Prac #7: Constructing a Food Web
Name:___________________
Introduction
To study an ecosystem, we begin by determining which organisms are present. However, understanding
the ecosystem requires us to do more than just compile a list of the plants and animals it contains.
To understand how an ecosystem functions, we need to know how the organisms of the ecosystem interact
with each other. Most interactions between species come down to an understanding of ‘who eats whom’.
In order to answer this question, we need to observe some feeding relationships. This is not always easy, as
many interactions are difficult to see; however, careful observation allows us to deduce the movement of
energy and nutrients from organism to organism through an ecosystem.
Energy and nutrients enter most ecosystems when plants capture light energy from the sun and, by the
process of photosynthesis, convert carbon dioxide and water into sugar. Photosynthetic organisms are
called producers.
All other organisms consume their food ready-made, so they are called consumers. Consumers include:
 Herbivores – consumers that feed directly on producers.
 Carnivores – consumers that eat other consumers (Carnivores include first-order carnivores, which
feed directly on herbivores, and second-order carnivores, which feed directly on first-order
carnivores.
 Scavengers and detritivores – consumers that eat dead plants and animals and their waste
products.
 Decomposers – consumers that break down dead material. Bacteria and fungi are decomposers.
The feeding relationship between organisms can be summarized in a food chain. Since one organism will
often have more than one food source, food chains will often interlink and form a food web. A food web
describes the feeding interactions of the community.
Purpose
1. To use a series of observations of feeding relationships to construct food webs for two communities
found in a suburban garden.
2. To make some comparisons between, and draw some inferences about, the feeding relationships in the
two communities.
Part A: Around a Grapevine
These observations were made at a grapevine in a suburban garden.
Organism
Grapevine
Vine moth
Yeast (fungi)
Vinegar fly
Blackbird
Earthworm
Snail
Silver-eye
Black house-spider
Bee
Observation
Deciduous vine, flowers in spring, fruit (grapes) ripens in autumn
Larva (caterpillar) feeds on vine leaves
Grows in juice of damaged grapes
Feeds on yeasts
Feeds on grapes and other soft fruits, vine moth larvae, earthworms, spiders and snails;
permanent resident of garden
Feeds on decaying leaves
Feeds on decaying leaves
Migratory bird which feeds on insects and fruits
Feeds on adult vine moths
Feeds on nectar and pollen
Procedure
1. Start by writing ‘grapevine’ at the bottom of a blank piece of paper. Write the names of, and draw
arrows to, those organisms that use the grapevine as a food source. The ‘grapevine’ includes leaves
on the vine, leaves on the ground, and grapes.
2. Add the names of the organisms that feed on those that use the grapevine as a food source. Draw
in arrows to indicate the feeding relationships. Make sure the arrowhead always points towards
the organism doing the eating. In this way it is possible to trace the flow of energy and nutrients
(matter) from one organism (the one being eaten) to the next (the one doing the eating).
3. Finally, check through the data and make sure you have not left out any observations about feeding
relationships. At this point, you may wish to redraw your food web in a clearer form.
4. From the food web:
a. Name two herbivores
b. Name an organism that acts as a detritivore
c. Write down a food chain in which the blackbird acts as:
i. A herbivore
ii. A first-order carnivore
iii. A second-order carnivore
d. Why type of consumer is yeast in this food web? Explain.
Part B: Around a Eucalypt
In a part of the garden nearby, eucalypts and other native species were growing. The following
observations were made in this area.
Organism
Eucalypts
Cup moth
Sawfly
Bee
Wattlebird
Silver-eye
Orb web spider
Beetle
Earthworm
Observation
Various species that flower from spring to autumn; flowers produce large amounts of pollen
Larvae feed on eucalypt leaves
Larvae feed on eucalypt leaves
Feed on nectar and pollen
Feed on nectar, pollen, fruits (including grapes) and insects
Migratory bird which feeds on insects and fruits
Feeds on flying insects
Feeds on eucalypts flowers
Feeds on decaying leaves
Procedure
5. Using the method from part A, construct a food web for this community
Discussion
The two sets of data suggest that some organisms move between the two communities in search of food.
This links the two food webs.
6. From the data, explain which organisms provide this link
7. It has been observed that insect species such as vine moths, vinegar flies and cup moths disappear
from the food web during winter. Suggest why this occurs and how they might survive to reappear
each spring.
8. While silver-eyes appear during the fruit season but leave afterwards, only to reappear the next
year, blackbirds remain as permanent residents. What feeding habits do blackbirds possess that
enable them to survive all year in this community?
9. Earthworms are also year-round inhabitants of the two communities. Explain how earthworms are
able to survive and remain active throughout the winter when many insects cannot.
Conclusion
Write a short paragraph detailing what you have learnt from this activity.
Discussion Answers
6. Earthworm, bee, silver-eye
7. Move to a warmer climate to avoid the cold
Could be the time of their life cycle where they lay eggs and die
8. Blackbirds eat earthworms, snails,. Vine moths, black house-spiders. Don’t rely on the fruit to survive, so when fruit
season is over, they can stay.
9. Live underground, therefore warmer
Feed on dead leaves from grapevine, so can still find food
Sample Conclusion
By completing this prac, we discovered that food webs are constantly changing due to animals leaving or arriving in
the area. This may occur due to weather conditions, food availability, availability of animals to breed with, and other
major changes. Not all food webs are closed webs because there is constant interaction between webs.