Food chains and food webs

Where the energy comes from

All living things need energy to move, grow, repair themselves and reproduce. For nearly every ecosystem on Earth, that energy starts as sunlight. The problem is that sunlight can't be stored or passed around. It has to be turned into something that can.

Plants, algae and some bacteria do this by photosynthesis. They use light energy to join carbon dioxide and water into a sugar called glucose, releasing oxygen:

carbon dioxide + water → glucose + oxygen (using light energy)

The energy is now stored in the glucose. The plant uses some of it straight away, builds some into new leaves, stems and roots, and stores the rest as starch. Every cell in every organism can then release the energy from glucose by respiration. You'll look at both processes in detail in Chapter 2.

Producers and consumers

• Producers make their own food by photosynthesis. They're also called autotrophs ("self-feeders"). Grass, gum trees, seaweed and tiny floating algae called phytoplankton are all producers.
• Consumers can't make their own food, so they must eat other organisms. They're also called heterotrophs ("other-feeders").

Consumers are named by what they eat:

• Herbivores eat only plants or other producers (koalas, grasshoppers, wombats).
• Carnivores eat only other animals (quolls, sharks, wedge-tailed eagles).
• Omnivores eat both (most people, emus, possums).

Then there's the clean-up crew. Detritivores are small animals such as worms, slaters, maggots and dung beetles that eat dead and decaying material. Decomposers, mostly fungi and bacteria, finish the job: they break dead matter down into simple chemicals that go back into the soil, water and air, where plants can take them up again. Without them, nutrients would stay locked up in dead bodies and the whole system would grind to a halt.

Food chains

A food chain shows one path that energy takes through an ecosystem, as a list of organisms joined by arrows.

Three rules for food chains:

• A food chain always starts with a producer, because producers are where the energy enters.
• Each arrow points from the food to the eater. Read it as "is eaten by". It shows the direction the energy flows. (A very common mistake is drawing the arrows backwards.)
• Each step is a trophic level (feeding level). The producer is the first trophic level, the animal that eats it is the first-order consumer (or primary consumer), the next is the second-order consumer (secondary), and so on. The consumer at the end, with nothing eating it, is the top predator.

Decomposers aren't usually drawn in a food chain, because they feed on dead material from every level.

Food webs

Real animals rarely eat just one thing. A magpie eats skinks, but also grasshoppers, worms and spiders. Eating several foods is a survival strategy: if one food runs short, there's something else. So each species usually belongs to many food chains at once. When you link all those chains together you get a food web, which is a much more accurate picture of who eats whom.

In a food web, one species can sit at more than one trophic level. A skink that eats ants is a second-order consumer, but a skink that eats a spider (which ate an ant) is a third-order consumer.

When a food web changes

Adding or removing one species affects the whole web. To predict what happens, check three directions:

• Predators above lose a food source. If it was their only food, they decline or leave. If they have other foods, they eat more of those instead.
• Prey below are eaten less, so their numbers usually rise, at least for a while.
• Competitors on the same level that ate the same food now have more of it.

Feral cats are a good example. Cats aren't part of Australia's native food webs, but they now live almost everywhere on the mainland. They compete with magpies and kookaburras for skinks and prey heavily on small birds, so where cats are removed, populations of skinks, wagtails, honeyeaters and other small animals bounce back.