Learning Objectives
 State key terms and definitions
 State that ecosystems are dynamic
 Describe how energy is transferred through
ecosystems
Starter: Card sort
Key definitions
Ecosystem –All the living organisms and non living
components in a specific habitat, and their interactions
Habitat – the place where an organism or population of
organisms lives eg a woodland, rocky shore.
Microhabitat – an area within a habitat with specific
conditions eg a rotting log.
Niche – The ‘role’ an organisms has in it’s environment
– where it lives, what it eats, where and when it feeds,
when it is active. Every species has its own unique
niche.
Key definitions
Community – All the populations of different species
that live in the same place at the same time, and who
can interact with each other.
Population - all of the organisms of one species, who
live in the same place at the same time, and can
breed together
Environment – The conditions surrounding an
organism, including both abiotic factors and biotic
factors
Key definitions
Abiotic factor – the effects of the non-living
components of an ecosystem eg temperature
Biotic factor – the effects of the living parts of the
ecosystem eg competition, predation
Autotrophs – organisms that can make their own
food.
Heterotrophs - organisms that can’t make their own
food.
Trophic level – the level at which an organisms feeds
in a food chain
Key definitions
Producers - autotrophic organisms that convert light
energy to chemical energy, which they then supply
to consumers
Consumers – living organisms that feed on other
living organisms
Decomposer – An organism that breaks down
complex organic molecules into simple inorganic
molecules that can be recycled. Also know as a
saprophyte.
Detritivore – An organism that feed on dead organic
matter.
Dynamic ecosystems
 Matter is constantly recycled within an ecosystem eg
carbon and nitrogen cycles
 Energy is not recycled it flows through ecosystems
Photosynthesis
Light energy
Respiration
Biotic
component
Nutrients
Abiotic
component
Heat energy
Simple Food Chain
Producer
First Consumer
Second Consumer
•The arrows in a food chain show the transfer of food energy from
organism to organism
•Food chains always begin with a PRODUCER. This is a green
plant which is able to make food using the energy of sunlight in
photosynthesis
•Each stem in the food chain is known as a TROPHIC LEVEL
•In this case the TOP CARNIVORE is the fox. Top Carnivores are
not eaten by anything else except decomposers (bacteria and fungi)
after they die.
Food Web
Simple food chains are not very realistic. In practice very few animals eat just one
thing. Most of them can eat several other animals or plants. A more realistic
picture is given by a FOOD WEB which is made up of interconnected food chains.
Barn Owl
Fox
Falcon
Thrush
Rabbit
Dormouse
Pigeon
Snail
Cabbage
Grain
Energy transfer in the food chain
What are pyramids of numbers?
Pyramids of numbers are a
quantitative way of
representing food chains.
They record the number of
organisms at each trophic
level in a specified area.
What are the problems of
representing food chains in
pyramids of numbers?
Pyramids of numbers only give an accurate impression of
the flow of energy in a food chain if the organisms are of
similar size. Measuring the biomass at each trophic level
can give a more accurate picture.
Numbers or biomass?
Understanding pyramids of numbers
In a pyramid of numbers, the length of each bar represents
the number of organisms at each trophic level in a specified
area.
As a single tree can support many organisms, this food
chain produces an unbalanced pyramid.
Understanding pyramids of biomass
In a pyramid of biomass, the length of each bar represents the
amount of organic matter – biomass – at each trophic level
in a specified area.
At each trophic level, the amount of biomass available is
reduced, giving a pyramid shape.
Drawbacks of pyramids of biomass
 Fresh mass is quite easy to assess, but the presence of
varying amounts of water makes it unreliable.
 The use of dry mass measurement overcomes this
problem but, because the organisms must be killed,
it is usually only made on a small sample and this
sample may not be representative.
 Different species can release different amounts of
energy per gram
Pyramids of energy
 Measures the amount of energy in the organism in
kilojoules per square metre per year (kJm-2yr-1).
 These pyramids are very difficult to measure but are
always true pyramids
Drawbacks of pyramids of biomass
 It’s a ‘snapshot in time’ - Biomass can vary with the seasons.
 Also, when the producer is a small organism which
multiplies very rapidly, the total biomass of the producers
present at any one time may be less than the total biomass
of the primary consumers.
 Over the course of a whole year, the mass of phytoplankton (plants)
must exceed that of zooplankton (animals), but at certain times of the
year this is not seen. e.g. In early spring around the British Isles,
zooplankton consume phytoplankton so rapidly that the biomass of
zooplankton is greater than that of phytoplankton.
Measuring Energy Transfer
 Pyramid of Biomass
 Bars represent dry mass
 Dry mass: evaporate all water at 80oC
 Scientists often measure wet mass and calculate
using prior data
 Pyramid of Energy
 Different species produce different amounts of
energy
 Use calorimeter to calculate heat released per gram
 Too destructive for most scientists
Plenary: Summary questions
State 2 advantages of using a pyramid of biomass rather than a
pyramid of numbers when representing quantitative information on
a food chain (2 marks).
In a pyramid of numbers: no account is taken of size (1 mark)
The pyramid of individuals of one species may be so great that it is
impossible to represent them on the same scale as other species in the food
chain (1 mark).
1.
2.
Explain how a pyramid of biomass for a marine ecosystem may
sometimes show producers (phytoplankton) with a smaller biomass
than primary consumers (zoo plankton) (1 mark).
At certain times of year (e.g. Spring) zooplankton consume phytoplankton so
rapidly that their biomass temporarily exceeds that of phytoplankton (1
mark).
3.
Name suitable units for the measurement of biomass (2 marks).
Grams per square metre (gm-2) (1 mark)
Grams per cubic metre (gm-3) (1 mark)