Human Involvement in Food Webs

Human involvement in food webs has been profound, bringing about enormous and disproportionate losses of large apex predators on land and in water. The losses have modified or even eliminated concatenations of indirect interactions propagating from predators to herbivores to plants, inter alia. Food webs are a synthesis of bottom-up energy and nutrient flow from plant producers to consumers and top-down regulation of producers by consumers. The trophic cascade is the simplest top-down interaction and accounts for a great deal of what is known about food webs. In three-link cascades, predators suppress herbivores, releasing plants. In longer cascades, predators can suppress smaller mesopredators, releasing their prey animals. Hunting, fishing, and whaling have brought parallel losses of large apex predators to food webs. Without apex predators, smaller mesopredators have often become superabundant, sometimes with unprecedented suppression of their prey, extinctions, and endangerment. Flourishing mesopredators also can reverse the web regulation and suppress apex predators that have become rare owing to hunting and fishing. This can prevent fisheries recovery and lead to persistent alternative ecosystem states. Although food-web modules of large animals are increasingly well understood, the parts of webs consisting of small inconspicuous organisms, such as mutualists and parasites, and webs in obscure places, such as in the soil, are much of the challenge of future research