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Pyramids of Biomass
Pyramids of Biomass
The biomass value of a trophic level depends on its position in a food chain. Using given data, pyramids of biomass can be constructed for any food chain.
Look at this garden food chain:
The buttercups are the producers (trophic level 1). Ladybirds and song thrushes are both carnivores. What trophic level are the song thrushes?
Take a look at this diagram.
Plants and algae are the producers - they produce the energy that gets passed through the food chain to the highest trophic level, in this case the song thrushes.
What do we mean when we say biomass?
So biomass is a measure of the mass of organic material that makes up either an individual, a whole population or even a whole community in an ecosystem. Which of these should we look at if we want to see how biomass is transferred between trophic levels in an ecosystem?
Measuring biomass is actually a very complicated process.
In practice, we normally use an estimate of the actual biomass of a population.
What do you minimally need to know to make the estimate?
A) The average biomass of an individual and the total population size. B) The biomass of every individual and the total population size.
Even measuring the individual's biomass is not straightforward.
An individual's mass will vary according to how much water is in their body. This needs to be ignored. The biomass needed is the dry biomass - so the mass once the samples have been dehydrated.
Once the average dry biomass per individual has been calculated, what do we need to know next to estimate the total biomass of a population?
Once the dry biomass of each trophic level has been found, it can be represented in a form of a graph.
We call it a pyramid of biomass.
What happens to the amount of biomass between each level as it moves up the food chain?
A) It increases. B) It decreases. C) It remains the same.
Biomass is lost from the food chain between each trophic level. This means that not all the biomass from one level makes it to the next level. Some is used for respiration etc. at each level.
In a healthy ecosystem the pyramid will be wide at the base and gradually get narrower. If the base was narrower than the other levels what would that suggest about that food chain?
The pyramid of biomass is a way of looking at the structure and health of a food chain, but it makes several assumptions and has some limitations on the accuracy of the data. What might some of these limitations and assumptions be?
You can select multiple answers
In the table, you can see that the ecologist collected data on the biomass of both the whole oak tree and its leaves separately. Which piece of data should be use in the lower level of the biomass pyramid?
A) The whole oak tree. B) The oak tree's leaves only.
The caterpillars only eat the leaves.
The rest of the tree's biomass doesn't matter for this food chain, so the lower level in the biomass pyramid for this food chain should only include the biomass of the oak tree's leaves.
