Showing posts with label animal-plant interactions. Show all posts
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7:44 PM
Animal-plant Interactions
Animal-plant Interactions
The ways in which certain animals and plants interact have evolved in some cases to make them interdependent for nutrition, respiration, reproduction, or other aspects of survival.

Ecology represents the organized body of knowledge that deals with the relationships between 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 the processes of pollination and seed dispersal.

A major example of animal-plant interactions involve the continual processes of photosynthesis and cellular respiration. Green plants are classified as ecological producers, having the unique ability, by photosynthesis, to take carbon dioxide and incorporate it into organic molecules.

Animals are classified as consumers, taking the products of photosynthesis and chemically breaking them down at the cellular level to produce energy for life activities. Carbon dioxide is a waste product of this process.


Mutualism

Mutualism is an ecological interaction in which two different species of organisms beneficially reside together in close association, usually revolving around nutritional needs.

One such example is a small aquatic flatworm that absorbs microscopic green algae into its tissues. The benefit to the animal is one of added food supply. The mutual adaptation is 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 to increased sunlight. This type of mutualism, which verges on parasitism, is called symbiosis.

Coevolution

Coevolution is an evolutionary process wherein two organisms interact so closely that they evolve together in response to shared or antagonistic selection pressure. A classic example of coevolution involves the yucca plant and a species of small, white moth (Tegitecula).

The female moth collects pollen grains from the stamen 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 flowers’ 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 seeds; thus, both species (plant and animal) benefit.

Although this example represents a mutually positive relationship between plants and animals, other interactions are more antagonistic. Predator-prey relationships between plants and animals are common. Insects and larger herbivores consume large amounts of plant material. In response to this selection pressure, many plants have evolved secondary metabolites that make their tissues unpalatable, distasteful, or even poisonous. In response, herbivores have evolved ways to neutralize these plant defenses.

Mimicry and Nonsymbiotic Mutualism

Mimicry
Mimicry
Inmimicry, an animal or plant has evolved structures or behavior patterns that allow it tomimic either its surroundings or another organism as a defensive or offensive strategy.

Certain types of insects, such as the leaf hopper, walking stick, praying mantis, and katydid (a type of grasshopper), often duplicate plant structures in environments ranging from tropical rain forests to northern coniferous forests. Mimicry of their plant hosts affords these insects protection from their own predators as well as camouflage that enables them to capture their own prey readily.

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 ambush the insects that visit these flowers.

In nonsymbiotic mutualism, plants and animals coevolve morphological structures and behavior patterns bywhich they benefit each other but without living physically together.

This type of mutualism can be demonstrated in the often unusual shapes, patterns, and colorations that more advanced flowering plants have developed to attract various insects, birds, andmammals for pollination and seed dispersal purposes. Accessory structures, called fruits, 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 parent plant. Some seeds must pass through the digestive plant of an animal to stimulate germination.

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 animal fur, feet, feathers, or beaks. Such strategies of dispersal reduce competition be- tween the parent plant and its offspring.

Pollinators

Pollinators
Pollinators
Because structural specialization increases the possibility that a flower’s pollenwill be transferred to a plant of the same species, many plants have evolved a vast array of scents, colors, and nutritional products to attract pollinators. Not only does pollen include the plant’s spermcells; it also represents a food reward.

Another source of animal nutrition is a substance called nectar, a sugar-rich fluid produced in specialized structures called nectaries within the flower or on adjacent stems and leaves.

Assorted waxes and oils are also produced by plants to ensure plant-animal interactions. As species of bees, flies, wasps, butterflies, and hawk-moths are attracted to flower heads for these nutritional rewards, they unwittingly become agents of pollination by transferring pollen from stamens to 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 copulate, 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 the insect’s male counterpart will attempt to mate with them, thereby encouraging precise pollination.

Among birds, hummingbirds are 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 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, some flowers instead focus upon the production of strong, fermenting, or fruit-like odors and abundant pollen rich in protein. In certain environments, bats and mice that are primarily nocturnal have replaced day-flying insects and birds as pollinators.
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9:27 PM
Biochemical Coevolution in Angiosperms
Biochemical Coevolution in Angiosperms
Flowering plants, or angiosperms, produce many compounds that are not directly related to growth and development. These secondary metabolites arise from primary metabolic pathways and act as antiherbivory mechanisms, allelochemicals, or attractants.

