Plant and animal interactions

Ecology represents the organized body of knowledge concerning the interrelationships of living organisms and their nonliving environments. Increasingly, the realm of ecology involves a systematic analysis of plant-animal interactions through the considerations of nutrient flow in food chains and food webs, exchange of such important gases as oxygen and carbon dioxide between plants and animals, and strategies of mutual survival between plant and animal species through pollination and seed dispersal.

Ecologists study both abiotic and biotic features of such plant and animal interactions. The abiotic aspects of any environment consist of nonliving, physical variables, such as temperature and moisture, that determine where species can survive and reproduce. The biotic (living) environment includes all other plants, animals, and microorganisms with which a particular species interacts. Certainly, two examples of plant and animal interactions involve the continual processes of photosynthesis and cellular respiration. Green plants are often classified as ecological producers and have the unique ability to carry out both these important chemical reactions. Animals, for the most part, can act only as consumers, taking the products of photosynthesis and chemically releasing them at the cellular level to produce energy for all life activities.

Plant-Animal Mutualism

One topic that has captured the attention of ecologists involves the phenomenon of mutualism, in which two different species of organisms beneficially reside together in close association, usually revolving around nutritional needs. One such example demonstrating a plant and animal association is a certain species of small aquatic flatworm that absorbs microscopic green plants called algae into its tissues. The benefit to the animal is one of added food supply; the adaptation to this alga has been so complete that the flatworm does not actively feed as an adult. The algae, in turn, receive adequate supplies of nitrogen and carbon dioxide and are literally transported throughout tidal flats in marine habitats as the flatworm migrates, thus exposing the algae favorably to increased sunlight. Ecologists report a similar example of mutualism in various reef-building corals, which are actually marine, colonial animals that grow single-celled green algae called zooxanthellae within their bodies. The coral organisms use the nutrients produced by these algae as additional energy supplies, enabling them to build the massive coral reefs associated with tropical waters more easily. In 1987, William B. Rudman reported a similar situation while researching the formation of such coral reefs in East African coastal waters. He discovered a type of sea slug called a nudibranch that absorbs green algae into its transparent digestive tract, producing an excellent camouflage as it moves about on the coral reefs in search of prey. In turn, the algae growing within both the coral and sea slugs receive important gases from these organisms for their own life necessities.

An example of plant-animal mutualism that has been documented as a classic example of coevolution involves the yucca plant and a species of small, white moth common throughout the southwestern United States. The concept of coevolution builds upon Charles Darwin’s theories of natural selection, reported in 1859, and describes situations in which two decidedly different organisms have evolved into a close ecological relationship characterized by compatible structures in both. Thus, coevolution is a mutualistic relationship between two different species that, because of natural selection, have become intimately interdependent. The yucca plant and yucca moth reflect such a relationship. The female moth collects pollen grains bearing sperm cells from the stamens of one flower on the plant and transports these pollen loads to the pistil of another flower, thereby ensuring cross-pollination and fertilization. During this process, the moth will lay her own fertilized eggs in the flower’s undeveloped seed pods. The developing moth larvae have a secure residence for growth and a steady food supply. These larvae will rarely consume all the developing plant seeds; thus, both species (plant and animal) benefit.

Defensive Mutualism

Although these examples demonstrate the evolution of structures and secretions that reflect mutual associations between plants and animals, other interactions are not so self-supportive. Plant-eating animals, called herbivores, have always been able to consume large quantities of green plants with little fear of reprisal. Yet, some types of carnivorous plants have evolved to capture and digest small insects and crustaceans as nutritional supplements for their normal photosynthetic activities. Many of these plants grow abundantly in marshlike environments, such as bogs and swamps, where many insects congregate to reproduce. Such well-known plants as the Venus’s-flytrap, sundews, butterworts, and pitcher plants have modified stems and leaves to capture and consume insects and spiders rich in protein. On a smaller scale, in freshwater ponds and lakes, a submerged green plant commonly known as the bladderwort partially satisfies its protein requirements by snaring and digesting small crustaceans, such as the water flea, within its modified leaves.

