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The structure of a community may be controlled from the bottom up by nutrients or from the top down

The structure of a community may be controlled from the bottom up by nutrients or from the top down

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The structure of a community may be controlled from the bottom up by nutrients or from the top down by predators. Simplified models based on relationships between adjacent trophic levels are useful for  discussing how communities might be organized. Consider three possible relationships between plants (V for vegetation) and herbivores (H).  H  H   H Arrows indicate that a change in biomass at one trophic level causes a change in biomass  at in the other trophic level. We can define two models of community organization. The  bottom-up model  postulates V   H linkages, in which the presence or absence of  mineral nutrients (N) controls plant (V) numbers, which control herbivore (H) numbers,  which control predator (P) numbers. A simplified bottom-up model is N   V   H   P.
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