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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%
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.