The food supplies of many herbivores vary seasonally

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The food supplies of many herbivores vary seasonally. Seasons of scarcity follow seasons of abundance. In many regions, dry seasons follow wet ones. In regions beyond the tropics, winters follow summers. Herbivores respond in several ways: Many put on extensive layers of fat during seasons of abundance that will at least partially carry them through seasons of scarcity. Some migrate. In Africa, herds of wildebeests and zebra are constantly moving to areas where food is available.The same is true of caribou and reindeer in the Arctic and was true of bison in the Great Plains of North America. Other herbivores hibernate or estivate, lowering their food requirements, during seasons of hardship. Finally, some herbivores respond to seasons of scarcity by making do with foods of relatively low nutritional value. Many nonmigratory songbirds in regions with cold winters switch diets from primarily insects to primarily seeds. Other herbivores, especially mammals, survive for months on nothing more than dead grass or leaves. Their diet is almost exclusively cellulose.To exploit these foods, they need the assis- tance of other organisms living in their intestinal tract—a type of interaction between organisms we will discuss more fully later. Plants respond to herbivores. One of the most common responses is simply to pro- duce enough tissue to feed herbivores and still survive. Many grasses, for example, seem to have an unlimited ability to grow new tissue. Graze (or mow) the grass and it grows right back.There are limits, of course, but grasses sustain heavy grazing and still survive. So, too, do many trees and bushes by replacing lost leaves. Many plants are less passive in their responses to herbivory.When faced with intense herbivore pressure, many “fight” back.A typical scenario might be as follows:A herbi- vore begins to feed on a plant’s leaves.In response either to the release of chemicals from damaged plant cells or herbivore saliva, the plant produces hormones that stimulate other leaves to produce chemicals that deter further feeding (Figure 15-21). Some such chemicals are distasteful and avoided by herbivores. Thus, certain willows discourage feeding by snowshoe hares. Other deterrent chemicals affect the herbivore’s physiology. Some, for example, interact with insect hormones, especially those of larvae, to slow or stop growth. Slowed down insect larvae are more susceptible to predators or may fail to develop into adults before winter. Some deterrent chemicals are downright toxic to herbivores: Chemicals in the leaves of poison ivy and the sap of milkweed are examples. Of course, evolution being what it is, herbivores respond in turn.Any herbivore that can tolerate a particular plant species’ defenses will thrive.Thus, a number of birds feed on poison ivy berries, and monarch butterfly larvae feed on milkweed with impunity. Not only can they tolerate the highly alkaloid milkweed sap, but they accumulate it in their own tissues,thereby becoming distasteful (and toxic) to their predators.Remarkably,they retain the alkaloids as adult butterflies and continue to fend off predators.
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