A fundamental question in community ecology is what controls the diversity and abundance of organisms at different trophic levels. A community could be controlled by each trophic level limiting the level above it "bottom-up control". Alternatively, the composition of a trophic level could be limited by predation, termed "top-down control". If a community exhibits bottom-up control for example, then the number of insects which feed on plants would be limited by the number of plants which are present (which might be limited, in turn, by nutrient or light availability.) In a community which exhibits top-down control, the number of insects would be limited by predation.

One of the first attempts to deal with the relative importance of top-down and bottom-up control was but forward by Hariston, Smith and Slobodkin (1960). They observed that the world was green, and interpreted this to mean that there are a great number of plants in the world which are not consumed by herbivores. This means that herbivore numbers are not limited by the amount of plant matter that they can feed on- there is much unused plant material available. Instead, they argued, herbivores must be controlled by their predators. In other words, top-down control must be prevalent.

They first proposed this idea in terms of terrestrial plant eating insects, and it seems likely that it does not apply to all locations. For example, bottom-up control may be very important in marine ecosystems. The relative importance of top-down and bottom-up control in terrestrial ecosystems is still debated, and some ecologists think that the "world is green" argument is a useless one. None the less, it is still commonly referred to in ecology, and it was one of the early attempts to understand community ecology.

The reference is: Hariston, N.G., Smith, F.E. and Slobdkin, L.B. 1960. Community structure, population control, and competition. American Naturalist. 44:421-425.

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