Secondary metabolites are biochemicals produced by plants in response to selection pressures. These pressures may be from herbivory, competition, or the need for pollination.

As plants produce compounds to enhance their survival, predators, competitors, and pollinators react and evolve means of adjusting to the plant’s efforts. Chemically simple secondary metabolites may be widespread throughout angiosperm (flowering plant) families, whereas more complex chemicals are often restricted to a single species.

Secondary metabolites are often under high selection pressures, causing individual compounds to have very limited distributions and making them useful in determining the evolutionary relationships between taxonomic groups. Presence of secondary compounds influences the activities of organisms interacting with the plants and, over long periods of time, influences evolution of those species.

Antiherbivory Mechanisms

Antiherbivory chemicals may have a wide range of effects on herbivores (plant-eating animals). Many compounds merely deter grazing. Crystals produced from calcium oxalate (raphides) may be ejected from the vacuoles of cells along with proteinaceous toxins, causing tissue swelling in the mouth of an offending herbivore.

Many monocotyledonous plant families, such as Liliaceae, Heliconiaceae, Rubiaceae, and Arecaceae, produce this type of antiforaging device. This type of defense is especially notable in young tissues that have not developed the toughness found inmature leaves as a herbivory deterrent. Red oaks produce tannins in response to gypsy moth attacks, reducing further herbivory.

Continued feeding on plants containing tannins would lead to slow starvation of a herbivore, as its digestive system could not absorb proteins. The same tannins are not deterrents to squirrels. Squirrels harvest acorns and bury them for later consumption, providing a food source for the squirrel and a dispersal mechanism for the acorn.

Beavers provide another example of the interaction between plant and animal evolution. Some of the beaver’s preferred foods include species that are unpalatable or toxic to other mammals, such as bracken fern, nettles, thistles, and skunk cabbage. This gives the beaver a largely uncontested food source that may involve a metabolic "cost" to the animal.

Other antiherbivory chemicals result in effects more severe than mere deterrence of feeding. Alkaloids such as caffeine, nicotine, and strychnine are potent anti herbivory mechanisms, causing convulsions, comas, and even death in herbivores. These effects may not occur in all herbivores.

Strychnine, for example, is produced by the fruit of some plants that may be eaten by birds without ill effects, but in mammals the same fruit causes failure of the central nervous system and induces seizures.The plant reduces herbivory by mammals, and the seeds get dispersed by birds that are able to detoxify the strychnine.

Grains and seed crops, such as wheat and peanuts, which are particularly attractive to animal and insect herbivores, often produce cyanogenic glycosides that release hydrogen cyanide as the tissues are digested. This compound inhibits cellular respiration, thus killing the herbivore. In each case members of a plant population are consumed by the herbivore, but future generations are spared by the loss.

Allelochemicals

Allelochemicals
Allelochemicals
Allelochemicals are compounds produced by an organism that interfere with the growth or development of another organism.Many phenolic acids act as allelo chemicals, inhibiting root growth of competing species. Many grains are known to release ferulic acid and caffeic acid into the soil, thus inhibiting the germination of weed species.

Phenolics may also act as antifungal compounds, increasing in concentration with fungal infection, thus protecting the plant from further attack. Phenolic compounds produced in tobacco and tomato leaves reduce the growth of these plants’ natural predator, tobacco hornworm, without affecting the growth or activity of the hornworm’s natural predator.

Allelochemicals produced in response to injury by herbivores may also attract predators of the herbivore.Wastes from many species of caterpillars induce the release of terpenoids from green leaves that attract parasitoid insects.

Production of specific combinations of volatiles on the part of the plant signals the predator, which will then reduce further herbivory. The plants have evolved the signal in response to herbivory, and the predators have evolved the ability to detect the signal indicating the location of their host.

Lectins are widely distributed carbohydrate-binding proteins, most commonly found in the Leguminoseae (legume) family. When found in the seeds, these compounds act as broad-spectrum insecticides, whereas in the roots of legumes they maintain bacterial relationships in nitrogen fixing nodules, providing the plant with a source of nitrogen unavailable to plants not producing nodules.

Attractants

Attractants
Attractants
Terpenoids and aliphatic compounds are often the components of essential oils of plants. The volatile nature of these compounds produces a distinctive odor that attracts pollinators. Composition of the volatile compounds often closely matches the natural pheromones produced by the pollinator, mimicking the chemical scent of a female insect in an attempt to attract male pollinators.

