Lesson video

Hello and welcome to today's lesson.

I'm Mr. Jarvis, and I'm gonna be taking you through this lesson from the unit, Maintaining Biodiversity and Human Impacts.

Today's lesson is all about transfers of biomass between trophic levels.

By the end of today's lesson, you should be able to explain biomass transfer between trophic levels in food chains, and calculate the efficiency of biomass transfers.

There are five key words to today's lesson.

They are producer, trophic level, biomass, pyramids of biomass and efficiency.

You can pause the video at this point if you want some time to read through the definitions, but we will go through them as we go through the lesson.

Today's lesson is broken down into two parts.

First, we're going to look at food chains and pyramids of biomass, and then we're gonna move on to calculate the efficiency of biomass transfer.

So if you're ready, let's get started with our first section, which is all about food chains and pyramids of biomass.

Food chain diagrams and food web diagrams are used to show feeding relationships between populations of organisms. And food chains always begin with a producer.

Producers make all of the food for all of the organisms in the rest of the food chain.

So here's an example, grass, the producer, is food for a snail.

Snail is food for a thrush.

And thrush is food for a buzzard.

The grass is the producer.

The snails, thrush, and buzzard all have to consume or eat other organisms to get the food that they need and they're all called consumers.

Producers use the process of photosynthesis to make food, and that food that the producers make is used for life processes, which includes growth.

As organisms grow, they increase their biomass.

You can see some plants growing in the image on the screen.

Biomass is the mass of living material in one or more organisms. And you can see from that animation, the plants have been growing, they've got bigger leaves, and that's increased their biomass.

That's increased the mass of living material in those organisms. Here's a check, which of the following explains what the term biomass means? Is it A, the mass of living material in one or more organisms? B, the mass of water in one or more organisms? Or C, the number of organisms present in a habitat? I'll pause for a few seconds and then we'll see what the right answer is.

The answer is A, biomass is the mass of living material in one or more organisms. Well done if you got that.

Food chain diagrams show how food or biomass, the mass of living material in one or more organism, is transferred from one organism to another.

Producers are eaten by consumers.

Producers here are in this picture are grass plants and the consumer is a sheep.

The producer is the grass plant, the consumer is a sheep.

And when we have a consumer eating a producer, the consumers are called herbivores.

Consumers are eaten by other consumers.

So here we've got a leopard eating the carcass of a deer.

So the consumer here is the leopard, and that's eating another consumer, the deer.

Where we have an animal eating another animal, we call their animal a carnivore.

Decomposers break down and feed on dead organisms and their waste.

And here in the picture we've got some apples which are beginning to decompose or rot.

They're being broken down by fungi and bacteria.

Food chains show the trophic level or position in the food chain that an organism occupies.

So a trophic level is the position of an organism in a food chain diagram.

Let's look at an example.

Phytoplankton occupied trophic level one.

It's food for fish, which occupy trophic level two.

The fish is food for seals, which is trophic level three.

And seals are food for killer whales, which occupy, in this food chain diagram, trophic level four.

Trophic level one is always a producer.

It's the organism that makes its own food and it also provides all the food for the rest of the food chain diagram.

Trophic level two is always a primary consumer, and it's also always a herbivore because trophic level two eats the organism in trophic level one, which is always a producer.

And so the fish are eating phytoplankton, which make them herbivores.

They also prey of the organism in trophic level three.

Trophic level three is always a secondary consumer and it's a carnivore because it eats other animals, other consumers, and it's also a predator because it hunts and catches the fish.

And trophic level four is always our tertiary consumer.

It's also a carnivore, and it's the apex or top predator in this food chain diagram.

The apex predator is the top predator, the last predator in a food chain diagram.

Biomass is often measured as dry mass of biological material in grammes.

And the main problem with measuring biomass's dry mass is that you have to kill the organism to dry it out.

So here's an example.

We have a parsley plant.

It's mass is three grammes.

You can see all of the soil is washed off the roots, so it's just the living organism that we are measuring.

If we dry that parsley out and we can crumble it down, the mass of that dry parsley or the dry biomass is 0.

3 grammes.

Parsley is about 90% water.

Instead, we can measure the total mass of living organisms in grammes.

And this is sometimes called the wet biomass because water makes up a large part of all organisms. But this measurement is less useful.

And the reason for this is that water content of species varies and it varies species by species.

It also varies at different times of the day and as a result of environmental conditions.

Measuring wet biomass is therefore less repeatable and less reliable.

Here's a question to check your understanding of that last section.

Which of the following statements about biomass is correct? Is it A, dry biomass is a more reliable measure of biomass? B, wet biomass is a more reliable measure of biomass? Or C, to measure dry biomass, you have to kill the organism? I'll pause for a few seconds and then we'll check to see which answer's right.

