One of the best examples of mutualism is known by the unusual name mycorrhiza, which is a "fungus root," or a fungus living in symbiosis with the roots of a vascular plant. (A vascular plant is any plant species containing a vascular system, which is a network of vessels for moving fluid through the body of the organism.) The relationship is a form of mutualism because, while the fungus benefits from access to carbohydrates, proteins, and other organic nutrients excreted by or contained in the roots of the host plant, the host plant benefits from an enhanced supply of inorganic nutrients, especially phosphorus, that come from the fungus.
The fungus carries out this function primarily by increasing the rate at which organic matter in the immediate vicinity of the plant root decomposes and by efficiently absorbing the inorganic nutrients that are liberated by this process-nutrients it shares with the plant. (The term organic refers to the presence of carbon and hydrogen together, which is characteristic not only of all living things but of many nonliving things as well.) The most important mineral nutrients that the fungus supplies to the plant are compounds containing either phosphorus or, to a lesser degree, nitrogen. (These elements are present in biogeochemical cycles—see The Biosphere.) So beneficial is the mycorrhizal mutualism that about 90% of all vascular plant families, including mustards and knotweeds (family Brassicaceae and Polygonaceae, respectively), enjoy some such relationship with fungi.
Many mycorrhizal fungi in the Basiodiomycete group develop edible mushrooms, which are gathered by many people for use in gourmet cooking. Mushroom collectors have to be careful, of course, because some mycorrhizal fungi are deadly poisonous, as is the case with the death angel, or destroying angel— Amanita virosa.
Perhaps the most famous of the edible mushrooms produced by mycorrhizae are the many varieties known by the name truffle. Among these mushrooms is Tuber melanosporum, which is commonly mycorrhizal on various species of oak tree. The spore-bearing bodies of the truffle fungi develop underground and are usually brown or black and covered with warts. Truffle hunters require the help of truffle-sniffing pigs or dogs, but their work is definitely worth the trouble: good truffles command a handsome price, and particularly in France the truffle industry is big business. Given the lucrative nature of the undertaking, one might ask why people do not cultivate truffles rather than hunting for them. To create the necessary conditions for cultivation, however, so much effort is required that it is difficult to make a profit, even at the high prices charged for truffles. The soil composition must be just right, and under conditions of cultivation this takes about five years.
Orchids are an example of a plant in an obligate mutualism: they can thrive only in a mycorrhizal relationship. Tiny and dustlike, orchid seeds have virtually no stored energy to support the seedling when it germinates, or begins to grow. Only with the assistance of an appropriate mycorrhizal fungus can these seedlings begin developing. Until horticulturists discovered this fact, orchids were extremely difficult to propagate and grow in greenhouses; today, they are relatively easy to breed and cultivate.
Some species of vascular plants do not contain chlorophyll, the chemical necessary for photosynthesis, or the conversion of light energy from the Sun into usable chemical energy in a plant. Such a plant is like a person missing a vital organ, and under normal circumstances, it would be impossible for the plant to survive. Yet the Indian pipe, or Monotropa uniflora, has managed to thrive despite the fact that it produces no chlorophyll; instead, it depends entirely on mycorrhizal fungus to supply it with the organic nutrients it needs. This obligate relationship is just one example of the critical role mycorrhizae perform in the lives of plants throughout the world.
Mycorrhizae are vital to plant nutrition, especially in places where the soil is poor in nutrients. Whereas many plant roots develop root hairs as a means of facilitating the extraction of water and nutrients from the soil, plant roots
Often, when species of trees and shrubs grown in a greenhouse are transplanted to a non-forested outdoor habitat, they exhibit signs of nutritional distress. This happens because the soils in such habitats do not have populations of appropriate species of mycorrhizal fungi to colonize the roots of the tree seedlings. If, however, seedlings are transplanted into a clear-cut area that was once a forest dominated by the same or closely related species of trees, the plants generally will do well. This happens because the clear-cut former forest land typically still has a population of suitable mycorrhizal fungi.
Plants' dependence on mycorrhizal fungi may be so acute that the plants do not do well in the absence of such fungi, even when growing in soil that is apparently abundant in nutrients. Although most mycorrhizal relationships are not so obligate, it is still of critical important to consider mycorrhizal fungi on a site before a natural ecosystem is converted into some sort of anthropogenic habitat (that is, an area dominated by humans—see Biomes). For example, almost all the tree species in tropical forests depend on mycorrhizae to supply them with nutrients from the soils, which are typically infertile. (See The Biosphere for more about the soil in rain forests.) If people clear and burn the forest to develop new agricultural lands, they leave the soil bereft of a key component. Even though some fungi will survive, they may not necessarily be the appropriate symbionts for the species of grasses and other crops that farmers will attempt to grow on the cleared land.
