Why does most communities have only about three or four trophic levels

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Explanation:

As you progress through each trophic, each organism on that level loses energy gained from the previous trophic. This is due to losing energy through wasted material (e.g. bones) and to life processes, 7 of which can be remembered by MRS GREN:

M ovement

R espiration

S ensitivity

G rowth

R eproduction

E xcretion

N utrition

This means that each time, energy is wasted and lost from the food chain. More than four trophics would result in so much energy being lost from the initial trophic (producer), that it would be largely pointless!