The correct answers are A and C.

Dry biomass is a more reliable measure of biomass, and to measure the dry biomass you have to kill the organism.

To estimate the biomass of a population, we need to know first of all the number of organisms in that population, and then the average biomass of the organisms in that population.

We can estimate the number of organisms in a population, and the average biomass of organisms by sampling.

And sampling is by taking a few organisms and taking measurements and applying those measurements to the rest of the population.

Once we've measured or estimated the average biomass of an individual and estimated the number of organisms in a population, we can calculate the biomass of the population.

So let's look at this example.

An individual parsley plant dry biomass is not 0.

35 grammes.

The number of parsley plants in the population is 10,000.

We've estimated that by sampling.

So the dry biomass of parsley in the population is 0.

35 grammes multiplied by the population size 10,000, and that gives us 3,500 grammes, which is the same as 3.

5 kilogrammes.

Having estimated the biomass at each trophic level of a food chain, we can construct a pyramid of biomass.

The pyramids base is always the first trophic level, the producer.

The second level is the biomass of primary consumers.

The third level is the biomass of secondary consumers.

And the fourth level is the biomass of tertiary consumers.

And you can see that it makes a pyramid shape.

Look at this food chain diagram.

Grass is food for rabbit, rabbit is food for fox.

The number of organisms in each population has been estimated, and the average dry biomass of each organism has also been calculated.

Here's a table of our results.

So for grass, we've estimated the population size to be 1 million.

And the average dry biomass of each individual is 0.

2 grammes.

So the dry biomass of the population is 1 million multiplied by 0.

2 grammes, and that gives us a dry biomass of 200,000 grammes.

The rabbit population size is just 100 organisms, and the average dry biomass of a rabbit is 350 grammes.

So to estimate the dry biomass of the population, we multiply the population size, 100, by the average dry biomass of each individual 350 grammes, and that gives us 35,000 grammes.

And finally, the fox, the population size of the fox is just 10, and the average dry biomass of each individual is 2000 grammes.

To find the dry biomass of that population, we multiply by 10, the number of individuals, by the average dry biomass of the individual, and we get 20,000 grammes.

We can draw a scaled pyramid of biomass.

And so to draw a scaled diagram of a pyramid of our biomass, we need to have a scale along the bottom that shows how many grammes of dry biomass there is.

Let's plot the three levels of this food chain.

Remember we start at the bottom with our producer, which is 200,000 grammes.

Then the primary consumer, the rabbit, 35,000 grammes, and then finally the fox just 20,000 grammes.

So we've got our pyramid of biomass, and you can see the pyramid shape for this food chain diagram.

Here's a check.

The diagram shows a pyramid of biomass, which trophic level is organism Y? Is it A, a producer, B, a primary consumer, C, a secondary consumer, or D, a tertiary consumer? I'll pause for a few seconds and then we'll see what the right answer is.

The correct answer is B, it's a primary consumer.

Remember, we start at the bottom of the pyramid of biomass with the producer and then work our way along the food chain.

So as this is the second level of the pyramid of biomass, it's the primary consumer.

Well done if you got that.

Let's move to our first practise task.

I'd like you to look at the food chain diagram and the data that I'm going to give you about each of these populations in this food chain diagram.

The food chain diagram is lettuce, slug, blackbird, cat.

Here's the data.

You can see that we've got estimated population sizes at each trophic level of this food chain, and we've calculated the average dry biomass of each individual within that population.

What I'd like you to do is to calculate the dry biomass of each population in the food chain.

And secondly, to draw a scaled pyramid of biomass for the food chain.

You'll need to pause the video at this point, copy the table down and work out the dry biomass of the population and then draw your pyramid of biomass.

Once you're ready, press play and we'll check to see how well you've done, good luck.

I hope you didn't find that too tricky.

Let's go through what the answers are.

First of all, I asked you to calculate the dry biomass of each population in the food chain.

So remember, what we're going to do to calculate the dry biomass of the population is to multiply the estimated population by the average dry biomass.

So for lettuce, it's 5,000 multiplied by five grammes, and that gives us 25,000 grammes as the dry biomass of the population.

The slug population is 1000 multiplied by 2.

5, that gives us 2,500 grammes.

The blackbird is 30 multiplied by 25 grammes, which is 750 grammes, and the cat is two multiplied by 250, which gives us 500 grammes.

Well done if you calculated those correctly.

The second part of the question was to draw a scaled pyramid of biomass for the food chain.

So our first task is to draw our scaled axes, so we need at least 25,000 grammes.

And you can see I've gone 15,000 grammes either side of my main axis.

That gives us a total of 30,000 across the whole of the scale.

