Figure 57.10 Bottom-up and Top-down Control of Food Webs (A) Production in an ecosystem can be viewed as mostly being controlled by limiting resources (bottom-up) or by consumers at higher trophic levels (top-down). (B) A three-level food web, in which the secondary consumer affects the primary consumer’s abundance, has higher NPP (represented by the size of the green circle) than a two- or four-level food web. (C) A three-level food web with omnivory has lower NPP than a food web without omnivory.