Pheromone mimicry is found primarily in members of the Orchidaceae, which are often dependent on single species of wasp for pollination. Other plants may mimic the odor of food. The smell of rotting flesh, attractive to flies, is produced via ammonia and alkylamines, such as cadaverine and putrescine. Methylesters may attract moth pollinators by mimicking the sweet smell of fruit.

Flavonoids often provide color to fruits and flowers and act as visual cues for pollination. Reds, blues, or yellows in varying patterns stand out against a background of green leaves, helping pollinators locate the flower.

Species may have minor chemical differences in their flavonoids that allow for the determination of identity, hybridization between species, and possible coevolution with pollinators. For example, tropical flowers tend to have a more intense red color from anthocyanins than do temperate flowers.

This difference correlates with differences in pollinator preferences, indicating a role by natural selection. Birds, such as hummingbirds, prefer red to yellow, whereas bees are not able to discern reds but are attracted to yellows. Carotenoids, such as xanthophyll and beta-carotene, give fruits and flowers distinctive yellow and orange colors.

Color patterns are also important in attracting pollinators. Butterflies are attracted to red/yellow color patterns. Flavonoid compounds not only impart color but also may modify color patterns by absorbing ultraviolet (UV) light.

Bees are capable of seeing in the UV range, so the presence of flavonoids may alter the bees’ perception of the flower. The patterns may also create cues as to the location of nectaries within the flower, guiding the pollinator to its reward.
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10:27 AM
Community, ecosystem Interactions
Community, ecosystem Interactions

Ecosystems are complex organizations of living and nonliving components. They are frequently named for their dominant biotic or physical features (such as marine kelp beds or coniferous forests).

Communities are groups of species usually classified according to their most prominent members (such as grassland communities or shrub communities). The interactions between species and their ecosystems have lasting impacts on both.

In an ecological sense, a community consists of all populations residing in a particular area. Examples of communities range in scale from all the trees in a given watershed, all soil microbes on an agricultural plot, or all phytoplankton in a particular harbor to all plants, animals, and microbes in vast areas, such as the Amazon basin or the Chesapeake Bay.

An ecosystem consists of the community of species as well as the environment of a given site. A forest ecosystem would include all living things along with climate, soils, disturbance, and other abiotic factors. An estuarine ecosystem, likewise, would include all the living things present, in addition to climate, currents, salinity, nutrients, and more.


Interactions between species in communities and ecosystems range from mutually beneficial to mutually harmful. One such category of interaction is mutualism, which usually involves two species. Both species derive benefit from a mutualism. Commensalism is used to describe a situation in which one species benefits without harming the other.

If the two species are neither helped nor harmed, a neutralism is said to occur, and an amensalism happens when one species is harmed while the other remains. During competition, both species involved are negatively affected.

A number of terms are used to describe a relationship in which one species benefits at another’s expense, including herbivory, predation, parasitism, and pathogenicity. The choice of term more often than not depends on the relative sizes of the species involved.

Competition

Plants typically compete for resources, such as light, space, nutrients, or water. One way an individual may outcompete its neighbors is to outgrow them, thus capturing more sunlight for itself (and thus producing more sugars and other organic molecules for itself).

Another way is to be more fecund than the neighbors, flooding the surroundings with one’s progeny and thereby being more likely to occupy favorable sites for reproduction.

For example, in closed forests tree fall gaps are quickly filled with growth from the canopy, thus shading the ground and making it more difficult for competing seedlings and saplings to survive.

Plants compete in the root zone as well, as plants with a more extensive root network can acquire more of the water and other inorganic nutrients necessary for growth and reproduction than can their competitors.

In semiarid areas, for example, trees often have trouble colonizing grasslands because the extensive root systems of grasses are much more effective in capturing available rain-water.

Sometimes plants resort to chemical “warfare,” known as allelopathy, in order to inhibit the growth of competitors in the surrounding area.

The existence of allelopathy remains a controversial topic, and simpler explanations have been offered for many previously alleged instances of the phenomenon. Allelopathy cannot be rejected outright; however, the controversy most likely proves only that many aspects of nature cannot be pigeonholed into narrow explanations.