We then need to plot our organisms on our pyramid of biomass.

So we can see we've got 25,000 grammes to plot for lettuce and lettuce are the producers.

So the producers are always at the bottom of this pyramid of biomass.

So let's plot our lettuce plants.

There we go.

We are then going to look at the slug population, and that biomass is 2,500 grammes, so that gives us a pyramid or a shape like this.

Then our blackbird just 750 grammes.

And then finally the cat just 500 grammes.

And you can see that we've got a nice pyramid shape.

I hope that you got something that looked like that.

Well done if you did.

That brings us to the second part of the lesson today, which is all about calculating the efficiency of biomass transfer.

So if you're ready, let's move on.

Producers only convert about 1% of the light that falls on the earth into food by photosynthesis, and that's enough to provide all of the food or all of the biomass for all of the other organisms in the food chains.

Not all biomass is passed on from one trophic level to the next, and this means that the amount of biomass reduces as you pass along the food chain.

So here's an example of how some of that biomass is converted into new organic matter in an organism.

But also how some of it is wasted and not passed on to the next trophic level.

So we've got our picture of a zebra, and the zebra is a consumer.

It's feeding on a producer, some grass.

So it is consuming biomass, it's consuming grass plants.

And the biomass that it consumes is digested.

Some of that food that's digested is converted into new zebra tissue, new biomass, the organism is growing, it's getting bigger.

However, some of the biomass is wasted.

Some of it is lost through faeces, some of it is lost when it's providing energy for life processes such as movement or growth.

Some of it is lost in the urine of the organism and some of it is lost through heat, which is lost to the surroundings as a result of cellular respiration.

So you can see that some of that biomass is used for life processes and that in essence wastes some of the biomass and returns it to the surroundings.

Here's some examples of how biomass is lost.

Respiration is a process that transfers heat to the surroundings.

Lots of biomass is used by organisms to provide energy to move.

And the more you move, the more biomass is lost from the food chain because it's not being used to grow.

Egestion is the biomass that's eaten by the organism, but not then used or digested, and that's lost in the faeces.

Excretion is the biomass that's lost through waste products such as carbon dioxide and urine.

And uneaten biomass is that some of the biomass may not be actually eaten.

So for example, the zebra may chop with its teeth some of the grass plants, but may drop some of that and that then goes into decomposing into the soil and returning that biomass to the surroundings, but the biomass doesn't enter the body of the organism.

Here's a check.

Which of the following shows ways that biomass is lost from a food chain? A, increased biomass of the organism, B, excretion of waste from the organism, or C movement of the organism to catch prey.

I'll pause for a few seconds and then we'll check the right answer.

The correct answer is B and C.

Excretion of waste from the organism and movement of an organism to catch prey are both ways that biomass is lost from a food chain.

So about 10% of biomass is transferred from one organism to another as you pass along a food chain.

So 10% of the biomass passes from one trophic level to the next.

And this explains why a pyramid of biomass is a pyramid shape.

Let's look at it.

Grass, some of the biomass is lost, is food for the mouse.

The mouse has a smaller amount of biomass.

The mouse is food for a fox, but some of that biomass is lost through things like movement, hunting for prey, et cetera.

And so the biomass in the fox population is much smaller still.

The bars on the pyramid get smaller as you move up the pyramid.

Here's another check.

What must be true about the bars on a pyramid of biomass? A, the producer is always at the top of the pyramid.

B, they decrease in size as you move up the pyramid.

Or C, they increase in size as you move up the pyramid.

I'll give you a few seconds and then we'll check your answer.

The correct answer is B, the bars on a pyramid of biomass, decrease in size as you move up the pyramid.

Well done if you got that.

Efficiency is how much of something is transferred compared with how much is available.

So the efficiency of a biomass transfer is how much biomass is transferred to a consumer compared to how much biomass is available within the organism.

And it's calculated using an equation.

The efficiency of biomass transfer is equal to biomass in the higher trophic level divided by biomass in the lower trophic level multiplied by 100 because efficiency is usually given in a percentage.

So let's look at an example.

Let's look at the part of the food chain diagram we saw earlier in this lesson.

Grass is food for a mouse.

The total biomass of a grass population is 19 kilogrammes.

The total biomass of the mouse population is 1.

8 kilogrammes.

So we're gonna use our calculation to work out the efficiency of the biomass transfer.

So the efficiency is equal to the biomass in the higher trophic level, which is 1.

8 divided by the total biomass in the lower trophic level, which is 19, multiplied by 100.

And that gives us 0.

0947 multiplied by 100, and that works out as 9.

47%.

There's 9.

47% of the biomass transferred from the grass to the mouse.

Let's look at another example.

