Harmony and Conflict in the Living World - Alexander F. Skutch [44]
Exploitation of Animals by Plants
Animals, which exploit plants on a vast scale, are rarely exploited by them. The carnivorous flowering plants, which by traps and pitfalls the most diverse capture and digest insects and other small
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invertebrates, and occasionally a diminutive vertebrate, are a very minor element of the flora that deserves a separate chapter. A family of fungi, the Entomophthoraceae, contains numerous species that infest the tissues of flies, caterpillars, and other small creatures, killing and devouring them. Other fungi exploit larger animals, including humans, causing irritating skin infections and more serious diseases.
Exploitation of Plants by Animals
The only animals that do not exploit plants directly are the carnivores or insectivores that do so indirectly, by devouring creatures directly nourished by plants; or sometimes, in long food chains, nutrients obtained directly from plants pass through diverse animal bodies before they reach the ultimate predator. Recent investigations of the upper levels of tropical rain forests suggest that the number of insect species may run into millions, and a large proportion of them devour the tissues of living plants. Most conspicuous is their damage to foliage; if they do not defoliate a whole tree, they may leave few intact leaves upon it. Swarms of locusts may devastate a wide area. Less noticeable but almost equally damaging to plants are the depredations of a host of small, seed-eating beetles that may ruin the seed crop of a tree. The relation of butterflies to plants is ambivalent: caterpillars devour much foliage; the winged adults into which they metamorphose make some amends by serving as pollinators, although often not of plants that nourished their larval stage, as would be just.
Vertebrate animals that exploit plants unilaterally include the grazers, the browsers, and the seed eaters. Horses and their kin are most at home on wide, open grasslands, over which they roam in small or large herds. Over the ages, typical grazers and their food plants have coevolved, not in a direct, one-species-to-one-species pattern but in a more diffuse manner; many of the grazers involved in this secular interaction have disappeared from Earth, and perhaps some of the grasses that nourished them have also become extinct. By the basal growth of their leaves, grasses are as well adapted to support grazing that is not excessive as, by dentition
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and digestive system, the grazers are to crop and digest them. By preventing the accumulation of dead blades that reduce photosynthesis and invite fires, moderate grazing appears to benefit well-adapted grasses.
Browsers gather foliage, often together with branches and flowers, above the ground. Probably the most highly specialized of extant browsers is the giraffe, whose long neck, with associated modifications of the vascular system, enable it to reach green shoots too high for all its four-footed competitors. Although primarily grazers, horses are often tempted to browse upon the foliage of shrubs or trees within their reach. Cows browse as readily as they graze, as do a number of kinds of antelope and deer. At higher levels, sloths and monkeys browse upon the foliage of a variety of trees and vines, which for some species, like howler monkeys, is a principal food. Fairly large birds, such as grouse in northern woodlands and guans in tropical American forests, browse freely on foliage and buds, often high in trees. Among reptiles, iguana lizards eat leaves. Grazing and browsing mammals serve the plants among which they move by dispersing seeds that cling to their hair by means of hooks or sticky secretions. Many of these animals vary their diet of green herbage when they find tasty fruits, with seeds that may pass unharmed through their alimentary canals and thus be widely spread.