Lecture #3 Ecological Pyramids and the 10% Rule

Unit 8: Ecology

Ecological pyramids• Food Chains can also be

represented by an ecological pyramid by arranging the organisms at various trophic levels diagrammatically.

• Notice that there are many more producers than consumers.

• Each trophic level consists of organisms that get their energy in the same way.

How is Energy Moved and Utilized in Ecosystems?

•Trophic levels are ‘steps’ in the transfer of energy or matter within a food web.

•The efficiency of energy transfer between different levels puts a limit on the number of trophic levels in an ecosystem.

Reminder from last time………..

The ‘Rule of 10’10% Rule (Energy / Biomass) - as an organismconsumes another organism, only about 10% ofthe energy in the food becomes part of theconsumers body.

• This 10% is all that is available to the next trophic level when one organism consumes another.

What happens to the other 90%?

Answer: In every energy transfer, 90% ofthe energy is lost to the environment asheat, due to the various metabolicprocesses of the consumer.

The 10% rule is WHY the shape of an ecological pyramid is a ‘pyramid’

Explanation- At eachtrophic level, the energystored by the organisms in a level is about 1/10th of that stored by theorganisms in thelevel below.

Let’s Review……..

What determinesthe width of eachblock in an ecological pyramid?

Answer 1.The width of each ‘block’ is determined by the amount of energy or biomass stored inorganisms at that trophic level.

2. The width of the block also shows you the relative population size of the organisms present at each level (Ex: there tends to be more zebra than lions in the savannah)

3. The width of the block can give you an indication of the biodiversity that exists at that tropic level. Example - there tends to be more types of primary consumers than Secondary consumers, etc.)

Only 10% of the Energy Present in Organisms of One Trophic Level Is Captured by Organisms of the Next

This limits the number of trophic levels.

Take 2 minutes to summarize the 10% rule and how it limits the number of trophic levels in

an ecosystem.

Ecological pyramid- a diagram that shows the relative amount of energy or matter contained within each trophic level in a given food chain or food web.

There are 3 Types

1.Energy pyramids

2.Biomass pyramids

3.Numbers pyramids

Energy Pyramids

These pyramids areused to show thetransfer of energy fromone level to another ina food chain.

• Calories are the unit of energy that is used.

Biomass pyramid A biomass pyramidshows the mass ofliving tissue (biomass)within each trophiclevel.

• Kilograms or grams are the unit of mass that is commonly used

Biomass DistributionBiomass Distribution

Numbers Pyramid

A numbers pyramidshows the totalnumber ofindividuals in anecosystem at eachfeeding level duringone year.

Exceptions to the rule…… Sometimes, the # ofindividuals in a trophic level does not reflectthe amount of energythat is present in a level.

Example: In a forest, there are more insectsfeeding on trees thanthe trees they feed on.

In that instance, ecologists would use a biomass pyramid rather than a numberspyramid to reflect the amount ofenergy present in each trophic level.

What is a vegetarian diet more common in the developing world? If you are the leader of a remote village in thedeveloping world and had 100,000 lbs of grainwith which to feed your (starving) people, wouldit make more sense to feed the grain directly toyour people, or to cattle, which you could thenfeed to your people? Assume that every personin your village weighs 100 lbs.

How many people would this support for a year?

10 people could survive for a year if the food was handled in this way

How many people would the grain support if it were fed directly to the people?

100 people could be kept alive for 1 year if the food was handled in this way!

Take 2 minutes to summarize the informationthat can be learned through studying

ecologicalpyramids, and why it is energetically your ‘best bet’ to feed from lower trophic levels.