7.4 - Food Chains & Food Webs

Food chains

• A food chain shows the feeding relationships within a community - what eats what. It's a simplified single line of who-eats-whom.
• Every food chain begins with a producer: a photosynthetic organism such as a green plant or an alga.
• A producer uses light energy to make glucose by photosynthesis and builds this into its tissues - the material called biomass.

The order of a food chain

• The producer is eaten by a primary consumer.
• The primary consumer may be eaten by a secondary consumer.
• The secondary consumer may be eaten by a tertiary consumer.
• Some chains have more steps and some fewer.

What the arrows mean

• The arrows always point from the organism that is eaten to the organism that eats it.
• They represent the transfer of energy (and the biomass that carries it) up the chain, so an arrow points in the direction the energy flows, not the direction of feeding.
• Each organism passes on only a fraction of what it received, so the amount of energy and biomass available falls at each step.

Food webs

• In a real ecosystem, an organism rarely eats - or is eaten by - only one other kind of organism, so individual food chains overlap and interconnect.
• A food web is this network of many interconnected food chains, showing lots of feeding relationships in a community at once.
• Read a web the same way as a single chain: each arrow points from an organism to the one that eats it - but now with many branching links rather than one line.

Organisms in a web are interdependent

• Because the organisms are linked, a change to one population has knock-on effects on the others.
• If one organism is removed - for example a predator dies out or a prey species is lost - the populations connected to it rise or fall in response.
• Those changes then pass along the web to organisms further away.
• This is why the members of a community are described as interdependent: what happens to one can affect many.

Predator-prey cycles

• A predator is an animal that kills and eats other animals; the animals it eats are its prey.
• Plot both populations against time and they rise and fall in repeating, linked cycles - a predator-prey cycle.
• This is seen in a stable community, where the populations stay roughly balanced over the long run.

Why the two cycles are out of phase

• The populations don't peak at the same moment - the predator change always lags slightly behind the prey change, so the cycles are out of phase.
• Following the cycle round:
  1. When predators are few, less prey is eaten, so the prey population rises.
  2. More prey means more food for predators, so a little later the predator population rises.
  3. With many predators, prey is eaten faster than it can breed, so the prey population falls.
  4. With less food, the predator population falls, letting the prey recover - and the cycle begins again.

Why they cycle rather than settle

• A population takes time to change size.
• Even with plenty of food, it takes several generations of breeding before predator numbers actually increase.
• So the predator response always arrives after the change in prey numbers that caused it - giving the lag and the repeating up-and-down pattern.