A food web is an ecological concept which represents the relationships between
producers and
consumers in an ecosystem. It is normally represented
graphically, where each species in a
community is connected to all other species it consumes (or which consume it). In more modern conceptions, the strength of the
trophic transfer (read: the amount of biomass consumed) is indicated by the thickness of the link in the graphic. The strength of the trophic transfer is normally determined by
bioenergetic modelling or by
radiotracers.
From the concept of the food web, biologists have classified organisms based on their relative position into four categories:
- Primary producers: These are the plants which convert solar energy into biological material available for consumption. Examples: algae, flowers, shrubs, trees.
- Primary consumers: These are the herbivorous animals which transform plant biomass into animal biomass. Examples: grasshoppers, cows, deer.
- Secondary consumers: These are the majority of predatory species, including the omnivores and carnivores. Examples: wolves, bears, wolverines.
- Tertiary consumers: These are the top predators, which consumer other predatory species, and are almost exclusively carnivorous. Examples: eagles, bears, lions.
An example of a food web (highly simplified) in a lake might be as follows:
Walleye
^ ^
/ \
/ \
/ \
minnow yellow perch
|\ /|
| \ / |
| \ / |
| \ / |
| * |
| /\ |
| / \ |
| / \ |
| / \|
Daphnia Copepods
\ /
\ /
\ /
\ /
\/
algae
|
|
dissolved nutrients
Where the walleye is a tertiary consumer, the minnows and perch secondary consumers, the zooplankton primary consumers and the algae the primary producers.
There are some problems with the concept of the food web, and as a result it has fallen out of favour in recent years. The entire microbial loop is ignored by this model. Also, the decomposition cycle, critical to the maintenance of plant biomass, is exluded. Scientists have tried to include these two important compartment, but the result is invariably a god-awful mess.