Top-down forces in food chains and webs are opposed to and attenuate those from the bottom-up. Top-down forces occur when consumers suppress the numbers, biomass, and affect the population dynamics of species that they consume. The major theme of this article is the frequency, strength, and nature of top-down forces in food webs and chains. Top-down effects are not necessary and are apparently absent from or very weak in a number of food chains and webs in nature. Top-down forces in food webs are negative feedback that suppresses population of food and attenuates bottom-up forces. They occur when consumers are sufficiently numerous and voracious to diminish the population of their providers. For example, if rabbits are so abundant as to graze herbs and grasses in their feeding grounds to levels so low that the growth and reproduction of the food plants are suppressed, then rabbits are a top-down force; their consumption of the food plants reduces the matter and energy that are available to be transferred to rabbits. While bottom-up forces are in every food web, top-down forces usually come into play when consumers suppress the availability of their food. Not all food webs have top-down forces because consumers do not always suppress their food populations. While bottom-up forces have been studied by ecologists since the early twentieth century, top-down forces are a more recent discovery and a more difficult scientific topic. The strength and scale of top-down forces can be quite variable, difficult to resolve, and their study is one of the most active research areas of ecology.
The first clear reference to top-down forces known to this author is from the late nineteenth century by the Italian Camerano in 1880, a reference lost until recently rediscovered by Joel Cohen. Camerano published a treatise that anticipated the distinction between food chains with and without top-down forces. In the former, birds ate herbivorous insects and thereby protected plants. In the latter,
... the number of birds is high particularly in those places where insects are very abundant. When the number of insects decreases, so does the number of birds. Regions with low insect abundance also have few birds. The amount of insects in a region depends essentially on the amount of plant food found in it...
Camerano observed that the popularity of top-down forces was declining among naturalists during the 1880s. It is interesting that food web ecology as developed by Charles Elton and Alfred North Lotka in the early twentieth century and at mid-century by Raymond L. Lindeman, H. T. Odom, and Eugene Odom lacked much, if any, appreciation of top-down forces. In 1960, the famous work of Hairston, Smith, and Slobodkin, described by Evan Preisser in Trophic Structure, introduced top-down forces to ecology.
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