Contemporary ecologists have gone beyond the purely descriptive observations of plant-animal interactions (initially within the realm of natural history) and have designed controlled experi ments that are crucial to the development of such basic concepts as coevolution. For example, the use of radioactive isotopes and the marking of pollen with dye and fluorescent material in field settings have allowed ecologists to demonstrate precise distances and patterns of pollen dispersal. Ecologists and insect physiologists have cooperatively studied how certain insects, such as bees, are sensitive to ultraviolet light. When some flowers are viewed under ultraviolet light, distinct floral patterns become evident to guide these insects to nectar pollen sources. Through basic research, Carolyn Dickerman reported in 1986 that animal color preferences vary throughout the season. Insect pollinators, who must feed every day, will adapt to these changes by shifting their foraging behavior. Research in the field has demonstrated that some species of flowers, such as the scarlet gilia, will produce differently colored flowers to accommodate shifts in pollinator species. Early in the growing season, this plant will produce long, red, tubular-shaped flowers to attract hummingbirds. As the hummingbirds migrate, the flowers will later become lighter in hue and be pollinated primarily by nocturnal hawk-moths.
In the laboratory, ecologists and biochemists have cooperatively analyzed the chemical composition of plant secretions and products. The chemical analysis of nectar indicates great variation in composition, correlating with the type of pollinator. Flowers pollinated by beetles generally have high amino acid content. The nectar associated with hummingbird-pollinated flowers is rich in sugar. Pollen also varies widely in chemical composition within plant species. Oils and waxes are major chemical products in the pollen of plants visited primarily by bees and flies. For bat-pollinated flowers, the protein content is quite high.
Research has also successfully analyzed how certain plants have been able to develop toxins as chemical defenses against animals. These protective devices include such poisons as nicotine and rotenone that help prevent insect and small mammal attacks. A more remarkable group of protective compounds recently isolated from some plants are known as juvocimines. These chemicals actually mimic juvenile insect hormones. Insect larvae feeding on leaves containing juvocimines are prevented from undergoing their normal development into functional, breeding adults. Thus, a specific insect population that could cause extensive plant damage is locally reduced.
Ecological interactions between plants and animals are diverse and varied. These plant-animal interactions can be viewed as absolute necessities for developing food chains and food webs and for maintaining the global balances of such important gases as oxygen and carbon dioxide. The interactions can also be very precise, limited, and crucial for determining species survival or extinction. By analyzing varied plant-animal interactions, from the microscopic level to the global perspective, one can more fully appreciate all the ecological relationships that exist on the earth.
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