Movement of materials through any community can be tracked by developing a/an for that community.

Movement of materials through any community can be tracked by developing a/an food web for that community.

Explanation:

Knowing that a food chain is used to give a description about the feeding connections of diverse organisms in a linear scheme when we talk about a food web we refer to a representation of many food chains, that is to say, many relationships and connections. Knowing what a community consumes is a way to identify the flow of materials within it.  

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  b.

there are isolated pockets of farming, generally around rivers and oases, which are used for irrigation.