Food Chains, Food Webs & Energy Flow

IB Topic 5.1

Recap … Some common problems

In food chains & food webs, arrows () are used to show which organism eats which

For example:Grass grasshopper toadThe grasshopper eats the grassNOT the grass eats the grasshopper

Food webs

Food webs are interconnected food chains

Why? Since one organism often eats more

than just one type of foodA food chain does not tell the whole

story Can be very complex …

There is no set number

Some food chains have six trophic levels, but most have four

The number of levels is limited to how much energy enters the ecosystem

A lot of energy is lost at each level Determining trophic levels in a food

web can be difficult … why?

Difficulties …

Some organisms occupy more than one trophic level or take their food from multiple trophic levels

Many ecosystems are not fully understood Ditto for feeding preferences Food webs do not show change over time

as a population changes Seasons Disturbances

Energy! That’s why organisms eat, right? Producers are the most important

organisms … why? Sunlight is the initial source of energy

for all vegetation Some food chains can start without

sunlight (deep sea hydrothermal vents rely on chemicals instead of light)

Think about the food you like to eat – can you trace it back to the sun?

Energy Flow

Light is absorbed by producers (photosynthesis)

Energy is transferred from one organism to another as carbohydrates, lipids, and proteins are digested

Energy Transfer

Only chemical energy can be used by the next trophic level (remember ATP?)

Only a small amount of energy which an organism absorbs is converted into chemical energy

No organism can utilize 100% of energy present in the organic molecules of the food it eats

Only 10-20% of the energy is used

Why only 10%?

Where does the energy go? Not all of the organism is swallowed as a

food source Not all swallowed food can be

absorbed/used (bones, hair, seeds)• Lost as fecal matter; spit up

Some organisms die before being eaten Considerable heat loss due to cellular

respiration• Moving, running,

Pyramid of Energy

Used to show how much and how fast energy flows from one trophic level to the next in a community

Units are:Energy per unit area per unit time Kilojoules per square meter per yearkJ m-2 yr-1

Why the shape?

Take a look at figure 5.3 page 130 Because energy is lost, each level is

always SMALLER than the one before itOrganisms cannot create energy, they

can only transfer it Each level is smaller by ~10%

Diagrams

Energy vs. Nutrients

Energy: Enters in the form of light chemical energy most

is lost as heat Not recycled (you can’t give energy back to the

burger you ate for lunch) Nutrients:

Recycled There is not enough in the biosphere to meet all the

needs So, elements like carbon and nitrogen are recycled Locked up in cells of organisms until they die Only available by eating and decomposition

Lets hear it for the decomposers How do we get those precious

nutrients stored in the cells of plants and animals?

DecayDetritivores & saprotrophs Break down the body parts of dead

organisms Important for soil (nitrogen – key

element for plants)

Review

Which diagram shows the flow of energy through a community with three trophic levels?

A . B .

C . D .

Review Which is the best prediction about biomass?

A. The biomass of X is more than the biomass of W.B. The biomass of X is less than the biomass of Y.C.The biomass of V + X + Z is equal to the biomass of W.D.The biomass of Y is less than the biomass of Z.

Review

Which of the species feed both as secondary consumers and as tertiary consumers?

A. I, II and IV only

B. I, III and V only

C. I, IV and V only

D. III, IV and V only