Yellowstone National Park Ecosystem
Food Chains and Food Webs
Background Information:
What happens to the energy in an ecosystem when one organism eats another? That energy moves
along a one-way path. Energy flows through an ecosystem in one direction, from the sun (or inorganic
compounds) to autotrophs and then to various heterotrophs. The relationships between producers
and consumers connect organisms into feeding networks based on who eats whom.
Food Chains:
The energy stored by producers can be passed through an ecosystem along a food chain, a series of
steps in which organisms transfer energy by eating and being eaten. For example, in a prairie
ecosystem, a food chain might consist of a producer, such as grass, that is fed upon by a herbivore,
such as a grazing antelope. The herbivore is in turn fed upon by a carnivore, such as a coyote. In this
situation, the carnivore is only two steps removed from the producer.
In some marine food chains, such as the one shown below, the producers are microscopic algae
(called phytoplankton) that are eaten by very small organisms such as shrimp (called zooplankton).
The zooplankton, in turn, are eaten by small fish, such as herring. The herring are eaten by larger fish
such as marlin, which is ultimately eaten by large fish, such as a shark. In this food chain, the top
carnivore is four steps removed from the producer.
Food Webs:
In most ecosystems, feeding relationships are more complex than can be shown in a food chain.
Consider, for example, the relationships in a salt marsh. Although some producers—including marsh
grass and other salt-tolerant plants—are eaten directly by water birds, grasshoppers, and other
herbivores, most producers complete their life cycles, then die and decompose. Decomposers convert
the dead plant matter to detritus, which is eaten by detritivores, such as zooplankton. The detritivores
are in turn eaten by organisms such as insects and small fish and so on. Add frogs, birds, larger fish
and hawks to the scenario, and feeding relationships can get very confusing!
When the feeding relationships among the various organisms in an ecosystem form a network of
complex interactions, ecologists describe these relationships as a food web. A food web links all the
food chains in an ecosystem together. The food web below shows the feeding relationships in a saltmarsh community.
Yellowstone National Park Ecosystem Food Chains and Food Webs
Producers
Herbivores
Omnivores
Carnivores
*TC=Top Carnivores
Aspen trees
Elk
Raven
Grey Wolf* (TC)
Cottonwood trees
Bison
Magpie
Coyote
Pine Trees
Deer
Hapless Merganser
Black Bear* (TC)
Spruce Trees
Voles
Canadian Geese
Grizzly Bear* (TC)
Fir Trees
Beavers
Loons
Bald Eagle* (TC)
Juniper Trees
Algae-eating Fish
Mountain Bluebirds American White Pelican
Wildflowers
Insects
Fish
Frogs
Perennials/Shrubs
Mouse
Sandhill Crane
Large Fish
Grasses
Squirrels
Cedar Waxwing
Lizards
Algae
Caddis Flies
Fish
Red Fox
Shrubs
Rabbits
Willows
River Otter
Scavengers: Vultures
Instructions:
1. Use the list of organisms above, on a blank white piece of paper, build a diagram showing the
position of all of them in a food web of the Yellowstone Ecosystem. Ideally, the producers are at
the bottom of the food web and the top carnivores are at the top. Show the scavengers in a
separate location on the food web. (30 points) HINT: Do your food web in pencil!
2. Next, show the energy flow by indicating who eats who with an arrow. For example, to show
that deer eat grass, show it like this: (15 points)
Grass → Deer
3. Finally, on the back of the food web, draw 3 different food chains. Make sure each food chain
starts with a producer and ends with a top carnivore. Label each of the organisms in the food
chain with one of the following labels: Producer, Primary Consumer, Secondary Consumer,
Tertiary Consumer, Quaternary Consumer, Top Carnivore. (15 points).
Yellowstone National Park Ecosystem
Reintroduction of the Wolves
Timeline of Events
Instructions:
1. Read the article “Reintroduction of the Wolves”. Then, using information from the article,
draw a timeline of events with information under each event. An example follows below:
Event:
Event:
→
____________________
____________________
____________________
____________________
____________________
____________________
____________________
____________________
Your timeline will start with the eradication of the wolves from Yellowstone in the 1920s. Under
each event, write a sentence or two about the effects of that event. Make sure your timeline
describes the changes and their effects on the following species:
Eradication of Wolves
Return of Wolves
Aspen Trees
Cottonwood Trees
Elk Populations
Beaver Populations
NOTE: I’m not going to give you an exact number of events you need to include, because it may
vary depending on how well you explain the effects of each event. I will tell you that you need
to have a minimum of 5 events, but you could have several more. Just make sure you explain
each event and its’ effects completely.