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Ecosystems & Food Webs: Putting in some numbers

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

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

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

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

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

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

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

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

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%

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

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’

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

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

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

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

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

Now a more complex food chain ...

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

Now, for 100 units of plankton algal growth ... … and there are only 0.001 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

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

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

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

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

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