Bottom-up control versus top-down control.

(a) Bottom-up control proposes that host plant quantity or quality limits the density of herbivores, which in turn sets limits on the abundance of predators. Taken together, this means that high quantity and quality of host plants would result in increased numbers of predators because of higher densities of the herbivores they prey on. (b) Top-down control proposes that plant densities are limited by their herbivores and that herbivores are limited by their predators. Taken together, this means that high levels of predation would result in high densities of host plants because there would be fewer herbivores.