Food chains, food webs and
decomposers
Pages 74-75
Food chains
Food chains are simple models the biologists use to show the feeding relationships between the
organisms in a habitat.
The arrows in food chains show the way in which biomass energy is moving. They do not show what eats
what.
* Biomass: biological material
Producer and consumer
1
2
A food chain always starts with a producer,
an organism that makes food. This is
usually a green plant, because plants can
trap energy from sunlight to make their
own food by photosynthesis.
A food chain ends with a consumer, an
animal that eats a plant or another
animal.
Prey and Predator
A predator is an animal that hunts and
eats other animals, and the prey is the
animal that gets eaten by the predator.
In this Food chain, the acacia tree is the producer, the
giraffe is the primary consumer and the lion is the
secondary consumer. The lion is also a predator (hunter)
and the giraffe is an example of prey (hunted).
Organism
How it gets its energy
Consumer
Animal that feeds on other organisms
Primary consumer
Eating plants
Secondary consumer
Eating primary consumers
Tertiary consumer
Eating secondary consumers
Top Carnivore
The final carnivore in a food chain
Herbivore
Eating plants
Carnivore
Eating other animals
Omnivore
Eating both plants and animals
Decomposer
Feeding on dead and decaying organisms, and on the undigested parts of plant and animal matter in faeces.
Trophic level
The different stages in the feeding
sequence are also referred to as trophic
levels. Producers are trophic level 1;
primary consumers are trophic level 2 and
so on.
The trophic level of an organism is the
position it occupies in a food web.
Biomass transfers
The arrows in a food chain show the
transfer of biomass energy from one
organism to another. An example of a
food chain is:
Acacia Tree → giraffe → lion
Some of the energy from the Sun
absorbed by Acacia Tree when
it photosynthesizes is transferred to the
giraffe when it eats the plant. So biomass
is transferred. Then some of the biomass
in the giraffe is transferred to the lion
when it is eaten and so on.
Food chains are rarely longer than six trophic levels
because of energy loss.
Food chains may have two, three, four or even five links.
Not all of the biomass is passed from the maize plants to the
chicken. In fact, only about ten per cent of the biomass is
transferred from each trophic level to the next. The remaining
90 per cent is used by the trophic level to complete life
processes. Biomass can be lost between stages because not all
of the matter eaten by an organism is digested (skeleton and
fur left behind). Some of it is excreted as waste such as
solid faeces, carbon dioxide and water in respiration and water
and urea in urine.
Because only around 10% of the biomass at each trophic level
is passed to the next, the total amount becomes very small
after only a few levels. So food chains are rarely longer than six
trophic levels.
Leaf
beetle larva
oriole
From chains to webs
Food chains are very simple models.
Most animals eat more than one kind
of organism.
A food web is another way of
modelling feeding relationships which
links several food chains together. Even
though food webs are still simple
models, they are more realistic than
food chains.
The circle of life
Plants tale mineral salts from the soil through their roots.
If these things continued, eventually all life would end.
There would be no mineral salts and other nutrients in the
soil, so plants would not grow. Without photosynthesis all
the food chains and webs of life would fail.
The good news is the decomposers. They enable the
bodies of organisms to be recycled,
Decomposers break down plant and animal material and
return nutrients to the soil. These nutrients are taken up
from the soil and used to help build new plants, the
starting point of a new food chain.
Thank You
Presented by Mrs. Dima Dayeh