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Ecosystems & Food Webs: Biomass and production

Introduction and terms Summary Relating production to biomass

Introduction and terms Summary Relating production to biomass

Plants are the ‘producers’ in ecosystems The amount of food for plant-eating consumers - ‘herbivores’ - is determined by... The amount of plants growing in a habitat - the ‘biomass’ The addition of new plant growth - the ‘production’

The amount of production at the top of a food web is determined by: the amount of producer biomass the rate at which it is being produced the efficiency of transfer through the food web

For an ecosystem to be stable over time - ‘sustainable’ - the removal of biomass must be balanced by new growth - production

Introduction and terms Summary Relating production to biomass

In most land ecosystems, plants are large and slow-growing, so that it takes a large biomass of plants to produce enough to sustain the population of herbivores

Similarly, it takes a large biomass of large, slow-growing herbivores to support the population of carnivores

Producer Secondary consumer - predator Primary consumer - herbivore

In the African savannah, plant growth doubles its biomass in a few months to a year In the open ocean, growth of plankton algae doubles its biomass in a few days to a week

There is high production of plant material, and efficient conversion to herbivore biomass Small herbivores also grow fast, so that in turn a small herbivore biomass is enough to sustain the predator population

Blue whale eats about 3 tonnes of krill per day

It takes about 120 tonnes of krill growing at their maximum rate to support one whale This means that the whale needs to hunt over an area of between 5 and 40 square kilometres

Algae produce about 20% growth per day, so it takes about 125 tonnes of algae to support the krill needed to support one whale The krill need to eat 25 tonnes of algae per day

For a given area of the Southern Ocean, the biomass at each level in the food chain is very similar This is not what you would expect in a land ecosystem

125 tonnes120 tonnes100 tonnes BIOMASS

However, the production of new material by each level is very different The algae produce about 20% new material per day, whilst the whale produces 0.02% growth per day

25 tonnes per day3 tonnes per day120 kilogrammes per day PRODUCTION

25 tonnes per day3 tonnes per day120 kilogrammes per day PRODUCTION

Introduction and terms Summary Relating production to biomass

These two examples show two very different ecosystems The ratio between producer-biomass and -production underpins these differences

These differences can be seen in a global comparison between land and ocean ….

30% of the planet’s surface 70% of the planet’s surface 99% of all living material Less than 1% of all living material 54% of the annual growth of new living material 46% of the annual growth of new living material Very big plants Very small plants LAND OCEAN

Large amount per unit area Very small amount per unit area Grow slowly - double biomass in months to years Grow very fast - double biomass in days to weeks ‘Classical’ pyramid of biomass Often small biomass of producers and herbivores Very big plants Very small plants LAND OCEAN

Biomasses of plants in land and ocean ecosystems differ Production by plants determines the food available for other levels in the food web You have seen that -

Production at high levels in the food web is always lower than production at low levels Biomass of producers is not always larger than that of consumers You have seen that -

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