Competition involves a cost in resources devoted to outgrowing or outreproducing the neighbors. Because of the cost, closely related, competing species will diverge in their ecological requirements. This principle is known as competitive exclusion.

Mutualism, Commensalism, and Parasitism

Many flowering plants could not exist without one of the most important mutualisms of all: pollination. In concept, pollination is simple: In exchange for carrying out the physical work of exchanging genetic material (in pollen form) between individual plants (thus enabling sexual reproduction), the carrier is rewarded with nutrients in the form of nectar or other materials.

Many types of animals are involved in pollination: insects such as bees, flies, and beetles; birds, particularly the humming birds; and mammals such as bats.

Mutualism between plant roots and fungal hyphae, or mycorrhizae
Mutualism between plant roots and fungal hyphae, or mycorrhizae

Another highly important mutualism is that between plant roots and fungal hyphae, or mycorrhizae. Mycorrhizae protect plant roots from pathogenic fungi and bacteria; their most important role, however, is to enhance water and nutrient uptake by the plant.

In fact, regeneration of some plants is impossible in the absence of appropriate mycorrhizae. Mycorrhizae benefit, in turn, by receiving nutrients and other materials synthesized by the host plant.

There are two types of mycorrhizae: ectomycorrhizae, whose hyphae may fill the space between plant roots but do not penetrate the roots themselves; and vesicular-arbuscular mycorrhizae, whose hyphae penetrate and develop within root cells.

Few people can envision a swamp in the south-eastern United States without thinking of bald cypress trees (Taxodium distichum) draped in ethereal nets of Spanish moss (Tillandsia usneoides), which is actually not a moss but a flowering plant in the monocot family Bromeliaceae. Tillandsia is an epiphyte, a plant that grows on the stems and branches of a tree.

Mistletoe, an example of parasitism
Mistletoe, an example of parasitism

Epiphytism is one of the most common examples of a commensalism, in which one organism, for instance the epiphyte, benefits without any demonstrable cost to the other, in this case the host tree.

Epiphytes are common in tropical rain forests and include orchids, bromeliads, cacti, and ferns. In temperate regions more primitive plants, such as lichens, aremore likely to become epiphytes.

Not all epiphytes are commensal, however. In the tropics, strangler figs, such as Ficus or Clusia, begin life as epiphytes but send down roots that in time completely encircle and kill the host. Mistletoes, such as Phoradendron or Arceuthobium, may draw off the photosynthetic production of the host, thus severely depleting its resources.

Herbivory and Pathogenicity

Arm with spines discourage grazers
Arm with spines to discourage grazers

Plants, because of their ability to harvest light energy from the sun to produce the organic nutrients and building blocks necessary for life, are the primary producers of most of the earth’s ecosystems.

Thus, they face an onslaught of macroscopic and microscopic consumers. If macroscopic, the consumers are generally regarded as herbivores (plant-eating animals); if microscopic, they are pathogens. Either way, herbivores and pathogens generally devour the tissues of the host.

Plants have evolved a number of defense mechanisms in response to pressure from herbivores and pathogens. Some responses may be mechanical.

For example, trees on an African savanna may evolve greater height to escape grazing pressures from large herbivores, but some large herbivores, specifically giraffes, may evolve to grow to greater heights as well. Plants may encase themselves in nearly indestructible outer coatings or arm them-selves with spines in order to discourage grazers.

Other responses may be chemical. Cellulose, one of the important chemical components of plant tissues such as wood, is virtually indigestible—unless the herbivore itself hosts a bacterial symbiont in its stomach that can manage the job of breaking down cellulose.

Other chemicals, such as phenols and tannins—the class of compounds that gives tea its brown color—are likewise indigestible, thus discouraging feeding by insects. Plants produce a wide range of toxins, such as alkaloids, which poison or kill herbivores. A number of hallucinogenic drugs are made from plant alkaloids.

Phytoalexins are another group of defensive compounds produced by plants in response to bacterial and fungal pathogens. Substances present in the cell walls of bacteria and fungi are released via the action of plant enzymes and spread throughout the plant.

The bacterial and fungal substances function as hormones to stimulate, or elicit, phytoalexin production. Hence, these substances are referred to as elicitors. The phytoalexins act as antibiotics, killing the infective agents. Tannins, phenols, and other compounds also serve to defend against pathogen attack.
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