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.
Herbivores and Their Food Plants
Bison, sheep, and other grazers - Succulent forbs, grasses,
Deer and other ungulate browsers - Leaves and twigs of woody plants such as willows, arborvitaes, yews
Beaver - Tree bark, young shoots, leaves
Rodents - Succulent forbs, grasses, grass-like plants
Rabbits - Succulent forbs, grasses, bark
Voles - Roots, bark
Caterpillars - Leaves; in some cases, of specific species
Monarch butterfly - Milkweeds
Gypsy moth - Oaks and other hardwoods
Aphids - Plant juices; in some cases, of specific species
Many birds - Seeds and fruits
Locusts - All plants; seeds, leaves, and stems
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. 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.
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 plant is an example of mutualism.
All is not well in the realm of pollinators. The age-old relationships between plants and pollinators is threatened, especially in urbanized and agricultural regions. Habitat destruction and fragmentation, pesticide abuse, and disease all have taken their toll on pollinators.
As more land is cleared for human habitation, bees, butterflies, bats, and birds are left homeless. Our gardens offer little to sustain them. They need a constant source of nectar and pollen throughout the entire season. The few flowering plants most people grow will not suffice.
A related problem is fragmentation of plant communities. Plants must be pollinated in order to set seed for the next generation. Without pollinators, no seed is set and the plants eventually die out, leading to local extinction. Isolated patches of forest, grassland, or desert are particularly vulnerable. A small patch may not sustain enough pollinators, or may be too far from other patches for pollinators to travel. As a result, plants do not reproduce.
Pesticides have also reduced pollinator populations. Bees are often killed by chemicals applied to eliminate other pests. Honeybees are being destroyed by diseases and parasitic mites. The crisis is not just affecting native ecosystems. Fruit trees and many other food crops depend on pollination for production. We stand to lose over three quarters of our edible crops if we lose pollinators.
What can be done? Encourage pollinators by planting a diverse mixture of adult and larval food plants in your garden. Erect bat and bird houses, as well as bee hives. Reduce or eliminate pesticide use. Help restore native plant communities not only in your yard, but also in parks and along roadways, and connect them through corridors to preserves and other natural areas.
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.
Some Animals and the Plants They Disperse
Ants - Many wildflowers, such as trilliums, bloodroot, violets
Birds - Fleshy fruits and grains, such as baneberry, viburnums, mountain ash
Clark's Nutcracker - Whitebark pine
Woodpeckers - Poison Ivy
Mammals - Fruits, grains, nuts, berries
Squirrel - Nuts, such as those of oaks, hickories, pines
Fox - Berries, such as blackberry, grapes
Humans - Weeds such as plantain, dandelion, lamb's-quarters
Reptiles - Fleshy fruits, especially berries such as strawberry, groundcherry, jack-in-the-pulpit
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. 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.
The Lovely Lady-slipper
The reason lady-slipper orchids are so hard to grow in a garden is that the needs of both the orchid and its fungus must be attended to. The growing conditions in the garden must duplicate exactly those in the orchid's native habitat.
Anyone who tries to cultivate these beautiful plants learns before long that the pink lady-slipper (Cypripedium acaule) is much harder to grow than the yellow lady-slipper (Cypripedium calceolus). This is because of the fungus. Yellow lady-slippers grow in slightly acidic, rich soils. Their associated mycorrhizal fungus thrives under the same conditions as those in woodland and shade gardens.
The pink lady-slipper, on the other hand, grows in sterile, acid soil, not the typical garden variety. Plant the pink lady-slipper in rich garden soil, and its associated fungus cannot survive. As a result, the pink lady-slipper slowly languishes and eventually dies. Most lady-slipper orchids are still collected from the wild, harming native populations. Buy them only from nurseries that propagate their plants.