8.2: A Local Ecosystem
Food Chains and Food Webs
Food Chains
What is a food chain? Click here to refresh your memory
Trophic levels
Organisms in food chains are grouped into trophic levels. The trophic level of an organism is its position in a food chain, trophic levels consist of either a single species or a group of species that are presumed to share both predators and prey, its level determines what it eats and what eats it
The trophic levels in the basic food chain in Figure 1 below are producers, primary consumers, secondary consumers, tertiary consumers, quaternary consumers and decomposers. The quaternary consumers are those that eat the tertiary consumers as shown on the terrestrial and marine food chain diagrams.
Decomposers
The decomposers are the final level or stage in the food chain. Decomposers are organisms that consume dead plants and animals, and, in doing so, carry out the natural process of decomposition. Decomposers use deceased organisms and non-living organic compounds as their food source. The primary decomposers are bacteria and fungi. The decomposers then return some of the energy, accumulated through the food chain back to the soil as nutrients.
Figure 1: Food Chain
(Image source http://en.wikipedia.org)
Food chains don’t always make sense!
It would be easy to assume that food chains follow a logical sequence with smaller animals being consumed by larger ones and so on up the chain. This is not necessarily the case, for example, the whale shark is the largest fish in the world but it feeds on tiny krill and plankton as shown in the marine food chain below (Figure 2).
Figure 2: Marine Food Chain
(Image sourcehttp://en.wikipedia.org )
The direct pathway shown in a food chain is very simplistic as it assumes that the animals shown in the diagram only consume one type of food and are in turn consumed by only one species. Therefore a food web is a more accurate way of showing the feeding relationships between plants and animals.
Food Webs
What is a food web? Click here to refresh your memory
Trophic levels in food webs.
Trophic levels exist in food webs also, although more complex than the simple food chain. For example in the aquatic food web shown below on an Australian river ecosystem, the primary producers in this food web are the algae and the sedge. This algae is then consumed by the primary consumers, the tadpole and daphnia in this example. The secondary consumers are the damselfly and back swimmer. The sedge is consumed by the maned duck, moth, dragonfly, and ant, these animals being the primary consumers. The secondary consumers are the frog, centipede and lizard and the tertiary consumer is the kookaburra. The decomposers (bacteria) are also shown on this food web, breaking down the decayed material and returning the nutrients to the soil to then be used by the sedge for growth.
Light energy is used by primary producers (plants and phytoplankton) to synthesize organic molecules through the process of photosynthesis. Energy then flows to higher trophic levels through the consumers in the food web. It is often the case that the biomass (plant and animal material that can be used for energy) of each trophic level decreases from the base of the chain to the top. This is because energy is ‘lost’ to the environment with each transfer, this loss can be in the form of waste (e.g. carbon dioxide, faeces), and heat and kinetic energy (e.g. constant body temperature of mammals, the energy used to move). Up to 90% of matter and energy can be lost at each level. Therefore only 10% of the organism's energy is passed on to its consumer in the next level of the food web. For example grass has a biomass of 400g/m², this is then consumed by a grasshopper with a biomass of 50g/m² and finally the grasshopper is consumed by the kookaburra with a biomass of 10g/m².
Outcome- 8.2.2: Students learn to explain trophic interactions between organism in an ecosystem using food chains, food webs and pyramids of biomass and energy.