Here's our food chain diagram, oak tree, caterpillar, robin, hawk.

Here's the biomass of each of those populations.

So the biomass of the oak tree population is 25 kilogrammes.

The biomass of the caterpillar population, 2.

1 kilogrammes.

Biomass of the robin population, 0.

19 kilogrammes and the biomass of the hawk population, 0.

021 kilogrammes.

Let's work out the efficiency of the biomass transfer.

And remember, what we do is we take the top level, so the top trophic level and divide it by the bottom trophic level.

So 2.

1 divided by 25 multiplied by 100 to make it a percentage that gives us 0.

084 multiplied by 100, and that's 8.

4%.

So 8.

4% of the biomass goes from the oak tree into the caterpillar.

Let's look at the caterpillar to the robin.

Here we have 0.

19 kilogrammes, the biomass in the top trophic level, the higher trophic level, divided by 2.

1 kilogrammes, the biomass in the lower trophic level multiplied by 100, and that gives us 0.

090 by 100 or 9%.

And finally, the hawk population.

So the top level is 0.

021 and that's divided by 0.

19, the level below, multiplied by 100 to get a percentage that gives us 0.

111 by 100 or 11.

1%.

It's usually only about 10% of the biomass at each trophic level, which is passed along the food chain.

Each trophic level has less and less biomass available, and we can look at that graphically.

So here's the biomass that's available that's in the first trophic level, in our producer.

Only about 10% of that biomass is passed on to the next trophic level, our primary consumer, which takes up trophic level number two.

And you can see how much less biomass is available.

As we go to bi trophic level number three, there's only 10% of trophic level two's biomass available.

And so we've got much, much less biomass and that goes on and on and on.

So trophic level four, even less biomass.

And by the time we get to trophic level number five, the biomass available comes very, very small indeed.

And then there isn't enough biomass for organisms to survive.

And so as a result, food chain diagrams are usually limited to about five trophic levels.

Sometimes it's a few more, you sometimes get a sixth trophic level, but very often they don't even reach five trophic levels because there's not enough biomass that's available to support organisms with the food that they need to provide the energy for life processes.

Here's a check.

Why do food chain diagrams rarely have more than five trophic levels? Is it because A, all food chains have to start with a producer, B, there are fewer prey than predators or C, the amount of biomass decreases at each trophic level? I'll pause for a few seconds and then we'll check to see what the right answer is.

The correct answer is C.

Food chain diagrams rarely have more than five trophic levels because the amount of biomass decreases at each trophic level, and by the time we get to five trophic levels, there's very little biomass available, and that means there's not enough for any other organisms to survive.

So let's move to our final task of today's lesson.

The amount of biomass contained within trophic levels of a food chain as shown in the table below.

I'd like you to first of all draw a food chain diagram for the organisms in the table.

Secondly, I'd like you to calculate the percentage of biomass of the shrimp that's transferred to the large fish.

Remember, the efficiency of biomass transfer is equal to the biomass in the higher trophic level divided by the biomass in the lower trophic level, multiplied by 100.

You might need a calculator to help you with that calculation.

Draw your food chain diagram, calculate the percentage of the biomass moving from the shrimp to the large fish.

And then when you're ready and you calculated your answer, press play and we'll see how well you've done, good luck.

So I asked you, first of all, to draw a food chain diagram for the organisms in the table.

You should have drawn algae, shrimp, small fish, large fish.

Secondly, I asked you to calculate the percentage of the biomass of the shrimp that's transferred to the large fish.

So I've got you to look at two trophic levels away.

How much of the biomass in the shrimp has transferred to the fourth trophic level, the top predator or the apex predator, the large fish.

So we know the biomass in the higher trophic level, and we know the biomass in the lower trophic level.

So it's a matter of slotting in those numbers into our equation.

So the efficiency is equal to 0.

017 divided by 1.

3 multiplied by 100, and that gives us 0.

1308 multiplied by 100, which is 1.

31%.

Well done if you got that right.

That brings us to the summary of the lesson today.

We've seen that food chains always start with a producer and producers make biomass or food, which is consumed by other organisms at each trophic level in the food chain.

Remember, a trophic level is the position of an organism within a food chain.

Biomass is usually measured as dry mass, which means to remove the water.

The issue with that is that you have to kill the organism to make that measurement.

The total biomass of organisms in a food chain can be used to draw a pyramid of biomass.

Biomass reduces at each trophic level due to respiration, movement, excretion and digestion.

And the efficiency of biomass transfer can be calculated, and usually this is about 10% between each trophic level.

That limits how long food chains can be.

I hope you've enjoyed today's lesson.

It's been great, as always, having you learn along with me, and I look forward to seeing you again soon, bye-bye.