1. Food Chains, Food Webs & Energy Flow
IB Topic 5.1

2. Recap … Some common problems
In food chains & food webs, arrows () are used to show which organism eats which
For example:
Grass  grasshopper  toad
The grasshopper eats the grass
NOT the grass eats the grasshopper

3. Food webs Food webs are interconnected food chains Why?
Since one organism often eats more than just one type of food
A food chain does not tell the whole story
Can be very complex …

4. There is no set number
Some food chains have six trophic levels, but most have four
The number of levels is limited to how much energy enters the ecosystem
A lot of energy is lost at each level
Determining trophic levels in a food web can be difficult … why?

6. Difficulties …
Some organisms occupy more than one trophic level or take their food from multiple trophic levels
Many ecosystems are not fully understood
Ditto for feeding preferences
Food webs do not show change over time as a population changes
Seasons
Disturbances

7. Energy! That’s why organisms eat, right?
Producers are the most important organisms … why?
Sunlight is the initial source of energy for all vegetation
Some food chains can start without sunlight (deep sea hydrothermal vents rely on chemicals instead of light)
Think about the food you like to eat – can you trace it back to the sun?

8. Energy Flow Light is absorbed by producers (photosynthesis)
Energy is transferred from one organism to another as carbohydrates, lipids, and proteins are digested

9. Energy Transfer
Only chemical energy can be used by the next trophic level (remember ATP?)
Only a small amount of energy which an organism absorbs is converted into chemical energy
No organism can utilize 100% of energy present in the organic molecules of the food it eats
Only 10-20% of the energy is used

10. Why only 10%? Where does the energy go?
Not all of the organism is swallowed as a food source
Not all swallowed food can be absorbed/used (bones, hair, seeds)
Lost as fecal matter; spit up
Some organisms die before being eaten
Considerable heat loss due to cellular respiration
Moving, running,

11. Pyramid of Energy
Used to show how much and how fast energy flows from one trophic level to the next in a community
Units are:
Energy per unit area per unit time
Kilojoules per square meter per year
kJ m-2 yr-1

12. Why the shape? Take a look at figure 5.3 page 130
Because energy is lost, each level is always SMALLER than the one before it
Organisms cannot create energy, they can only transfer it
Each level is smaller by ~10%

13. Diagrams

14. Energy vs. Nutrients Energy: Nutrients:
Enters in the form of light  chemical energy  most is lost as heat
Not recycled (you can’t give energy back to the burger you ate for lunch)
Nutrients:
Recycled
There is not enough in the biosphere to meet all the needs
So, elements like carbon and nitrogen are recycled
Locked up in cells of organisms until they die
Only available by eating and decomposition

15. Lets hear it for the decomposers
How do we get those precious nutrients stored in the cells of plants and animals?
Decay
Detritivores & saprotrophs
Break down the body parts of dead organisms
Important for soil (nitrogen – key element for plants)

16. Review
Which diagram shows the flow of energy through a community with three trophic levels?

17. Review Which is the best prediction about biomass?
A. The biomass of X is more than the biomass of W.
B. The biomass of X is less than the biomass of Y.
C. The biomass of V + X + Z is equal to the biomass of W.
D. The biomass of Y is less than the biomass of Z.

18. Review
Which of the species feed both as secondary consumers and as tertiary consumers?
A. I, II and IV only
B. I, III and V only
C. I, IV and V only
D. III, IV and V only