|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.
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|
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|
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 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.