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Ecosystems & Food Webs:
Comparing land and ocean
Land and ocean ecosystems obey the
same basic rules
However, there are some key differences
between the two
Understanding these differences
helps us to understand how
ecosystems work
Comparing land and ocean ecosystems
Examining an ocean ecosystem
Summary
Comparing land and ocean ecosystems
Examining an ocean ecosystem
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’
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
Image from the SeaWiFS sensor, courtesy ORBIMAGE
Land covers about 30% of the planet’s surface
Colours show the amount of biomass:
Blue and purple = low
Yellow and orange = high
There are also plants in
the oceans, as this
satellite image shows
This is a chain-forming diatom - a common
type of plankton alga
Most vegetation in the ocean
is microscopic plankton
Each cell is about 0.05 mm long
The average depth of the oceans is more
than 3.5 kilometres, and 84% of the
oceans are at least 2 kilometres deep
Enough light for plant growth only reaches
to about 100 metres depth
This means that rooted plants can only
grow in shallow seas - less than 5% of total
ocean area
Enough light here for growth
In the open ocean, algal
plankton grow only in the top
100 metre-thick layer
surface
100 m
500 m
1000 m
Too dark for growth
3500 m
Plankton algae can grow
rapidly
Unlike trees, they are almost
completely edible
There is a small mass of algae
per unit area
but this is relatively productive
Comparing land and ocean ecosystems
Examining an ocean ecosystem
Summary
The Southern Ocean provides an
example of a simple oceanic ecosystem
Food chains tend to consist of only a
few levels
Plankton algae:
Producers
Baleen whale:
Consumer - predator
Krill (crustacea):
Consumers - herbivores
This simple food chain is an extreme
example of an oceanic ecosystem
It has only two links from producer to
the ‘top-level’ consumer
There are huge differences in size
between the different components
We can visualise the size differences more easily if we equate
each living thing to familiar objects
Making each phytoplankton cell 1000 times bigger means that it
is about the size of a squash ball
Making the krill 1000
times bigger means
that it becomes as
long as a small bus
Making the whale
1000 times bigger
means that it is still
huge - it would stretch
across the Isle of
Wight
In a simple ecosystem such as this, plant growth is
transferred efficiently from producer to top-level
consumer because ….
Primary consumers (herbivores) can convert most of
their plant food to tissue growth and reproduction
There are few steps in the food chain, so most plant
biomass can be transferred to the top-level consumer
Comparing land and ocean ecosystems
Examining an ocean ecosystem
Summary
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
You have seen that Land ecosystems have large, slowgrowing plants
It takes a large biomass of
plants to support herbivores
You have seen that Ocean ecosystems have small,
fast-growing plants
In the oceans, a small biomass of
plants supports high herbivore
production
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NOTES for USERS
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