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Ecological pyramids

Ecological pyramids are quantitative ways of representing relationships between organisms in an ecosystem, built up from bars stacked one above another.

It is standard practice to place producers at the bottom of the pyramid, with primary consumers, secondary consumers, tertiary consumers etc above them.

The width of each bar should be proportional to the quantity that is being displayed, and the height is usually the same for each bar.

It is possible to produce pyramids to describe either a food chain or a food web.

In the case of a food web, all the producers, primary consumers, secondary consumers, tertiary consumers are lumped together (into the appropriate categories!)

Food pyramids are often used in order to show the efficiency of transfer from one trophic level to the next. Just as in constructing a building, a broad and firm foundation is necessary if the upper layers are to be properly supported.

However some situations do not give a typical pyramid with a broad base, tapering towards the top, so they need to be interpreted carefully.

Why are there no arrows in an ecological pyramid?
>Because it is assumed that producers are on the bottom and the various consumers are above

Consider the simple food chain:

Grass

rabbits

foxes

This could be represented by different pyramids as follows:

Pyramid of numbers

How would data be collected in order to make this pyramid?

> Count organisms (in a particular area)

How would you ensure that the data were valid?

> Study area beforehand

> Get list of species

> Check you can identify species

Can you envisage any problems with this (or other food chains/webs)?

> Some animals are only active at night

> Definition of the area under study

> (e.g. foxes will move around more than rabbits or grass)

> Seasonal changes

> Numbers will change as rabbits are born/killed

> Time scale (counting information is just a snapshot in time)

> If you add another trophic level (foxes have fleas, so do rabbits) you may not get a pointed pyramid

Advantages to this pyramid	Disadvantages to this pyramid
> Easy to count	> Ignores sizes of organisms
> No organisms killed	> Difficult to convert grass plant leaves to numbers which are worth comparing to others

Units used

Simply numbers of organisms

Pyramid of biomass

Definition of biomass:

total mass of organisms at each trophic level - often dry mass (obtained by weighing [samples] after drying to constant mass in oven at 105 °C)

How would data be collected in order to make this pyramid?

> Catch, weigh, collate data

How would you ensure that the data were valid?

> Estimate/catch representative samples

Can you envisage any problems with this?

> Catching, weighing techniques

> Stress to organisms, disturbance to environment

> Some plants contain more water: animals don't vary so much

> Need to (kill, weigh, dry, reweigh and) calculate dry mass

Advantages to this pyramid	Disadvantages to this pyramid
> Shape always gets narrower nearer the top	> Impossible to catch/weigh all the organisms

Units used

Mass units (per area [per time]) g, kg, tonnes (per m², km² [per year])

Pyramid of energy

How would data be collected in order to make this pyramid?

> Catch sample, kill, dry ( from numbers & biomass data as above), then burn and measure heat output using calorimeter
OR Make an estimate based on someone else's data

How would you ensure that the data were valid?

> Representative samples

> Take account of long time scale (sun's energy varies during the year)

Can you envisage any problems with this?

> Many practical problems

Advantages to this pyramid	Disadvantages to this pyramid
> Shows efficiency of conversion	> Some organisms killed at some stage
	> The most involved calculation!

Units used

KJ (Kilojoules) or Kcal (older unit-Kilocalories) per m² per year

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