A form of ecological interaction commonly classified as mimicry can be found worldwide in diverse environments. In such situations, an animal or plant has evolved structures or behavior patterns that allow it to mimic either its surroundings or another organism as a defensive or offensive strategy. Certain types of insects, such as leafhoppers, walking sticks, praying mantids, and katydids (a type of grasshopper), often duplicate plant structures in environments ranging from the tropical rainforests to the northern coniferous forests of the United States. Such exact mimicry of their plant hosts affords these insects protection from their predators as well as camouflage that enables them to capture their own prey readily. In other examples of mimicry, some insects will absorb unpalatable plant substances in their larval stages and retain these chemicals in their adult forms, making them undesirable to birds as food sources. The monarch butterfly demonstrates this type of interaction with the milkweed plant. The viceroy butterfly has evolved colorations and markings like those of the monarch, thereby ensuring its own survival against bird predators. Certain species of ambush bugs and crab spiders have evolved coloration patterns that allow them to hide within flower heads of such common plants as goldenrod, enabling them to grasp more securely the bees and flies that visit these flowers.

Nonsymbiotic Mutualism

Many ecologists have been studying the phenomenon known as nonsymbiotic mutualism: different plants and animals that have coevolved morphological structures and behavior patterns by which they benefit each other without necessarily living together physically. Plant-pollinator mutualisms are demonstrated in the unusual and bizarre shapes, patterns, and colorations that flowering plants have developed to attract various insects, birds, and small mammals for pollination and seed dispersal purposes. Around 170,000 plant species and 200,000 animal species engage in this type of mutualism, which is critical for Earth's ecosystems. Pollination essentially is the transfer of pollen grains (and their enclosed sperm cells) from the male portion of a flower to the egg cells within the female portion of a flower. Pollination can be accomplished by the wind, by heavy dew or rains, or by animals, and it results in the plant’s sexual production of seeds that represent the next generation of new embryo plants. Accessory structures, called fruits, often form around seeds and are usually tasty and brightly marked to attract animals for seed dispersal. Although the fruits themselves become biological bribes for animals to consume, often the seeds within these fruits are not easily digested and thus pass through the animals’ digestive tracts unharmed, sometimes great distances from the original plant. Other types of seed dispersal mechanisms involve the evolution of hooks, barbs, and sticky substances on seeds that enable them to be easily transported by animals’ fur, feet, feathers, or beaks to new regions for possible plant colonization. Such strategies of dispersal reduce competition between the parent plant and its new seedlings for moisture, living space, and nutritional requirements.

The evolution of flowering plants and their resulting use of animals in pollination and seed dispersal probably began in dense, tropical rainforests, where pollination by the wind would be cumbersome. Because insects are the most abundant form of animal life in rainforests, strategies based upon insect transport of pollen probably originated there. Because structural specialization increases the possibility that a flower’s pollen will be transferred to a plant of the same species, many plants have evolved a vast array of scents, colors, and nutritional products to attract many insects, some birds, and a few mammals. Not only does pollen include the plant’s sperm cells, but it is also rich in food for these animals. Another source of animal nutrition is a substance called nectar, a sugar-rich fluid often produced in specialized structures called nectaries within the flower itself or on adjacent stems and leaves. Plants also produce assorted waxes and oils to ensure plant-animal interactions. As species of bees, flies, wasps, butterflies, and hawkmoths are attracted to flower heads for these nutritional rewards, they unwittingly become agents of pollination by transferring pollen from male portions of flowers (stamens) to the appropriate female portions (pistils). Some flowers have evolved distinctive, unpleasant odors reminiscent of rotting flesh or feces, thereby attracting carrion beetles and flesh flies in search of places to reproduce and deposit their own fertilized eggs. As these animals consummate their own relationships, they often become agents of pollination for the plant itself. Some tropical plants, such as orchids, even mimic a female bee, wasp, or beetle so that its male counterpart will attempt to mate with it, thereby ensuring precise pollination.

