In nature, short-term survival depends mainly on the organism finding sufficient food for its needs, and preventing itself becoming dinner for something else.

Animals will choose and prefer foods which are for them easily caught, palatable and of a suitable size for eating. The food must provide more energy than is expended in its hunting, capture and eating. For example, a lion eats large prey such as zebra or young elephant, not ants, whereas a spider may eat tiny prey such as flies.

Food chains
A food chain is a simple, graphic way of showing a food relationship between organisms. It shows how energy and nutrients are transferred from plants (producers) to herbivores and carnivores and through to decomposers.

All food chains start with a producer. The arrows in the food chain below depict the direction in which energy and nutrients flow, i.e. the arrow always points from the eaten to the eater.

Here is an example of a food chain:

Grass

grasshopper

field mouse

snake

owl

The grass is the producer. It makes sugars using carbon dioxide, water and sunlight, and then uses other minerals from the soil to convert these into proteins and other organic substances.

The first consumer is always a herbivore (in this instance, the grasshopper) and it is called a first order consumer, with successive members of the food chain being called in turn second, third and fourth order consumers (field mouse, snake, owl).

Affecting the food chain
Improved environmental conditions (more rain, adding fertilisers, fencing off kangaroos, and so on) will increase the amount of grass, providing food for increased numbers of grasshoppers, and then more food for all higher order consumers in the chain. Conversely, a mouse eradication program would also decrease the snake and owl populations unless they have alternate food sources.

Any break in the food chain will have repercussions at all higher levels of the chain (for example, starvation) as well as resulting in higher numbers at the lower levels of the chain (because they are not being eaten).

More food chains

Algae

water beetle

dragonfly nymph

fish

heron

Phytoplankton

small crustaceans

fish

dolphins

killer whale

Algae

tadpole

small fish

Murray cod

human

Energy in the food chain
At each level in the food chain, energy is used up by the organism's life functions such as growing, moving and so on. This energy is ultimately lost as heat to the environment, which means it is unavailable to organisms at the next level in the food chain. Because of this progressive loss of energy, food chains are rarely more than 6 members long.

It is less wasteful of energy for humans to eat producers such as grains, fruit and vegetables, than to eat consumers such as meat, fish and eggs. In third world famine situations, therefore, eating meat is a luxury.

Food webs
In nature, feeding relationships are more complex. Most consumers eat several different foods, and are in turn eaten by many different higher-order consumers.