Unit: A Local Ecosystem
Topic 6: Trophic Interactions
Part of the Local Ecosystems Module
Biology in Focus, Preliminary Course
Glenda Childrawi and Stephanie Hollis
DOT Point
 Explain trophic interactions between organisms in an
ecosystem using food chains, food webs and pyramids of
biomass and energy.
Introduction
Ecological interactions are the
exchanges and flows of energy
and matter, and these
interactions are determined by
the ways in which organisms
obtain their food. Ecosystems
are often described in terms of
their trophic or feeding
relationships.
Autotrophs and Heterotrophs
Autotrophs: (producers) are organisms that make their own
food by converting inorganic molecules to organic compounds.
 The majority of autotrophs are green plants and algae that use
sunlight for photosynthesis. A small group of autotrophs that
do not carry out photosynthesis are chemosynthetic bacteria.
Autotrophs and Heterotrophs
Heterotrophs: (consumers) must consume other organisms in
order to gain the organic molecules they need for life.
 All other organisms are heterotrophs or consumers.
Autotrophs and Heterotrophs
There are several types of consumer organisms:
 Primary consumers —herbivores: organisms that eat plants
only (e.g. koala)
 Secondary or tertiary consumers —carnivores:
organisms that eat animals only (e.g. crocodile) —omnivores:
organisms that eat both plants and animals (e.g. ants).
Primary consumers are eaten
by secondary consumers, and
secondary consumers are eaten
by tertiary consumers.
Autotrophs and Heterotrophs
Among the heterotrophs there are also organisms that feed on
dead organisms and organic waste from different trophic
(feeding) levels. These are called degraders.
Autotrophs and Heterotrophs
Degraders include:
■ Scavengers—animals that eat dead organisms
■ Detritivores—animals that ingest organic litter or detritus
(and then digest it)
■ Decomposers—fungi and
bacteria that cause chemical
decay of organic matter and
absorb the broken-down
material.
Food Chains and Food Webs
Food chains: show the energy movement from one living thing
to another.
 They describe the feeding order in which plants or animals eat
or are eaten by other animals. A simple food chain is
comprised of:
producer → herbivore → carnivore
Food Chains and Food Webs
The trophic level of an organism is its position in a food chain,
the sequence of feeding and energy transfer through the
environment.
 Arrows indicate the direction of the flow of energy from one
organism to the other. This diagram represents examples of
food chains where the energy flows up each trophic level.
Food Chains and Food Webs
Food chains are not isolated in
ecosystems; they are more
realistically shown as a food web.
Food webs: show complex food
interactions in an ecosystem. They
are made up of two or more food
chains.
 Even food webs may not be
isolated in ecosystems. Trophic
interactions usually take place
within a particular habitat however
these sometimes overlap.
Ecological Pyramids
An ecological pyramid is graphical
representation designed to show the
biomass or biomass productivity at
each trophic level in a given ecosystem.
They begin with producers on the
bottom (such as plants) and proceed
through the various trophic levels (such
as herbivores that eat plants, then
carnivores that eat herbivores, then
carnivores that eat those carnivores,
and so on). The highest level is the top
of the food chain.
Ecological Pyramids
Pyramid of numbers: shows graphically the population of each
level in a food chain.
Ecological Pyramids
A Pyramid of biomass: shows the amount of biomass through
each level of the food chain. biomass is the total amount of living
material present at any one time.
 The amount of food at any trophic level depends, in part, on its
biomass. Typical units for a biomass pyramid could be measured
in weight.
Ecological Pyramids
At each level, energy (heat) and matter (food and wastes) are
lost (90 per cent). Only 10% is passed on to the next trophic level.
 Diagrams showing the biomass of trophic levels in an
ecosystem are more frequently pyramidal in shape than
diagrams of numbers of organisms.
Ecological Pyramids
When an ecosystem is found to be unstable (where biomass from
one level cannot support the next) then the biomass pyramid shape
moves away from the pyramidal shape. For example, if only 4 g
of eucalypt leaves has to support a 21g leaf beetle, then the
biomass pyramid is unstable; the lower trophic level must have a
larger biomass than the higher levels.
Ecological Pyramids
Pyramids of Energy:
indicates the flow of energy
(food) through each trophic
level. This is more accurate
than either numbers or
biomass. We can therefore
represent this energy flow
diagrammatically in pyramids
of energy flow. A pyramid of
energy is never inverted.
Ecological Pyramids
Some ecologists illustrate ecological pyramids as a stepped shape
(as represented in Figs 2.15, 2.16 and 2.17); however, others
prefer to simplify it further to a triangular non-stepped shape.
The following diagrams summarise the three different types of
ecological pyramids illustrated as the simplified, triangular nonstepped shape.
Activity
-Students to complete Constructing Food Chains and Food
Webs