11.1 Food Chains & Food Webs

Ecology is the study of how living things interact with each other and with their surroundings, and its most famous diagram is the food web. Food webs are a named topic on ICAS Papers B and C, and the skill of reading them - tracing what eats what, and predicting what happens when something changes - is tested at every level of the exam right through to the senior papers.

Producers, Consumers and Decomposers

Every food chain begins with a producer: a living thing that makes its own food from sunlight, water and carbon dioxide by photosynthesis. On land the producers are plants; in rivers and oceans they are algae and floating phytoplankton. Producers do not eat anything - they capture energy straight from the Sun, which is why every food chain on Earth is really a chain of captured sunlight.

Everything that eats other living things is a consumer, and consumers are sorted by their diet:

The third group is easy to overlook but essential. Decomposers - mainly fungi and bacteria - feed on dead plants, dead animals and animal waste, breaking them down into simple nutrients that return to the soil or water. Those nutrients then feed the producers, which is why decomposers are called nature's recyclers. Without them, dead material would pile up and the nutrients locked inside it would never reach new plants.

What the Arrows Mean - The Most Common Error

In a food chain diagram such as grass → rabbit → fox, the arrow does NOT mean "is eaten by" and it does not point at the food. The arrow shows the direction of energy flow: it points from the organism being eaten to the organism that eats it. Grass → rabbit means "energy stored in the grass moves into the rabbit when the rabbit eats the grass". Reading the arrows backwards is the single most common food-web mistake students make, and ICAS papers are designed to catch it. Before you answer any food-web question, say to yourself: the arrows point the way the energy travels.

A worked land example

Take the chain grass → rabbit → fox. The grass is the producer. The rabbit is a herbivore - because it eats the producer directly it is also called a primary consumer (first-level consumer). The fox is a carnivore eating the primary consumer, so it is a secondary consumer. If an eagle then ate the fox, the eagle would be a tertiary consumer (third level). Labelling the levels like this - producer, primary, secondary, tertiary - is exactly the language ICAS questions use, so practise naming each level in any chain you are shown.

Energy Loss Up the Chain

Here is the big idea that explains so much about nature: only about ten per cent of the energy stored at one level of a food chain is passed on to the next level. Where does the other ninety per cent go? The rabbit uses most of the energy from its grass for hopping, keeping warm, digesting and simply staying alive, and that energy is eventually lost as heat. Some material - fur, bones, droppings - is never eaten by the fox at all, and goes instead to the decomposers.

A handy way to picture it is an energy pyramid. If the grass in a paddock stores 100 000 units of energy, the rabbits together might store about 10 000, and the foxes only about 1 000. Each step up divides the available energy by roughly ten. That is why food chains are short - rarely more than four or five links - and why top predators are always rare. A single wedge-tailed eagle needs a huge territory full of rabbits to support it, while the rabbits need a still larger mass of grass. If a question ever asks why there are thousands of grass plants, hundreds of rabbits and only two or three foxes in an ecosystem, energy loss is the answer.

From Chains to Webs: Predicting Change

Real ecosystems are never single chains. A rabbit eats many grasses, a fox eats rabbits, mice and birds, and eagles eat rabbits too. Link all the chains together and you get a food web. Webs matter because a change to one species ripples along every chain connected to it.

Consider a simple marine web: phytoplankton (producers) are eaten by small fish and by krill; small fish are eaten by penguins and by seals; krill are eaten by penguins and by whales; and seals are eaten by orcas. Now imagine overfishing removes most of the small fish. Work through the arrows:

• The penguins lose one food source, so they eat more krill instead - penguin numbers may fall if krill alone cannot support them.
• The seals lose their food and decline, so the orcas that prey on seals go hungry too.
• With fewer small fish eating them, the phytoplankton may actually increase - a knock-on effect in the opposite direction.

That last point is the trap ICAS loves to set: students expect every effect to go the same way, but removing a consumer can cause the level below it to rise. When you face an if-then question, trace the arrows one step at a time, ask "who eats this?" and "what does this eat?", and remember both directions of effect: predators of the removed species tend to decrease, and prey of the removed species tend to increase.

Exam tactic

In a food-web stimulus you cannot see pictures in this guide, so practise reading webs described in words or tables. Draw a quick sketch on your working paper - organism names with arrows - before you look at the options. Thirty seconds of sketching turns a confusing web into a map you can trace with a finger, and it stops the most common error of all: reading an arrow the wrong way around.