (secondary
  consumers)

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• Decomposers
• Act on dead organisms, faeces and excretory
  products and enable the materials locked up in
  them to be returned o the physical environment to
  be used by plants
• E.g. Fungi, bacteria and earthworms

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Fig 21.13 Flow of energy and nutrients through
an ecosystem
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Food Chain and Food Web

• A series of organisms through which energy is
  transferred in material form (food) constitutes a
  food chain

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Food Chain
A food chain
Primary Consumer (herbivore)
Secondary Consumer (carnivore)
Tertiary Consumer (carnivore)
Producer (green plant)
grasshopper
spider
green plant
bird
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Food Chain Food Web
A food chain
Primary Consumer (herbivore)
Secondary Consumer (carnivore)
Tertiary Consumer (carnivore)
Producer (green plant)
A food web
caterpillar
green plant
grasshopper
spider
bird
aphid
aphid
ladybird
ladybird
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• Each stage in a food chain is known as a trophic
  level
• A predator is an animal that feeds on another
  animal called the prey
• In a community, food chains are linked up
  together to form a food web or food cycle

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Ecological Pyramids

• Pyramids of numbers
• The number of organisms at each trophic level can
  be used to construct a pyramid of numbers
• The pyramid will be broad at the base and narrow
  towards the top

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Grass ? rabbits ? snakes ? Hawks
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Pause and ponder

• In a pyramid of numbers, all the organisms at
  each trophic level are counted, regardless of
  size and stage of development
• This might cause the pyramid of numbers to be
  inaccurate

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Pyramid of biomass

• Allows us to compare the mass of organisms in
  each trophic level at a particular time
• The pyramid of biomass is constructed based on
  the dry mass of organisms in each trophic level
  at any one time.
• The pyramid of biomass has a similar shape to
  that of the pyramid of numbers

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How can we construct a pyramid of biomass?

• E.g. there are 1000 rabbits in a given area at
  one time. To determine the biomass of rabbits at
  that time
• Dry 10 rabbits in an oven at 100 C until a
  constant mass is obtained. If the dry mass is A
  gram, then 1000 rabbits is 100A gram.
• Repeat step 1 for all other trophic levels.
• Construct a pyramid of numbers using information
  obtained in steps 1 and 2.

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Variations in ecological pyramids

• A pyramid of numbers may be upside down or
  inverted if
• Organisms of one trophic level are parasitic on
  organisms of another trophic level.
• Many small organisms of one trophic level feed on
  a large organism of another trophic level. Eg.

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• 2. Pyramid of biomass for rapidly producing
  organisms are also not pyramid-shaped. This is
  because the pyramid of biomass does not take into
  account the rate of reproduction of organisms.
  E.g.
• Phytoplankton ? zooplankton ? small fish ? large
  fish

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Pyramid of biomass for food chain involving
phytoplankton
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Pyramid of Energy

• Total energy in the various trophic levels of a
  food chain being represented
• Need to determine the total energy content in
  each trophic level over a period of time.
• Have to take into consideration the rate at which
  the organisms in each trophic level reproduce
• Average energy content of each trophic level can
  then be calculated using special techniques

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Pyramid of Energy

• A lot of energy is lost to the environment as
  food is transferred from one trophic level to the
  next. Energy level lost
• As heat during respiration at every trophic level
• In uneated body parts
• Through undigested matter egested by consumers
• Through excretory products (urea)
• Heat energy is wasted energy as it cannot be
  recycled in anyway in the ecosystem
• Total energy level decreases progressively along
  the food chain

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• A pyramid of energy is always broad at the base
  and narrow towards the top.
• A large number of producers may be required to
  support just a few consumers at the end of a food
  chain

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• About 90 of energy lost when it is transferred
  from one trophic level to the next
• Greatest amount of energy lost from producer to
  primary consumer
• A shorter food chain means more energy is
  available to the final consumer

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Non-cyclic Energy Flow in the Ecosystem

• Impt pts
• Ultimate source of energy is the sun
• Flow of energy through the eco-system is
  non-cyclic in nature
• Energy lost as heat energy
• Heat energy cannot be recycled in any way

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Energy Flow and Its Relation to Nutrition and
Respiration in the Ecosystemc
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respiration
excretion
Secondary consumers (carnivores)
egestion
energy lost in uneaten body parts, faeces and
excretory products (usable energy)
feeding (holozoic nutrition)
heat lost to environment
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Primary consumers (Herbivores)
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respiration
excretion
egestion
heat lost to environment
feeding (holozoic nutrition)
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faeces and excretory products dead bodies
of organisms
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respiration
Producers (Green plants)
decomposition
photosynthesis
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CO2
CO2
Sun (light energy)
heat released to environment
1
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The Carbon Cycle
respiration
Animals
Green plants
photosynthesis
feeding
respiration
Dead bodies and faeces decomposing
death
decomposition
Bacteria and fungi act on dead animals
combustion
Coal, oil, gas, etc
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Release of carbon dioxide into the atmosphere

• Carbon dioxide is released into the environment
  in 3 ways
• Respiration
• Combustion
• decomposition

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Importance of carbon cycle

• Ensures that there is continuous supply of
  inorganic carbon dioxide for plants to carry out
  photosynthesis
• Enables a linear flow of energy through the
  ecosystem. The carbon compounds are the vehicles
  through which trapped solar energy passes from
  one organism to organism in the food chains

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Structured Question Wk Bk

• 1 (a)
• Kilogram per unit area
• (b)
• Pyramid of numbers maybe inverted if organisms of
  one trophic level are parasitic on organisms of
  another trophic level. Hence it is not an
  accurate representation of the transfer of energy

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• (c) (i) Trees
• (ii) Sparrows