1. Flow of Energy Through an Ecosystem (Food chains and Food webs)
Unit 6 Notes
Flow of Energy Through an Ecosystem (Food chains and Food webs)

2. Overview
We learned about different symbiotic relationships (mutualism, commensalism, parasitism) that illustrate how organisms interact.
Another way to study the interactions of organisms within an ecosystem is to follow the energy that flows through it
Organisms differ in how they obtain energy; they are divided into two categories: autotrophs and heterotrophs

3. Autotrophs
Autotrophs are organisms that collect energy from the sunlight to produce food
Also called producers (produce their own food)
Green plants and other organisms that create their own food from abiotic sources
Organisms that contain chlorophyll (the green pigment found in plants that captures sunlight) absorb energy during photosynthesis and convert it to food (like glucose, a type of sugar)
If sunlight isn’t available (deep in the ocean, for example), organisms can use inorganic substances like carbon dioxide to make their food

4. Heterotrophs
Heterotrophs – organisms that get energy by eating another organism
They are called consumers
A primary consumer eats only plants,
a secondary consumer eats primary consumers (herbivores),
a tertiary consumer eats secondary consumers (carnivores)
Different types:
Herbivores – eat only plants
Carnivores – eat other animals
Omnivores – eat both plants and animals
Detritivores – eat dead matter and return nutrients to the soil so they can be reused (worms, fungi) – also called decomposers

5. Quick Check for Understanding
In your notes (summary section):
List at least 3 different:
autotrophs and
heterotrophs
herbivores,
carnivores,
omnivores, and
detritivores

6. Models of Energy Flow
Ecologists use food chains and food webs to model the energy flow through an ecosystem
They are simplified representations of the flow of energy
Each step in a food chain or food web is called a trophic level
Autotrophs make up the first trophic level
Get their energy from the sun (usually)
Heterotrophs make up the remaining levels, and get their energy from the trophic level before it

7. Food Chains
Food chains show the flow of energy from one organism to another in a single line (simpler diagrams than a food web)
Arrows represent the one-way energy flow which usually starts with an autotroph and moves to heterotrophs
Each organism uses a portion of the energy it gets from the organism it eats for cellular processes to build new cells and tissues
The remaining energy is released into the surrounding environment (usually as heat) and is no longer available to the organisms

8. Food Webs
A food web is a model representing the many interconnected food chains and pathways in which energy flows through a group of organisms
Show the flow of energy through connected food chains
Food webs are usually more complex than a single food chain because most organisms feed on more than one species

9. Quick Check for Understanding
Use the following information to make a food web in a meadow ecosystem:
Red foxes feed on raccoons, crayfishes, grasshoppers, red clover, meadow voles, and gray squirrels
Red clover is eaten by grasshoppers, muskrats, red foxes, and meadow voles
Meadow voles, gray squirrels, and raccoons all eat parts of the white oak tree
Crayfishes feed on green algae and dead material, and they are eaten by muskrats and red foxes
Raccoons feed on muskrats, meadow voles, gray squirrels, and white oak trees
Identify all the herbivores, carnivores, omnivores and detritivores in the food web
Describe how the muskrats would be affected if disease kills the white oak trees

10. Ecological pyramids
Ecological pyramids are diagrams that can show the relative amounts of energy or numbers of organisms at each trophic level in an ecosystem
Only 10 percent of all energy is transferred to the level above it
Most of the energy contained in the organisms at each level is consumed by cellular processes or released to the environment as heat
Usually the amount of living matter at each trophic level decreases as you move up a step because there is less energy available to support organisms (there are more mice than foxes, for example)

11. Quick Check for Understanding
Draw an energy pyramid in the question column of your notes for a food chain made up of:
Grass, a caterpillar, a beetle, lizard, snake, and a roadrunner (bird)
Assume that 100 percent of the energy is available for the grass.
At each stage, show how much energy is lost and how much is available to the next trophic level