Mycorrhizae are just one example of the ways that mutualism brings into play interactions between widely separated species—in that particular case, between members of two entirely different kingdoms, those of plant and fungi. In some cases, mutualism may bring together an organism of a kingdom whose members are incapable of moving on their own (plants, fungi, or algae) with one whose members are mobile (animals or bacteria). An excellent example is the relationship between angiosperm plants and bees, which facilitate pollination for the plants (see Ecosystems and Ecology.)
Another plant-insect mutualism exists between a tropical ant ( Pseudomyrmex ferruginea ) and a shrub known as the bull's horn acacia ( Acacia cornigera ). The latter has evolved hollow thorns, which the ants use as protected nesting sites. The bull's horn acacia has the added benefit, from the ant's perspective, of exuding proteins at the tips of its leaflets, thus providing a handy source of nutrition. In return, the ants protect the acacia both from competition with other plants (by removing any encroaching foliage from the area) and from defoliating insects (by killing herbivorous, or plant-eating, insects and attacking larger herbivores, such as grazing mammals).
A much less dramatic, though biologically quite significant, example of interkingdom mutualism is the lichen. Lichen is the name for about 15,000 varieties, including some that are incorrectly called mosses (e.g., reindeer moss). Before the era of microscopy, botanists considered lichens to be single organisms, but they constitute an obligate mutualism between a fungus and an alga or a blue-green bacterium. The fungus benefits from access to photosynthetic products, while the alga or bacterium benefits from the relatively moist habitat that fungus provides as well as from enhanced access to inorganic nutrients.
In contrast to these cross-kingdom or interkingdom types of mutualism, there may be intrakingdom (within the same kingdom) symbiotic relationships between two very different types of animal. Often, mutualism joins forces in such a way that humans, observing these interactions, see in them object lessons, or stories illustrating the concept that the meek sometimes provide vital assistance to the mighty. One example of this is purely fictional, and it is a very old story indeed: Aesop's fable about the mouse and the lion.
In this tale a lion catches a mouse and is about to eat the little creature for a snack when the mouse pleads for its life; the lion, feeling particularly charitable that day, decides to spare it. Before leaving, the mouse promises one day to return the favor, and the lion chuckles at this offer, thinking that there is no way that a lowly mouse could ever save a fierce lion. Then one day the lion steps on a thorn and cannot extract it from his paw. He is in serious pain, yet the thorn is too small for him to remove with his teeth, and he suffers hopelessly—until the mouse arrives and ably extracts the thorn.
Many real-life examples of this strong-weak or big-small symbiosis exist, one of the more well-known versions being that between the African black rhinoceros ( Diceros bicornis ) and the oxpecker, or tickbird. The oxpecker, of the genus Buphagus, appears in two species, B. africanus and B. erythrorhynchus. It feeds off ticks, flies, and maggots that cling to the rhino's hide. Thus, this oddly matched pair often can be seen on the African savannas, the rhino benefiting from the pest-removal services of the oxpecker and the oxpecker enjoying the smorgasbord that the rhino's hide offers.
Humans engage in a wide variety of symbiotic relationships with plants, animals, and bacteria. Bacteria may be parasitic on humans, but far
All important agricultural plants exist in tight bonds of mutualism with humans, because human farmers have bred species so selectively that they require assistance in reproducing. For example, over time, agricultural corn, or maize ( Zea mays ), has been selected in such a way as to favor those varieties whose fruiting structure is enclosed in a leafy sheath that does not open and whose seeds do not separate easily from the supporting tissue. In other words, thanks to selective breeding, the corn that grows on farms is enclosed in a husk, and the kernels do not come off of the cob readily. Such corn may be desirable as a crop, but because of these characteristics, it is incapable of spreading its own seeds and thereby reproducing on its own. Obviously, agricultural corn is not on any endangered species list, the reason being that farmers continue to propagate the species through breeding and planting.
Another example of human-animal mutualism, to which we alluded earlier, is the relationship between people and their pets, most notably dogs ( Canis familiaris ) and house cats ( Felis catus ). Fed and kept safe in domestication, these animals benefit tremendously from their interaction with humans. Humans, in turn, gain from their pets' companionship, which might be regarded as a mutual benefit—at least in the case of dogs. (And even cats, though they pretend not to care much for their humans, have been known to indulge in at least a touch of sentimentality.) In addition, humans receive other services from pets: dogs protect against burglars, and cats eradicate rodents.
Where insects and symbiosis are involved, perhaps the ideas that most readily come to mind are images of parasitism. Indeed, many parasites are insects, but insects often interact with other species in relationships of mutualism, such as those examples mentioned earlier (bees and angiosperms, ants and bull's horn acacia plants). Additionally, there are numerous cases of mutualism between insect species. One of the most intriguing is the arrangements that exists between ants and aphids, insects of the order Homoptera, which also are known as plant lice.
