Transcript Chapter 21 Ecology

Chapter 21
Ecology
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
• 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
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
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)
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
Fig 21.13 Flow of energy and nutrients through an ecosystem
Food Chain and Food Web
•A series of organisms through which energy is
transferred in material form (food) constitutes a
food chain
Food Chain
A food chain:
Producer
(green plant)
green plant
Primary
Consumer
(herbivore)
grasshopper
Secondary
Consumer
(carnivore)
Tertiary
Consumer
(carnivore)
spider
bird
Food Chain & Food Web
A food chain:
Producer
(green plant)
Primary
Consumer
(herbivore)
Secondary
Consumer
(carnivore)
Tertiary
Consumer
(carnivore)
A food web:
caterpillar
green plant
grasshopper
spider
bird
aphid
ladybird
ladybird
• 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
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
Grass 
rabbits
 snakes  Hawks
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
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
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:
1.
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.
2.
3.
Variations in ecological pyramids
1. 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.
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
Large Fish
Small Fish
Zooplankton
Phytoplankton
Pyramid of biomass for food chain involving
phytoplankton
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
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
•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
• 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
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
Energy Flow and Its Relation to Nutrition and
Respiration in the Ecosystemc
5
excretion
Secondary
consumers
egestion
respiration
(carnivores)
energy lost in uneaten body
parts, faeces and excretory
products (usable energy)
5
4
3
feeding
(holozoic nutrition)
excretion
Primary consumers
egestion
(Herbivores)
3
faeces and excretory products
+ dead bodies of organisms
feeding
(holozoic nutrition)
heat lost to
environment
4
respiration
heat lost to
environment
4
Producers (Green plants)
respiration
decomposition
2
+ CO2
CO2
heat released to environment
photosynthesis
1
Sun (light
energy)
The Carbon Cycle
respiration
photosynthesis
respiration
Green plants
Animals
feeding
Dead bodies
and faeces
decomposing
Coal, oil, gas, etc
Bacteria
and fungi
act on
dead
animals
Release of carbon dioxide into the atmosphere
• Carbon dioxide is released into the
environment in 3 ways
• Respiration
• Combustion
• decomposition
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
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
(c) (i) Trees
(ii) Sparrows