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The slide show was designed and produced for the NMA by STEP, the
Science Training & Education Partnership

2. Ecosystems & Food Webs: Putting in some numbers

3. Drawing a food web describes the structure of an ecosystem, but does not explain fully how it works

4. Managing resource species, such as fisheries or timber
We need a more complete understanding of ecosystems for a number of reasons. Examples include …..
Managing resource species, such as fisheries or timber
Understanding food webs includes quantifying biomass and production
Understanding the role of biological production in climate change

5. What happens within a food-web level
Losses along a food chain
Summary

6. What happens within a food-web level
Losses along a food chain
Summary

7. Not all food eaten by an animal is turned into new body tissue - ‘growth’- or is devoted to reproduction
A good reminder is to think of the amount of food in the ‘weekly shop’. Even children who are growing fast do not accumulate much food as growth - most is converted to energy

8. In fact, for many animals, very little food contributes to growth
Some is indigestible, and is lost as faeces
Much food is digested, but then burnt up to provide energy

9. An example: krill feeding on plankton algae
This leaves 10 milligrammes of food that can be used for growth and reproduction
A 1-gramme krill needs about 5% of its body mass per day as an energy supply, so it ‘burns up’ 50 milligrammes of food
70 milligrammes of algae eaten per day
These are real numbers. Krill can grow very fast by comparison with land herbivores - up to 3% of body mass per day. But they also use a lot of energy swimming, so a large amount of their food is devoted to respiration
10 milligrammes - about 15% - is indigestible and lost as faeces

10. 10 milligrammes to growth
= 15% of food eaten
70 milligrammes
‘in’
50 milligrammes
plus
10 milligrammes
‘to waste’

11. A growth rate that is 15% of the food eaten is high
The final conversion efficiency is high compared with the benchmark value of 10% quoted in most textbooks
To achieve this, the krill is adding 1% of its body mass per day

12. The ratio is less for animals which use a lot of energy to stay alive
More typically, the ratio between the amount of food eaten and the amount of growth is around 10%
The ratio is less for animals which use a lot of energy to stay alive
Warm-blooded animals have to devote more food to energy generation in order to maintain a constant body temperature
Warm-blooded animals such as mammals typically have ratios less than 5%

13. What happens within a food-web level
Losses along a food chain
Summary

14. The loss of food material (to provide energy and as indigestible faeces) means that only part of the growth of plants can be eaten by secondary consumers - ‘predators’

16. As the food web becomes more complicated, with longer food chains, less plant production reaches the top-level predators

17. The animated sequence build various food chains through the Southern Ocean food web

18. We can contrast two food chains in the Southern Ocean food web

19. Blue whale converts just 4% of krill to growth
Krill converts 15% of algal food to growth

20. For 100 units of plankton algal growth ...
… there are 15 units of new krill growth ...
These losses represent the conversion of food to energy (and loss of indigestible material)
… and there are only 0.6 units of new whale growth

21. Now a more complex food chain ...

22. Seal converts 4% of food to growth
Squid converts 10% of food to growth
Krill converts 15% of copepod food to growth
Fish converts 10% of food to growth
Copepod converts 15% of algal food to growth

23. Now, for 100 units of plankton algal growth ...
… and there are only units of new seal growth
There are more links in this food chain, so more losses
99.999% of the algae eaten by the copepods has been lost

24. Long food chains are inefficient
Short food chains give an efficient transfer between producers and top-level consumers
Long food chains are inefficient

25. What happens within a food-web level
Losses along a food chain
Summary

26. You have seen that -
Much of the food eaten by animals is either converted to energy or lost as faeces
Only about 10% of food is converted to growth

27. You have seen that -
Because of the low conversion efficiency, much plant production is lost within a food web
Long food chains are less efficient than short ones

29. Science Training & Education Partnership NOTES for USERS
The material in this slide show is designed to support the teaching of science at Key Stages 3-4
A full description of the slide show, and linked activities for students, can be found on the National Marine Aquarium (NMA) web-site:
Teachers are free to amend the slide show in whatever way they feel fit, or to use slides in other contexts. However, please note that neither the NMA nor the designers will accept responsibility for modifications, and original material remains copyright of the NMA
Individual images used in the slides are copyright of NMA or STEP,
except where acknowledged separately
The slides have been set up to display as A4 landscape format. If they are incorporated into other slide sequences with different display settings, change in aspect ratio and text location will occur
The slide sequence contains the minimum of effects and transitions. However, there are some automated animations, and teachers will wish to make sure that they are familiar with the sequence before use in class
Use the PowerPoint notes viewer to obtain additional information for some slides
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