1. Teacher Information! Necessary materials:
Presentation (ppt.)
Teacher Information!
Necessary materials:
PowerPoint Guide
If teacher has internet access, the following YouTube video would be a good tool at Slide 12
“Trophic Level Cascades Complete” at
**all relevant terms are bolded.
(**Unit 2 does not use Managing Our Natural Resources)

2. Ecosystem Productivity
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Ecosystem Productivity
PowerPoint adapted from:
 very good online resource that further explains primary productivity.
Principles of Ecology

3. Students will be able to…
Intro to Range--OBJ 4: PPT
Students will be able to…
Discuss trophic levels and energy flow in ecosystems

4. Intro to Range--OBJ 4: PPT
Food chains
Definitions reviewed:
Producers  autotrophs make up the base of an ecosystem
Consumers  eat other living organisms
Detrivores/decomposers  consume dead organisms and fecal wastes
Review terms from “Food Chains & Webs” PowerPoint.

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Primary productivity
The rate at which producers capture & store energy in their tissues
Gross = total
Net = after respiration
The most productive ecosystems in the world  estuaries, swamps, marshes, tropical rain forest
Temperate forests are also highly productive.

6. Net primary production per unit area of the world’s common ecosystems
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Net primary production per unit area of the world’s common ecosystems
Figure from:

7. Factors influencing primary productivity
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Factors influencing primary productivity
Climate & nutrients
Morphology & size of organism
Rainfall
Temperature
Season
Soil (mineral & nutrient availability)
Numerous factors influence primary productivity, especially climate and nutrient availability.

8. Pathways of energy flow
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Pathways of energy flow
Energy from primary productivity can flow through 2 categories of food webs
Grazing food webs
Producer  Primary consumer  Secondary consumer  tertiary consumer…
Detrital food webs
Energy flows from producers to detrivores & decomposers
Grazing food webs are what we usually think of when we hear the term food web. The producer (plant) is eaten by the primary consumer (herbivore) which is eaten by the secondary consumer (carnivore) which is eaten by the tertiary consumer (carnivore) and so on.
Detrital food webs focus on the energy flow from the producer to detrivores and decomposers—returns nutrients to soil.
These two food webs overlap when, i.e., mushrooms (detrivore) are eaten by a consumer.
How do people disrupt the flow of food chains and webs?  clearing habitat for urban/industrial/farm development, introducing (accidentally or intentionally) invasive, nonnative organisms, chemicals from industry and agriculture, plastic waste in ocean gets consumed by fish which in turn get consumed by baby seabirds, baby seabirds can’t pass or digest the plastic so they die. There are many examples of human disruption of food webs.

9. Intro to Range--OBJ 4: PPT
Trophic levels
Feeding levels with respect to primary source of energy
Producers & consumers each occupy a different trophic level
Energy is lost at each level
See next slide for diagram.

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Image from:
Note that energy is lost as heat at each trophic level. Ask students if they think there can be more than four trophic levels in an ecosystem. There can be, but this doesn’t occur often (more than five trophic levels would be even more uncommon) because of the amount of energy that is lost by the time the tertiary consumer is reached.

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Biomass
The total weight of all living organisms
Biomass at each trophic level  biomass pyramid
Biomass pyramid
(grams/m2)
809 37 11
1.5
Producers
Herbivores
Primary carnivores
Top carnivores
Detrivores/
decomposers 5
If you weighed up all the biomass at each trophic level you could plot it as a “biomass pyramid” of the ecosystem.
Keep in mind that herbivores = primary consumers, primary carnivores = secondary consumers, etc.
Again, note how energy is lost at higher trophic levels.
Measuring biomass is a way to measure the productivity of an ecosystem.

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Energy flow pyramid
The amount of energy in each trophic level can also be estimated and plotted in a pyramid
Energy flow pyramid
(kcal/m2/year)
20,810
3,368
383 21
Producers
Herbivores
Primary carnivores
Top carnivores
Detrivores/decomposers
5,060
For most ecosystems, the amount of energy and biomass decreases at higher trophic levels.
Based on the energy flow and biomass pyramids, how would you expect the number of higher-level consumers (top predators) to compare to lower trophic-level organisms (producers and primary consumers)?  There are fewer predators than herbivores, and fewer herbivores than producers. People are generally the top predator of all food webs. The world human population is over 6 billion. What implications could this have for food chains?
The number and type of primary producers supported by an ecosystem will control the type and number of consumers in the system. Alternately, top consumers in the system can have exceptional control over the structure of their ecosystem  Bottom up vs. top down control.
See YouTube video “Trophic Level Cascades Complete” at for discussion on bottom up vs. top down effects (about 5 minutes long).

13. Why do energy and biomass decrease at higher trophic levels?
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Why do energy and biomass decrease at higher trophic levels?
Not all biomass is consumed from one trophic level to next
Not all that is consumed is turned into biomass
Shorter food chain/web = less loss of energy
Supports idea that vegetarianism is the best way to feed a large population…
Not all biomass is consumed from one trophic level to the next
Not all that is consumed is turned into biomass
A. Most energy is used for body activity (energy is released as heat)
B. Some biomass is lost as inorganic nutrients (CO2)
C. Some that is consumed is not digested and is lost as undigested fecal waste
D. Result: The shorter the food chain or web, the less the loss of usable, high quality energy.  this supports the idea that man should be vegetarian in order to feed a large population. But see next slide…

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Vegetarianism
Results in a decrease of human position on food chain
This won’t solve world hunger
Only 25% of earth’s land can be farmed
We need ruminants
So if we more efficiently use energy by placing ourselves lower on the food chain (by being vegetarians), couldn’t this increase our food supply (increase energy)? It wouldn’t work. Non-farmable pastures and grasslands are important energy sources to us…
Humans can’t eat grass (or corn stalks or cottonseed hulls  byproducts of crop production) but ruminants (eg., cows) can. Ruminants convert grasses and crop residue into an energy source that we can use (meat, milk). Less than half the dry matter produced by crops is edible by humans. The remaining residues (stalks, leaves, husks) can be converted to human food only by animals like cattle.

15. The “Cellulose Dichotomy”
Cellulose  most abundant, naturally-occuring organic molecule on earth
Humans can’t digest it
Ruminants can digest it
Cattle, sheep, goats
Deer, bison, antelope, moose, elk
“Hind-gut fermentors” can digest cellulose
Horses, rabbits, some rodents
cellulose

16. Intro to Range--OBJ 4: PPT
Review
Discuss trophic levels and energy flow in ecosystems