Food web

In the syllabus and the exam

A food web is formed by linking together several overlapping food chains in the same ecosystem, since most real organisms eat more than one type of food and are eaten by more than one predator, unlike the single fixed pathway shown in an isolated food chain. Exam questions typically present a food web diagram and ask candidates to extract one or more individual food chains from within it, to identify an organism's trophic level (which can differ between different chains within the same web), and to predict what happens to several other populations if one species is removed, added, or increases sharply in number. A key exam skill is recognising that a food web is more resilient to the loss of one species than a single food chain would be, because predators and prey usually have alternative feeding pathways available within the web; candidates should use this reasoning explicitly rather than assuming any species loss is automatically catastrophic. A common error is only tracing one chain through the web and ignoring the other connections that would also be disturbed, missing indirect effects such as a decline in one prey species indirectly benefiting a competing prey species by reducing shared predation pressure. Candidates should also state clearly that arrows in a food web, like in a food chain, always represent the direction of energy or feeding flow, from prey to predator.

Example

In a forest food web, a hawk may eat both a snake and a squirrel, and a squirrel may eat nuts as well as insects, so removing one species affects several interconnected feeding pathways at once, not just a single chain.

Related terms

Food chainProducerEcosystem

Source:SRC-DSKP-EN