EXAMPLE OF FOOD CHAIN
sparrow hawk (tertiary consumer)
level
(third-order consumer)
(top carnivore)
thrush
(secondary consumer)
4th trophic
3rd trophic
level
(second-order consumer)
(first level carnivore)
snail
(primary consumer)
2nd trophic
level
(first-order consumer)
(herbivore)
lettuce
level
(producer)
(autotroph)
1st trophic
FOOD CHAINS and FOOD WEBS
RECALL:
FOOD CHAINS show predator-prey relationships.
WOLF
FOX
FIELD MOUSE
GRASS SEED
FOOD WEBS are more complex, showing more interrelationships. They give a more accurate picture of the
ecosystem.
PIKE
SUNFISH
SHINER
PLANKTON
MUSKIE
LARGEMOUTH BASS
SHAD
PERCH
ALGAE
TRY THIS ONE:
Using the following organisms, construct a food web: grass,
raccoons, fox, caterpillars, snakes, squirrels, oak trees, toads,
rabbits, red-tailed hawks, earwigs.
FOOD WEBS: A Florida River Community
You can best begin to understand a biotic community by studying one in the field. The
following description is of one community that has been studied. This example is located in the
short rivers along the west coast of Florida. The focus of the study was on the river turtle.
Adult river turtles are first-order consumers. They eat many of the species of green plants
that grow in the rivers, though they do not eat all species in equal amounts. Their favourite food
is tape grass. When plentiful, some other kinds of plants are eaten.
Unlike the adults, young river turtles are not entirely first-order consumers. They eat
snails (which are first-order consumers), aquatic insects (second-order consumers) and worms
(third-order consumers). Thus depending what they eat, they can be second-, third-, or even
fourth-order consumers.
Turtle eggs are laid on land in holes dug by the females. The nests are frequently raided
by skunks, raccoons, or snakes, all of which eat the eggs. The unhatched eggs can also be killed
by moulds that live in the soil and grow through the shells. If the eggs survive, the young may be
picked up by snakes or herons. Once in the water, they may be eaten by some kinds of fish, by
otters, or by a species of snapping turtle. When river turtles become larger, few organisms can
kill them except for alligators or garfish.
Leeches attach themselves to turtles to suck their blood, but do not kill them. Within the
turtles live parasitic worms and microorganisms that obtain their food from the turtles. A turtle
that dies becomes food for decomposers, which return the substances in the turtle's body to the
non-living world.
Each of the community relationships we have described so far is a direct relationship
between the river turtle and another organism. River turtles also have many indirect relationships
with organisms in the community. In the river live spiral-shelled snails, which are fond of tape
grass. The tape grass population varies greatly: sometimes it is plentiful and sometimes scarce.
When tape grass becomes scarce, both the turtles and the snails use the same food supply for a
while. Eventually the snails turn to eating algae, which are tiny green plants that grow on the
rocks. They can scrape these off the rocks while turtles cannot. On the other hand, turtles can
reach tape grass that the snails cannot reach.
The musk turtle eats nothing but snails. By reducing the number of snails that feed on
tape grass, the musk turtles have an indirect effect on the river turtles. The musk turtles never eat
all the snails because they cannot find them all. Moreover, many animals that kill musk turtles
also kill young river turtles. Thus, the more musk turtles there are in the river, the less likely the
young river turtles are to be caught.
When tape grass is abundant, another species of turtle, the pond turtle, enters the rivers
and temporarily joins the community. Since the pond turtle leaves as soon as the tape grass
supply declines, it has very little influence on the river turtle.
This study has looked at the community through the eyes of the river turtle. If a different
organism had been selected, we may have seen a slightly different picture. To understand a
community, we must look at all of the interrelationships that exist. Clearly, the study of an
ecosystem is not easy, but even a simple community like this one helps us better understand our
entire biosphere.
