Food chains, food webs and trophic levels
A grazing food chain starts with plants; a detritus food chain starts with dead organic matter. Linked chains form a food web; each feeding level is a trophic level.
Producers and consumers
Green plants are the producers. On land, the main producers are herbaceous and woody plants; in water, they are phytoplankton, algae and higher plants. All animals depend on plants, directly or indirectly, for food, so they are consumers, also called heterotrophs. Primary consumers (herbivores) feed on plants. Common herbivores are insects, birds and mammals on land and molluscs in water. Secondary consumers (primary carnivores) eat herbivores, and tertiary consumers (secondary carnivores) eat primary carnivores.
Grazing food chain vs detritus food chain
The two chains are linked: some organisms of the DFC are prey for animals of the GFC.
Food web
Food chains in nature do not stand alone. Some organisms of the detritus food chain are eaten by animals of the grazing food chain. Many animals, such as cockroaches and crows, are omnivores that eat both plants and animals. These natural interconnections of food chains form a food web.
Trophic levels in a grassland
| Trophic level | Who is there | Example |
|---|---|---|
| First | Producers | Grass |
| Second | Primary consumers (herbivores) | Grasshopper, rabbit, goat |
| Third | Secondary consumers (primary carnivores) | Frog, which eats grasshoppers; fox, which eats rabbits |
| Fourth | Tertiary consumers (secondary carnivores) | Snake, which eats frogs |
Standing crop, and a trophic level is a role
Each trophic level has a certain mass of living material at a given time, called the standing crop. It is measured as biomass or as the number of organisms in a unit area. Biomass can be given as fresh or dry weight. Dry weight is more accurate, because the water content of organisms varies a lot. A trophic level is a functional level, not a species. A sparrow is a primary consumer when it eats seeds, fruits and peas, and a secondary consumer when it eats insects and worms.
When you eat rice and dal, you are a primary consumer; when you eat fish curry, you are a secondary or higher consumer. Humans feed at several trophic levels, so we are omnivores in many food webs.
2 marks: “Why is the dry weight of biomass a more accurate measure than fresh weight?” Water content varies widely between organisms and with conditions, so fresh weight is unreliable. Dry weight measures only the actual organic matter.
2 marks: “A sparrow eats seeds and also insects. At which trophic levels does it function?” Second trophic level (primary consumer) when it eats seeds; third trophic level (secondary consumer) when it eats insects.
3 marks: “Distinguish between a grazing food chain and a detritus food chain. Give an example and state where each dominates.” GFC: starts with living producers, e.g. grass → goat → man; major energy path in aquatic ecosystems. DFC: starts with detritus; decomposers like fungi and bacteria; most energy on land flows through it.
Grazing starts Green; Detritus starts Dead.