1. ENERGY FLOW Topic 4.2 IB Biology Miss Werba
Ecosystems require a continuous supply of energy to fuel life processes and to replace energy lost as heat.
Topic 4.2
IB Biology
Miss Werba

2. SPECIES, COMMUNITIES & ECOSYSTEMS
TOPIC 4 – ECOLOGY
4.1
SPECIES, COMMUNITIES & ECOSYSTEMS
4.2
ENERGY FLOW
4.3
CARBON CYCLING
4.4
CLIMATE CHANGE

3. THINGS TO COVER U.1 U.2 U.3 U.4 U.5 U.6 U.7 Statement Guidance
Most ecosystems rely on a supply of energy from sunlight.
U.2
Light energy is converted to chemical energy in carbon compounds by photosynthesis.
U.3
Chemical energy in carbon compounds flows through food chains by means of feeding.
U.4
Energy released from carbon compounds by respiration is used in living organisms and converted to heat.
U.5
Living organisms cannot convert heat to other forms of energy.
U.6
Heat is lost from ecosystems.
U.7
Energy losses between trophic levels restrict the length of food chains and the biomass of higher trophic levels.

4. THINGS TO COVER S.1 Statement Guidance NOS 2.2
Quantitative representations of energy flow using pyramids of energy.
Pyramids of number and biomass are not required. Students should be clear that biomass in terrestrial ecosystems diminishes with energy along food chains due to loss of carbon dioxide, water and other waste products, such as urea.
Pyramids of energy should be drawn to scale and should be stepped, not triangular. The terms producer, first consumer and second consumer and so on should be used, rather than first trophic level, second trophic level and so on.
The distinction between energy flow in ecosystems and cycling of inorganic nutrients should be stressed. Students should understand that there is a continuous but variable supply of energy in the form of sunlight but that the supply of nutrients in an ecosystem is finite and limited.
NOS
2.2
Use theories to explain natural phenomena — the concept of energy flow explains the limited length of food chains.

5. FOOD CHAINS & WEBS
U.1
U.2
U.3
Sunlight is the initial energy source for most ecosystems.
Autotrophs convert light energy into chemical energy in carbon compounds by photosynthesis.
Heterotrophs consume this chemical energy in carbon compounds by feeding.

6. FOOD CHAINS & WEBS
U.3
A food chain represents the feeding relationship between species in an ecosystem.
The transfer of energy and matter are represented by arrows:
they point in the direction of energy flow
ie. to the eater!

7. FOOD CHAINS & WEBS
Trophic level: the position that an organism occupies in a food chain or web
In general:
1st TL = producer or autotroph
2nd TL = primary (1°) consumer, first-order consumer or herbivore
3rd TL = secondary (2°) consumer, second-order consumer or carnivore

8. FOOD CHAINS & WEBS
Food chains represent a linear relationship between organisms.
Food webs represent interconnected relationships between organisms.
Food webs are more realistic as there is rarely one predator to any one prey.

9. ENERGY FLOW
U.4
U.5
U.6
Only ~10% of the energy is passed from one trophic level to the next
The rest is lost!
How is it lost?
The chemical energy released by respiration is used in living organisms and converted to heat.
Living organisms cannot convert heat energy to other forms of energy.
Heat is lost from ecosystems.
Some parts (faeces, bones, etc) are also not consumed.

10. ENERGY FLOW ◼ Energy available in producers
Energy ingested
Energy available
Energy assimilated Y 
 X
◼ Energy available in producers
◼ Not all of the leaves/plant is eaten by herbivore
◼ After digesting and absorbing some of the plant material it has eaten, some will pass as faeces
◼ The herbivore moves and uses some of its consumed energy
◼ The remaining energy is used for growth. This is what will be available to the next trophic level.
J WERBA – IB BIOLOGY 10

11. Theories can be used to explain natural phenomena.
ENERGY FLOW
NOS 2.2
Theories can be used to explain natural phenomena.
The concept of energy flow explains the limited length of food chains.

12. ENERGY FLOW
U.7
Because 90% of the energy is lost between trophic levels, it has a limiting effect on the number of organisms that can exist in a food chain.
Results in competition for resources
Therefore, fewer organisms can be supported in each successive trophic level.

13. ENERGY PYRAMIDS
S.1
Energy pyramids are used to represent the amount of energy at each level of a food chain
Appear as a pyramid because higher trophic levels always have ~90% less energy than preceding levels.
The units of energy are kJ m-2 yr-1

14. ENERGY PYRAMIDS
S.1
Note: Biomass diminishes with energy along food chains in terrestrial ecosystems due to loss of carbon dioxide, water and other waste products, such as urea.

15. ENERGY PYRAMIDS
S.1
Note: There is a difference between energy flow and the cycling of inorganic nutrients in ecosystems.
There is a continuous (but variable) supply of energy in the form of sunlight but that the supply of nutrients in an ecosystem is finite and limited.
ie. energy can enter and leave an ecosystem, but nutrients have to be recycled through ecosystems

16. ENERGY PYRAMIDS Constructing a pyramid of energy:
always stepped, not triangular
producers are always placed on the lowest bar
the width of the bars must be drawn to scale and the height should be uniform
the bars must be centred
the terms producer, first-order consumer and second-order consumer and so on should be used

17. ENERGY FLOW
Q1 The data below was obtained from an Arctic tundra ecosystem on Devon Island in northern Canada: Construct a pyramid of energy for this ecosystem.
Trophic level
Energy flow (kJ m-2 y-1)
Producers
4925
Primary consumers 24
Secondary consumers 4
J WERBA – IB BIOLOGY 17

18. V = 500 kg, W = 600 kg, X = 5000 kg, Y = 50 kg, Z = 5 kg
ENERGY FLOW Q2
In an area of an ecosystem, a biologist estimated the following biomasses (V to Z represent populations of particular species):
V = 500 kg, W = 600 kg, X = 5000 kg, Y = 50 kg, Z = 5 kg
Which of the following represents a possible food chain in this ecosystem?
V ➝ W ➝ X ➝ Y
Z ➝ Y ➝ V ➝ X
Z ➝ Y ➝ X ➝ W
X ➝ V ➝ Y ➝ Z
J WERBA – IB BIOLOGY 18

19. seagrass (pop1) ➝ fish (pop2) ➝ pelicans (pop3)
ENERGY FLOW Q3
The diagram shows three populations that make up a food chain.
seagrass (pop1) ➝ fish (pop2) ➝ pelicans (pop3)
If a new predator moves in that also feeds on fish, what might happen to the numbers in the three populations?
Numbers in all three populations decline.
Numbers in population 2 decline but 1 and 3 increase.
Numbers in population 1 increase but 2 and 3 decline.
Numbers in population 2 decline but 1 and 3 stay the same.
J WERBA – IB BIOLOGY 19

20. ENERGY FLOW Q4
Identify the producers, primary consumers, and top-order consumer.
Draw a food chain for this food web that shows the seal in the position of: i. secondary consumer ii. tertiary consumer
Use an example from this food web to explain why food webs are more stable than food chains.
J WERBA – IB BIOLOGY 20

21. ENERGY FLOW Q5
Explain why the carnivores in an ecosystem tend to be fewer in number and have a smaller biomass than the herbivores in the same ecosystem. (2)
Explain why carnivores tend to be more affected by biomagnification than organisms lower down the food chain. (3)
J WERBA – IB BIOLOGY 21