A FLORIDA RIVER COMMUNITY
Questions:
1. Summarize the relationships in the river - turtle community by completing the food web below.
2. From the food web find an example of the following: producer, consumer, herbivore, carnivore,
omnivore.
3. Young river turtles may be first, second, third, or even higher order consumers. Explain.
YOUNG
RIVER
TURTLES
ADULT
RIVER
TURTLES
RIVER
TURTLE
EGGS
- parasitic
relationship
A FLORIDA RIVER COMMUNITY (Answers)
Pond
Turtle
Algae
Musk
Turtle
(H)
Snapping
Turtle
Snails
Tape
Grass
Green
Aquatic
Insects
Worms
Otters
YOUNG
RIVER (O)
TURTLES
Fish
Plants
(P)
ADULT
RIVER
TURTLES
Heron
Leeches
Alligators
RIVER
TURTLE
EGGS
Garfish
Skunks
(Co)
Snakes
(Ca)
Parasitic
Worms
Raccoons
MicroOrganisms
- parasitic
relationship
Mold
FOOD PYRAMIDS
Food pyramids show relationships between trophic levels. They are
another way of showing food chains. Pyramids are used to compare the
total numbers, biomass, or energy at each trophic level within the
ecosystem.
PYRAMID OF NUMBERS
15
Shrews
165
Grasshoppers
20000
Grass plants
PYRAMID OF BIOMASS
0.007 kg
Mosquitoes
22 kg
Squirrels
2076 kg
Oak tree
PYRAMID OF ENERGY
3.2 X 105 J
Largemouth bass
4.8 X 106 J
Shiners
5.0 X 107 J
Algae
WHAT IS A FOOD PYRAMID?
Background
Biologists in the year 2014 observe a food chain that is part of an ecosystem on a distant planet.
They find that it has many characteristics of food chains on earth. Here is what they discover.
On a particular area of land, a small, plant-like organism capable of photosynthesis, called a
vortek, serves as a source of food for an animal called a snive. The snive, in turn, is preyed upon by a
kluke. There are 100 vorteks, 50 snives, and 25 klukes. The food chain may be diagrammed as follows:
Building a Food Pyramid
By completing the activity described below, you will better understand interactions occurring in food
chains.
1. On a blank sheet of paper, list the members of this food chain by:
(a) placing them in order, with the members at the bottom of the chain near the bottom of the page;
(b) spacing them 5 cm apart.
2. For each, beginning with the bottom one draw a horizontal block around it to represent the numbers
of each member inside the block following these guidelines:
(a) height: Each block should be 5 cm tall, so that it touches the bottom of the block above it.
(b) width: Make the width to scale according to the numbers of each organism in the food chain 10 organisms in 1 cm. (The block around the vorteks, for example, will be 10 cm wide.)
(c) centering: Center each block on the page.
3. Draw lines to connect the edges of the blocks. What shape is formed?
4. Each block represents a trophic level. Label the organisms in each trophic level as to which is a:
(a) producer (b) primary consumer (c) secondary consumer.
Explain why you choose these labels for each one.
5. This food chain is similar to many on earth. What factors do you think cause certain food chains to
have this shape?
6. Turn your diagram upside down. Describe a food chain on earth that might have a shape similar to
this one.
7. How would the numbers of each member of the food chain - vorteks, snives, and klukes - change
over the course of time in each following situation? Assume there are no other predators or food
sources unless otherwise indicated.
(a) the biologists visiting the planet consume all of the vorteks for food;
(b) a deadly disease wipes out all the klukes;
(c) animals called skunkolas travel to the area being studied from another region of the planet. The
skunkolas prey upon the vorteks; the klukes, however, dislike the taste of skunkolas and refuse
to eat them.
(d) the biologists remove all the snives for study;
(e) more vorteks are planted;
(f) a tertiary consumer called a joon is introduced.
8. It has been stated that it is more energy efficient to eat "lower on the food chain." Look again at the
shape of the food chain you drew and explain why this statement is true.