Ecology is the study of interactions of organisms with one another as well as with their environment. Plants, with their sedentary existence and need to attract pollinators or prevent herbivores from consuming them whole (because they can't run away from them), have evolved a different set of behavior patterns than have animals.
Competition results when an individual plant interferes with the needs of another plant for the same environmental resource (such as light, minerals, space) or when members of one population interfere with members of another for the same environmental resource. In plants, competition generally is indirect, through the resource, not direct, one‐on‐one (plants don't engage in leaf‐to‐leaf combat). Plants with the same life form and growth requirements are often in competition but surviving in slightly different microenvironments. This generally leads to a better utilization of the resource and, with natural selection in operation over time, a greater diversification of the community.
Allelopathy is a particular form of direct competition in which one plant species (or a fungus like Penicillium) produces a substance toxic to another. In some instances, the substance inhibits the development of the producer's own seeds or spores. The compounds may leach from the roots into the soil or accumulate in the ground around the plant as leaves drop and decay. Some are terpenes that volatilize and are spread through the air as aerosols. The essential oils of members of the mint family are toxic to numerous plants, as is the oil of black walnuts. Caffeine produced by tea and coffee plants inhibits the growth of seedlings of many species.
Chemical warfare of another kind is waged by plants that produce secondary metabolites—chemical substances that protect the plants from being eaten by herbivores. Plants and their predators undoubtedly coevolved, with changes in one instigating reactions and further evolutionary changes in both.
Some of the metabolites are not merely deterrents, but are chemicals that imitate hormones, enzymes, or other essential compounds of animal bodies. One metabolite interferes with insect metabolism by inhibiting the juvenile growth hormone. Others, like the alkaloids morphine and cocaine, affect the human nervous system; and caffeine, although a stimulant to humans, in plants is toxic and lethal to insects and fungi. The estrogens produced by some plants have no known role in the plants, but their importance to human reproduction is well known—and a cause for concern when humans eat vegetables.
Defense substances of a different kind protect plants from bacteria and fungi attacks. These substances, called phytoalexins, act as natural antibiotics and protect the plant from bacteria and fungal pathogens when leaves are damaged or stems wounded. Nicotine in tobacco plants is synthesized in response to wounding.
In a symbiosis, two different kinds of organisms live together in an intimate and more or less permanent relationship. Lichens are the classic example of a symbiosis between a fungus and a cyanobacterium or an alga. Mycorrhizae, too, are examples of fungi and the root cells of vascular plants in a symbiosis. If the interactions between the symbionts are of mutual benefit, the symbiosis is termed a mutualism; if one partner benefits and the relationship is of no significance to the other, it is a commensalism; parasitism is a symbiosis in which one partner benefits and the other is harmed.
Mutualism. Seed plants have developed all manner of mutualisms, the most highly developed being the interactions between insects, birds, bats, and a few other animals that ensure pollination of flowers, especially by cross‐fertilization. Pollinators are attracted to the flowers by colors, scents, and nectars and once on‐site, all manner of structural floral adaptations insure the pollinator gets a dusting of pollen to take to the next flower it visits. The pollinator gets food, and the plant gets a messenger service more effective than chance winds.
Seed and fruit dispersal mechanisms also are well‐developed, co‐evolved mutualisms. Succulent edible fruits with their scents and colors are great dispersal devices geared to larger animals and often found on plants that produce seeds with hard seed coats. The coat may be so difficult for water to penetrate that germination is not possible unless some mechanical abrasion or chemical solvent is applied. The gizzard of birds is an effective grinder, and the stomach acids of mammals take off much of the seed coat before hard‐coated seeds are expelled in the feces.
Parasitism. Bacteria, viruses, and fungi have not spared the plants as hosts for their parasitic lifestyle nor have vascular plants that parasitize other vascular plants. The lines among mutualism, commensalism, and parasitism are often blurred because the definitions are based on value judgments, that is, on the degrees of harm or benefit to the symbionts. About 3,000 species of vascular plant parasites are worldwide in their distribution. Some of these have lost the ability to photosynthesize entirely, but others attach to the vascular system of their hosts and divert the water and minerals in transit to their own photosynthesis.