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Food Webs andEcological Pyramids

Every species interacts with other speciesand with its environment in a unique way.

These interactions define the ecological niche of aspecies - the role of a species within an ecosystem.

Ecological Niche

For example, the ecological niche ofa black bear is as follows:

– black bears feed on nuts andberries as well as insects andother small animals

– bears carry seeds over longdistances in their digestivesystems before they areexpelled and germinate

Ecological Niche

For example, the ecological niche ofa black bear is as follows:

– bears hibernate during thewinter

– they have few predators, butare fed on by blood-suckinginsects and other parasites

A key feature of any ecosystem is thefeeding roles of each species.

We have already distinguished betweenproducers and consumers, howeverconsumers can be further subdivideddepending on what types of organismsthey eat.

Ecological Niche

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animal that feeds on the remains ofanother organismscavenger

animal that eats both plants and animalsomnivore

animal that eats other animalscarnivore

animal that eats plants or other producersherbivore

DefinitionFeeding Role

Ecological NicheThe most common interactions betweenspecies are through feeding relationships.

The easiest way to display theserelationships is with food chains.

Food chains illustrate who eats whomin an ecosystem.

Food ChainsFood chains show how energy passesthrough an ecosystem.

Energy is continually lost from all levelsof the food chain.

Food Chains

Ecologists refer to the trophic level, orfeeding level, to describe the position ofan organism along a food chain.

Trophic Levels

Tertiaryconsumers(carnivores)

Secondaryconsumers

(carnivores oromnivores)

Primaryconsumers(herbivores)

Primary producers

1st TrophicLevel

2nd TrophicLevel

3rd TrophicLevel

4th Trophic Level

Producers occupy the lowest, or first, trophic level.Herbivores occupy the second trophic level, andcarnivores occupy the third and fourth trophic levels.

Trophic Levels

Tertiaryconsumers(carnivores)

Secondaryconsumers

(carnivores oromnivores)

Primaryconsumers(herbivores)

Primary producers

1st TrophicLevel

2nd TrophicLevel

3rd TrophicLevel

4th Trophic Level

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Food chains do not exist in nature.They are only used to show simplefeeding relationships.

Food chains are part of more complexsets of relationships that exist amongspecies.

Food Webs

A more accurate, butstill incomplete, way toillustrate interactions iswith a food web. A foodweb shows a seriesof interconnecting foodchains.

Food Webs

Food webs are highly complex, withconsumers feeding on many differentspecies.

The large number of interactions tends toreduce the vulnerability of any one speciesto the loss or decline of another species.

Food Webs

Food webs are useful tools to figure outwhat may happen when a species isremoved from or added to an ecosystem.

Food Webs

For example, if a species is removed froma food web, the species it feeds on mayincrease dramatically in numbers.

Conversely, thepopulation of a newlyintroduced speciesmay disrupt the entirefood chain.

Food Webs

For this reason, complex food webs arethought to be more stable than simplefood webs.

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Before After

Another way ecologists illustrate how ecosystemsfunction is through ecological pyramids.

Ecological Pyramids

Ecological pyramids display relationshipsbetween trophic levels in an ecosystem.

Ecological Pyramids

The three types of ecological pyramidsare energy, numbers and biomass.

Ecological Pyramids

energy pyramid pyramid of numbers pyramid of biomass

The size of each layerin the energy pyramidrepresents the amountof energy available atthat trophic level.

Energy PyramidsAn energy pyramid illustrates energy loss andtransfer between trophic levels.

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Only about 10% of the energy taken in by theindividuals at one trophic level is passed on toindividuals at the next trophic level.

Energy Pyramids

1000 kJ

100 kJ

10 kJ

Species in the highest trophic levels haveless energy available to them than speciesnear the bottom.

Energy Pyramids

This often results in theirpopulations being much smallerthan species lower in the foodchain (i.e. an ecosystem will havefewer predators than herbivores).

Biomass is the total mass of all living organisms ina given area. A pyramid of biomass shows thetotal mass of organisms in each trophic level.

Pyramid of BiomassA pyramid of numbers shows the numberof individuals of all populations in eachtrophic level.

Pyramid of Numbers

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Food Web Gizmo