Question

what is the difference between food Web and food chain

Answer 1

A food web consists of many food chains whilst a food chain is just one path of energy flow through an ecosystem. 
A food chain is a linear sequence that involves links in a food web starting off with a producer (like a plant), primary consumer (herbivore), Secondary consumer and tertiary consumer. So it is this link that involves one organism eating another organism and so on.

A food web, however, is the natural connection of food chains through a graphical representation of what organism eats what in an ecological community. This is different to a food chain because, in a food web, a particular organism has the probability of being eaten/eat more than just one organism.
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Answer 2

A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community. Another name for food web is consumer-resource system.