2.2.2 Food Chain & Food Web

Learning Objectives

1. Construct a food web by identifying food chains

2. Identify organisms in each trophic level

3. Summarise ecological pyramids

Losses of energy and organic matter along a food chain 

Open systems such as ecosystems are supported by continual input of energy, usually from the Sun

• There is never 100% transference of organic matter from one trophic level to the next.

  • Not all the food available to a given trophic level is harvested – for example, sheep eat the leaves of grass but not the roots.

  • Not all that is harvested is consumed – for example, the bones and teeth of animals are not usually eaten.

  • Not all that is consumed is absorbed – for example, some parts of the food cannot be digested such as the cellulose in plant matter.

  • Not all that is absorbed is stored – for example, some is lost as heat through cellular respiration.

Typically 10% or less of the energy flowing to a trophic level is available to the next level. The number of trophic levels in ecosystems is therefore limited due to energy losses.

There are 3 types of ecological pyramids:

1. Pyramid number

   • numbers of producers and consumers coexisting in an ecosystem can be shown by counting the numbers of organisms in an ecosystem and constructing a pyramid.

2. Pyramid biomass

   • quantifies the amount of biomass present at each trophic level at a certain point in time and represents the standing stock of each trophic level

   • measured in grams of biomass per metre squared (g m–2) or units of energy, such as joules per metre squared (J m–2)

   • Although pyramids of biomass are usually pyramid-shaped, they can sometimes be inverted and show greater quantities at higher trophic levels. 

   • This is because, as with pyramids of numbers, they represent the biomass present at a given time (i.e. they are a snapshot of the ecosystem). The standing crop biomass (the biomass taken at a certain point in time) gives no indication of productivity over time. 

   • For example, a fertile intensively grazed pasture may have a lower standing crop biomass of grass but a higher productivity than a less fertile and ungrazed pasture (because the fertile pasture has biomass constantly removed by herbivores). This results in an inverted pyramid of biomass. 

   • In a pond ecosystem, the standing crop of phytoplankton (the major producers) at any given point will be lower than the mass of the consumers, such as fish and insects. This is because phytoplankton reproduce very quickly. 

   • Inverted pyramids sometimes result from marked seasonal variations.

3. Pyramid energy (productivity)

   • consider the rate of production over a period -- each level represents energy per unit area per unit time

   • show the flow of energy through an entire ecosystem over a year

   • no inverted pyramids of energy

   • relative energy flow within an ecosystem can be studied and different ecosystems can be compared