Plants and animals evolved together, so it is not surprising that there are many complex plant/animal relationships. This process of interdependent evolution of two or more species is called coevolution. Some relationships are beneficial to both parties, while others have a clear benefit for one at the expense, or even death, of the other. Four important plant/animal interactions are explored here: plant/herbivore, plant/pollinator, plant/disperser, and other examples of mutualism.
Herbivory is an interaction in which a plant or portions of the plant are consumed by an animal. At the microscopic scale, herbivory includes the bacteria and fungi that cause disease as they feed on plant tissue. Microbes that break down dead plant tissue are also specialized herbivores. Browsers and grazers, from aphids and caterpillars to deer and bison, are more familiar herbivores. Even insects and animals that eat seeds are considered herbivores.
Some herbivores consume entire plants, or enough to kill them. Others only eat a portion of the plant, and so the plant can recover. The plant/herbivore relationship traditionally has been seen as lopsided, with the animal as the beneficiary and the plant as the loser. Current research, however, is revealing that herbivory has some potential benefits to plants. One example is canopy grazing by insects, which allows more light to penetrate into the lower layers of the forest. Gypsy moth grazing on canopy trees in some areas of Virginia's Blue Ridge Mountains, for instance, has resulted in more light penetration and therefore a more diverse and productive ground layer.
Plants and Their Pollinators
Pollination is the transfer of the pollen from one flower to the stigma, or female reproductive organ, of another, which results in fertilization and, ultimately, the formation of seeds (see Plant Reproduction). The earliest plants were pollinated by wind, and for some modern plants this is still the most expedient method. Many trees, all grasses, and plants with inconspicuous flowers are designed for wind pollination. Bright, showy flowers evolved for another purpose - to attract a pollinator.
Many plants depend on animals for pollination. Insects, birds, even bats are important for perpetuating plants. The flowers of these plants evolved in concert with their pollinators, and their form reflects the form and habits of their pollinators. Bee-pollinated plants are often irregular in shape, with a lip that acts as a landing pad to facilitate the bee's entry into the flower. Butterfly-pollinated flowers are often broad and flat, like helicopter pads. The flowers of many plants are brightly colored to attract their insect pollinators, and many offer nectar as an enticement. Hummingbirds, with their long beaks, pollinate tubular flowers. Bats require open flowers with room for their wings, such as those of the saguaro cactus. In the tropics, birds and bats take the place of insects as pollinators. Hummingbirds and honeycreepers, for example, have distinctive beaks that have evolved to exploit flowers. Often, a beak may be so specialized that it is only effective on a small group of flowers. (For more on plants and their pollinators, see Plant Reproduction)
The pollinators, in turn, have evolved to take advantage of the flowers. A successful pollinator typically has good color vision, a good memory for finding flowers, and a proboscis, or tongue, for attaining nectar.
Animal pollination has obvious advantages for plants. Many pollinators cover great distances, which insures genetic diversity through outcrossing, or the transfer of pollen to unrelated individuals. The pollinator benefits as well by gaining access to a source of food. The relationship of pollinator and plant is an example of mutualism.
Plants and Their Dispersers
No two plants can occupy the same spot. In order to have room to grow, seeds must be dispersed away from the parent plant. Seed dispersal is accomplished by a variety of means, including wind, water, and animals. Animal dispersal is accomplished by two different methods: ingestion and hitch-hiking. Animals consume a wide variety of fruits, and in so doing disperse the seeds in their droppings. Many seeds benefit not only from the dispersal, but the trip through the intestine as well. Digestive acids scarify seeds, helping them to break out of thick seed coats.
Some seeds are armed with hooks and barbs that enable them to lodge in the fur of animals that brush past them. Beggar's ticks and bur marigold are two examples. Eventually, the seeds are rubbed or scratched off, and may find a suitable spot on which to germinate and grow. People are important for dispersing plants, too. The common weed plantain was called "white man's footsteps" by Native Americans because wherever settlers walked, the plantain came in the mud on their shoes.
Mutualism is an obligate interaction between organisms that requires contributions from both organisms and in which both benefit. There are many examples in nature. Pollination and dispersal, discussed above, are mutualistic because both plant and pollinator or disperser benefit from the relationship. The relationship between mycorrhizal fungi and many higher plants is another common example of mutualism. The bodies of the fungi, called hyphae, live on or in the tissues of plants, and make nutrients available for the plants to absorb. The plants provide the fungi with amino acids and other complex compounds. One of the most celebrated examples is the orchids. Whereas some plants may support as many as 100 different fungi, orchids have quite specific mycorrhizal associations (see "The Lovely Lady Slipper"). Different plant communities have different mycorrhizal associations. The microflora of a grassland is different from that of a forest. These differences, at least in part, may influence the distribution of plant communities.