The food chain is undoubtedly one of the easiest ways to show feeding relationships in an ecosystem. In a given food chain, if an arrow is present, it means that eats it. Suppose a food chain is given.
Grass → grasshopper → lizard.
Here the grass is the producer. It is a producer because it photosynthesizes and creates food materials. The grasshopper is a primary consumer in the food chain above, which means it eats the grass. The grasshopper is also a herbivore. The lizard in that situation is called a secondary consumer. It eats the grasshopper, which is a primary consumer, and it is called a carnivore. The contrasting stages in the food chain (producer, primary, secondary consumer) are called trophic levels.
Many other food chains have more than three stages in them.
filamentous algae → mayfly nymph → caddis fly larvae → salmon
The extra trophic level in the food chain given above makes the salmon a tertiary consumer.
plankton → crustacean→ fish → ringed seal → polar bear
The extra trophic level above food chain makes the polar bear a quaternary consumer.
Food chains are one of the most convenient ways to show feeding relationships between different organisms quite easily. Sometimes the situation gets very simple.
In a natural freshwater ecosystem, many other mammals eat the same organism, and many different trophic levels are present. In complex situations, a food web is shown.
Food webs give us information much more than the food chain could. But they do not provide information about how many organisms or what mass of organisms is present.