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Structure & function of food
chains
We need a more complete understanding of
ecosystems for a number of reasons. Examples
include:
•Managing resource species, such as
fisheries or timber
•Understanding the role of biological production
in climate change
Introduction - land and ocean
Ocean food chains
Are there pyramids in the ocean?
Summary
Introduction - land and ocean
Ocean food chains
Are there pyramids in the ocean?
Summary
First, let’s start with a familiar ecosystem
Lion - carnivore (consumer)
Wildebeest - herbivore
(consumer)
Grass - producer
In the ocean, the food chain is built the same
way, but the plants and animals look different
FISH
COPEPOD
Carnivore
ALGAE
Herbivore
Producer
This food chain might not stop here ...
…. or it it might be surprisingly short ….
Plants are the ‘producers’ in nearly all
ecosystems
The amount of food for plant-eating consumers ‘herbivores’ - is determined by ...
The amount of plants growing in a habitat their ‘biomass’
The addition of new plant growth - their
‘production’
Plants on land
On land, large plants
are the dominant part
of most ecosystems
Much of this biomass
consists of woody
tissue
Wood is more or less
indigestible, and
persists for a long time
Because so much land plant material is difficult
to digest, herbivores need to eat very large
amounts
A four-tonne elephant will eat plants equivalent to
nearly 10% of its body weight per day
This means that you need a lot of plants to feed
one elephant
There are a few marine habitats where living
things form the framework for the ecosystem
Kelp
Coral
Image from the SeaWiFS sensor, courtesy ORBIMAGE
There are obvious differences in plant biomass on land
Plant biomass in the ocean is much lower, and also variable
Colours show the amount of biomass:
Blue and purple = low
Yellow and orange = high
Enough light here for growth
surface
100 m
500 m
1000 m
In the open ocean, algal plankton grow
only in the top 100 metre-thick layer
Too dark for growth
3500 m
Plankton algae can grow rapidly
Unlike trees, they contain little
inedible material
There is a small mass of algae per
unit area
but this is relatively productive
Land and ocean ecosystems have very different
plants at the base of the food web
We can summarise the main differences ….
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
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
Introduction - land and ocean
Ocean food chains
Are there pyramids in the ocean?
Summary
Not all food eaten by an animal is turned into
new body tissue - ‘growth’- or is devoted to
reproduction
In fact, for most 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
10 milligrammes - about
15% - is indigestible and
lost as faeces
10 milligrammes to
growth
= 15% of food eaten
50 milligrammes
70 milligrammes
‘in’
plus
10 milligrammes
‘to waste’
A growth rate that is 15% of the food eaten is high
To achieve this, the krill is adding 1% of its body mass per day
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 such as mammals typically have ratios less than 5%
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
We can contrast two food chains in the this
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 ...
… 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
99.999% of the algae eaten by the
copepods has been lost
Short food chains give an efficient transfer
between producers and top-level
consumers
Long food chains are inefficient
Introduction - land and ocean
Ocean food chains
Are there pyramids in the ocean?
Summary
This ecosystem demonstrates a clear pyramid
of biomass
Secondary
consumer predator
Primary
consumer
- herbivore
Producer
What about the open ocean?
Plankton algae:
Producers
Baleen whale:
Consumer - predator
Krill (crustacea):
Consumers - herbivores
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
to support one whale
The krill need to eat 25 tonnes of algae per
day
Biomass in this system is similar at all
three levels in the food chain
125 tonnes
120 tonnes
BIOMASS
100 tonnes
Production is a different story
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
So, a pyramid of production is real in both
ocean and land ecosystems
On the other hand, a pyramid of biomass (or
numbers) is not a useful representation of
ecosystem function in either environment
Introduction - land and ocean
Ocean food chains
Are there pyramids in the ocean?
Summary
You have seen that There are similarities and
contrasts between ocean and land
ecosystems
Relationship between plant
biomass and production is
different in the two systems
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
Difficult to digest
Easy to digest
You have seen that Very little plant production
contributes to consumer growth
Short food chains are efficient,
long food chains are inefficient
You have seen that Ocean food webs rely on the high
production rate of plankton algae
Only a pyramid of production
provides a real picture of ocean
or land ecosystems
www.justaddh2o.tv
www.national-aquarium.co.uk
NOTES for USERS
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