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Ecosystems & Food Webs:
Biomass and production
Introduction and terms
Relating production to biomass
Summary
Introduction and terms
Relating production to biomass
Summary
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
Relating production to biomass
Summary
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
Secondary
consumer predator
Primary
consumer
- herbivore
Producer
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 tonnes
120 tonnes
BIOMASS
100 tonnes
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 day
3 tonnes per day
PRODUCTION
120 kilogrammes per
day
25 tonnes per day
3 tonnes per day
PRODUCTION
120 kilogrammes per
day
Introduction and terms
Relating production to biomass
Summary
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 ….
LAND
OCEAN
Very big plants
Very small plants
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
LAND
OCEAN
Very big plants
Very small plants
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
You have seen that 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
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NOTES for USERS
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