In the seqeunce above, AB and BC are direct relationships, whereas AC is an indirect relationship. You need to know the two direct relationship to accurately model the sequence of relationships, but you need to take into account indirect effects, i.e. the complete picture, in order to understand the system. If you consider only part of the system, you explain nothing. If I pay a killer and the killer kills you, the judge will condemn me, and the killer to a lesser punishment. But ecologists think tha indirect relationship do not exist. They will codnemn only the killer, not the instigator. Consider for instance this interesting paper on Nature about the determinants of productivity. It says that productivity is directly related to size and age and indirectly related to climate. This is a major insight, since it retrieves the killer and the instigator. But in the paper the indirect relationship is considered in some way unreal.
Ecologists are slowly recognizing that often different factors are at play at the same time. But they accept only parallel chains of caustions, like that below in the diagram, and not series of causation like that below. But "closer" does not mean "stronger". In the chain above, if the rate of AB is slower than the rate of BC, the bottleneck is AB and not BC. We should consider the complete picture, and stress not the "closer" or the stronger relationship, but the bottlenects. In the case of parallel systems this is slowly getting recognized (consider for instance this paper on Ecology Letters)
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