Among the bird species, hummingbirds are probably the best examples of plant pollinators. Various types of flowers with bright, red colors, tubular shapes, and strong, sweet odors have evolved in tropical and temperate regions to take advantage of the hummingbirds’ long beaks and tongues as an aid to pollination.

Because most mammals, such as small rodents and bats, do not detect colors as well as bees and butterflies do, flowers that use them as pollinators do not usually rely upon color cues in their petals but, instead, focus on the production of strong, fermenting, or fruitlike odors and abundant pollen rich in protein to attract them. In certain environments, bats and mice that are primarily nocturnal have replaced day-flying insects and birds in these important interactions between plants and animals.

Experiments with Plant-Animal Interaction

Contemporary ecologists have gone beyond the purely descriptive observations of plant-animal interactions (initially within the realm of natural history) and have designed controlled experiments 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 hawkmoths.

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.

In a study investigating the potential existence of an antagonistic plant-animal relationship built on competition, researchers collected data on the carnivorous sundew plant, sheet‐web and ground‐running spiders, and their common prey, springtails. The data revealed that in areas with relatively low populations of springtails, sundew populations were higher. Also, in these areas, spiders were less commonly found than in areas with fewer sundews, and the spiders living near large sundew populations were smaller than those in other areas. Spiders living away from sundew populations experienced less competition from the plants and had less springtail activity and, therefore, fewer feeding opportunities. This finding suggests an asymmetrical competition between carnivorous plants and spiders.

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 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 are critical for developing food chains and food webs and for maintaining the global balance 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 on Earth.

The Ecology of Interaction

The ecological importance of plant-animal interactions cannot be stressed enough. Modern-day agriculture owes its existence to the activities of such insect pollinators as honeybees regarding the production of domestic fruits, vegetables, and honey. It is increasingly evident to many ecologists and forestry scientists how important certain bird species, such as blue jays and cedar waxwings, are in the natural reforestation of burned and blighted areas through their seed dispersal strategies. The plant horticulture and floral industries are also developing an appreciation of specific plant-animal interactions that produce more viable natural strains of flowers and ornamental shrubbery. The study of natural chemical defenses produced by some plants against animal invasions is most promising. The renewed interest in earlier efforts to extract such plant products as nicotine, rotenone, pyrethrum, and caffeine may produce natural compounds that can be effective insecticides without the long-term environmental hazards associated with such human-made pesticides as malathion, chlordane, and Dichloro-diphenyl-trichloroethane (DDT).

Humankind is realizing that it is essential to understand and protect plant-animal interactions and their environments, particularly Earth's tropical regions, to maintain the global balance of oxygen and carbon dioxide, biodiversity, and support emerging or endangered species. These tropical areas represent the last natural environments for the continuation of important plant species that produce secretions and products with favorable medicinal qualities for humans and domestic livestock. By maintaining these populations and understanding how certain animals interact with them, humans can be guaranteed a viable supply of beneficial plant species whose medicinal values can be duplicated within the laboratory.

Principal Terms

Cellular Respiration: The release of energy in organisms at the cell level, primarily using oxygen

Chlorophyll: One of several forms of photoactive green pigments in plant cells that is necessary for photosynthesis to occur

Coevolution: A mutualistic relationship between two different organisms in which, because of natural selection, the organisms become interdependent

Cross-Pollination: The transfer of pollen grains and their enclosed sperm cells from the male portion of a flower to a female portion of another flower within the same species

Food Chain: A diagram illustrating the movement of food materials from green plants (producers) through various levels of animals (consumers) within natural environments

Natural Selection: The survival of variant types of organisms because of adaptability to environmental stresses

Pistil: A female portion of a flower that produces unfertilized egg cells

Stamen: A male portion of a flower that produces pollen grains and their enclosed sperm cells

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