Plant: Ecology
Cooperation and Competition
Many plant species exist as separate male and female plants, and pollen from male flowers must reach the female flowers in order for pollination and seed development to take place. The agent of pollination is sometimes the wind (a part of the physical environment), but in many cases it is an insect, bat, or bird (members of the biological environment). Plants may also rely on agents for dispersing their seed. Thus, after pollination, cherry trees develop cherries that attract birds, which ingest the fruit and excrete the cherry stones in more distant terrains.
Plants have evolved many other mutually beneficial relationships, such as the nitrogen-fixing bacteria that occur in the nodules on the roots of legumes (see Nitrogen Fixation). Many prairie grasses and other plants that flourish on open land depend on various herbivores to keep forests from closing in and shading them.
In the competition among plants for light, many species have evolved such mechanisms as leaf shape, crown shape, and increased height in order to intercept the sun's rays. In addition, many plants produce chemical substances that inhibit the germination or establishment of seeds of other species near them, thus excluding competing species from mineral resources as well as light. Walnut species, for example, use such an allelopathy, or chemical inhibition.
The Food Web
Because plants are autotrophs—organisms that are able to manufacture their own food—they lie at the very foundation of the food web. Heterotrophs—organisms that cannot manufacture their own food—usually lead less sedentary lives than plants, but they ultimately depend on autotrophs as sources of food. Plants are first fed upon by primary consumers, or herbivores, which in turn are fed upon by secondary consumers, or carnivores. Decomposers act upon all levels of the food web. A large portion of energy is lost at each step in the food web; only about 10 percent of the energy in one level is stored by the next. Thus, most food webs contain only a few steps.
Plants and Humans
From the prehistoric beginnings of agriculture until recent times, only a few of the total plant species have been taken from the wild and refined to become primary sources of food, fiber, shelter, and drugs. This process of plant cultivation and breeding began largely by accident, possibly as the seeds of wild fruits and vegetables, gathered near human habitations, sprouted and were crudely cultivated. Plants such as wheat, which possibly originated in the eastern Mediterranean region more than 9,000 years ago, were selected and replanted year after year for their superior food value; today many domesticated plants can scarcely be traced back to their wild ancestors or to the original plant communities in which they originated. This selective process took place with no prior knowledge of plant breeding but, rather, through the constant and close familiarity that preindustrial humans had with plants.
See also Dicots; Diseases of Plants; Fruit; Monocots; Nut; Plant Propagation; and Poisonous Plants.