In discussing the ant-aphid mutualism, scientists often compare the aphids to cattle, with the ants acting as protectors and "ranchers." What aphids have that ants want is something called honeydew, a sweet substance containing surplus sugar from the aphid's diet that the aphid excretes through its anus. In return, ants protect aphid eggs during the winter and carry the newly hatched aphids to new host plants. The aphids feed on the leaves, and the ants receive a supply of honeydew.
In another mutualism involving a particular ant species, Formica fusca, two organisms appear to have evolved together in such a way that each benefits from the other, a phenomenon known as coadaptation. This particular mutualism involves the butterfly Glaucopsyche lygdamus when it is still a caterpillar, meaning that it is in the larval, or not yet fully developed, stage. Like the aphid, this creature, too, produces a sweet "honeydew" solution that the ants harvest as food. In return, the ants defend the caterpillar against parasitic wasps and flies.
As the old saying goes, "One man's meat is another man's poison"—in other words, what is beneficial to one person may be harmful to another. So it is with symbiotic relationships, and often a creature that plays a helpful, mutualistic role in one relationship may be a harmful parasite in another interaction. Aphids, for instance, are parasitic to many a host plant, which experiences yellowing, stunting, mottling, browning, and curling of leaves as well as inhibiting of its ability to produce crops.
One particular butterfly group, Heliconiinae (a member of the Nymphalidae, largest of the butterfly families) furnishes another example of the fact that a mutualistic symbiont, in separate interaction, can serve as a parasite. Moreover, in this particular case the heliconius butterfly can be a mutualistic symbiont and parasite for the very same plant. Heliconius butterflies scatter the pollen from the flowers of passionflower vines (genus Passiflora ), thus benefiting the plant, but their females also lay eggs on young Passiflora shoots, and the developing larva may eat the entire shoot. As an apparent adaptive response, several Passiflora species produce new shoots featuring a small structure that closely resembles a heliconius egg. A female butterfly that sees this "egg" will avoid laying her own egg there, and the shoot will be spared.
Years ago a National Geographic article on the Indian city of Calcutta included a photograph that aptly illustrated the idea of commensalism, though in this case not between animals or plants but between people. The photograph showed a street vendor in a tiny wooden stall with a window, through which he sold his wares to passers-by. It was a rainy day, and huddled beneath the window ledge (which also served as a counter-top) was another vendor, protecting himself and his own tray of goods from the rain.
The photograph provided a stunning example, in microcosm, of the overpopulation problem both in Calcutta and in India as a whole—a level of crowding and of poverty far beyond the comprehension of the average American. At the same time it also offered a beautiful illustration of commensalism (though this was certainly not the purpose of including the picture with the article). The vendor sitting on the ground acted in the role of commensal to the relatively more fortunate vendor with the booth, who would be analogous to the host.
The relationship was apparently commensal, because the vendor on the ground received shelter from the other vendor's counter without the other vendor's suffering any detriment. If the vendor in the booth wanted to move elsewhere, and the vendor on the ground somehow prevented him from doing so, then the relationship would be one of parasitism. And, of course, if the vendor with the booth charged his less-fortunate neighbor rent, then the relationship would not be truly commensal, because the vendor on the ground would be paying for his shelter. To all appearances, however, the interaction between the two men was perfectly commensal.
Plants that grow on the sides of other plants without being parasitic are known as epiphytic plants.
Among these plants are certain species of orchids, ferns, and moss. By "standing on the shoulders of giants," these plants receive enormous ecological benefits: the height of their hosts gives them an opportunity to reach a higher level in the canopy (the upper layer of trees in the forest) than they would normally attain, and this provides them with much greater access to sunlight. At the same time, the hosts are not affected either negatively or positively by this relationship.
Another commensal relationship, known as phoresy, is a type of biological hitchhiking in which one organism receives access to transportation on the body of another animal, without the transporting animal being adversely affected by this arrangement. The burdock ( Arctium lappa ) is one of several North American plant species that produce fruit that adheres to fur and therefore is dispersed easily by the movement of mammals. The burdock is special from a human standpoint, however, inasmuch as the anatomical adaptation that makes possible its adhesion to fur provided designers with the model for that extremely useful innovation, Velcro.
As with the illustration of the street vendors in Calcutta, it is always possible that commensalism, through a slight alteration, may yield a relationship in which the host is affected negatively. There are instances in which individual animals may become loaded heavily with sticky fruit from the burdock (or other plants that employ a similar mechanism), thus causing their fur to mat excessively and perhaps resulting in significant detriment. This is not common, however, and usually this biological relationship is truly commensal. Furthermore, phoresy should not be confused with parasitic relationships in which a creature such as a tick attaches itself to the body of another organism for transport or other purposes. (For much more about parasitism, see Parasites and Parasitology.)
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