Food Chains (Biology GCSE)

Uploaded by freeeschool on 04.09.2012

GCSE Biology – Food Chains
Hello! Welcome to a video that’s about Pyramids of Biomass & Food Chains or Food Chains & Pyramids
of Biomass. I have written it there.
Let’s start by looking at a very simple food chain. Over here, we have some grass.
The grass is eaten by a rabbit in this food chain. These rabbits can be eaten by the fox
in this food chain. The only thing we need to complete this food chain is to get the
arrows in the diagram as well. The arrows, once I complete this food chain, it is really
important that you remember the direction of the arrows. They go in the direction in
which the food is eaten. So the grass goes into the rabbits, the rabbits go into the
fox when they eat the rabbits. The reason why that is so important is because these
arrows represent the flow of energy through this food chain.
Now, grass has energy inside of it in terms of the living tissue that it has, but where
does it get the energy to produce this living tissue? You are probably aware already that
it gets it from sunlight and the process is photosynthesis. Photosynthesis is a process
that converts light energy into chemical energy and this can be converted into proteins or
used to produce proteins, fats, and carbohydrates that the plant needs and all of these can
then be used to produce new living tissue. That living tissue has special names and the
important keyword is called “biomass”.
Biomass is the living tissue inside living things. Not just in plants, but all living
things. Now, we can represent this food chain by using what we call a pyramid of biomass.
The pyramid of biomass represents how much the mass of living tissue at each stage in
the food chain and the way we represent it is by using blocks.
Imagine here is our first block. Image that we use this block to represent the biomass
of the grass. The size of the block represents the amount of biomass. A larger block represents
more biomass and a smaller block represents less biomass. So here for the grass, we have
the biomass represented by this bar. The rabbits, in fact, will have a slightly smaller bar
because the amount of bar in the rabbits is less and we will look at the reason for that
in a moment. If we then move on to the fox, you’ll see that the amount of biomass in
the fox or all the foxes in this particular environment or food chain would be even less.
The first layer here represents the grass. The second layer is the rabbits. The last
layer is the fox or the foxes; there might be more than one fox in this food relationship.
But here, we have the pyramid of biomass.
Why is it that each level in the pyramid of biomass gets smaller and smaller? What we
got to remember is that not all the biomass that enters the rabbit from the grass is passed
on to the fox. Why is that? Partly because rabbits carry out its process called respiration
and that is used for movement, so some of the biomass is used for the process called
respiration and that is used for movement and that is lost in the environment. The rabbits
keep their body temperature at a certain level because they are warm-blooded, so their biomass
is used to convert into heat which is lost to the environment. They also produce faeces
and urine which is not actually passed on to the fox. Some of the biomass is lost there.
You can also imagine that the fox might not necessarily eat every single part of the rabbit.
It might not eat fur or stuff like that. So that is not taken into the body of the fox.
The reason for this bar getting smaller at each level is because some of that biomass
is used for various things or some of it is not passed on to the next level.
Generally, when we look at food chains, they are usually quite short. It is very unusual
to find a food chain that is more than maybe four. Tough to find one that is five levels,
but food chains are said to be very short because at each stage in the food chain, the
biomass and energy is used up or is not actually passed on. It is important that you are able
to describe how this works.
This is a simple, primitive biomass for this food chain. What you might be required to
do is to come up with and draw your own pyramid of biomass. Here’s some data that has to
do with a food chain that goes like this. It is leaves, worms, hedgehogs and foxes.
Here, we got some information: the number of individuals. That’s on the biomass and
that’s the actual number of individuals for the food chain: 2,000 leaves, 200 worms,
15 hedgehogs and 2 foxes. More importantly, the biomass is described by this column here.
So we got 200 kgs. of leaves, 20 of worms, 12 of hedgehogs and 6 of foxes.
Now, to represent this accurately, you might have a clue here. We got some graph paper
here or some grid-lined paper that can help you represent this. But we have to think of
a scale of how we are going to represent this. What we do and I said that we have to think
about this before drawing or making the video. After thinking about it, we can actually use
one block as representing 20 kg. For example, this block here is 20, then 40, 60, 80, 100,
120, 140, 160, 180 and 200. That’s 200 kgs. represented there is represented by this rectangle
here or this bar here and I’ve said that one block equals 20 kg. That means that if
we’ve got this bar representing the leaves, the worms would then have to be 20 kg., so
that’s one block. We’ve got 12 kg. which is more or less if one block is 20, each one
of these must be 20, so we need 12, that’s the bar representing the hedgehogs and 6,
so that’s 1 ? blocks, it is about there. So a slightly wonky pyramid of biomass, but
that’s what it looks like based on the data we’ve been given. The height of this actually
doesn’t matter as much as the width. That’s the important thing which is just for consistency. This layer here, the
last thing to do here is to label it. This is the leaves, this are the worms, hedgehogs
and foxes.
This is our pyramid of biomass for this particular data and what you should be able to do if
asked is explain why each of the bars gets smaller as you go up the pyramid and that
was explained in the previous slide.
Thanks for watching and see you again soon.
[end of audio – 08:15] GCSE Biology – Food Chains