PARASITISM

Parasitism may be regarded as a special form of predation in which the predator is much smaller than the prey and remains closely associated with it. Parasitism is harmful to the prey organism and beneficial to the parasite. The concept of parasitism seems obvious, but individual instances are often surprisingly, difficult to distinguish from predation and from various other kinds of symbiosis.
Many instances of parasitism are well known; for example, vertebrates are parasitized by members of many different phyla of animals and protists. Invertebrates also have many kinds of parasites that live within their bodies. However, bacteria and viruses are often not considered parasites, even though they fit our definition precisely. Lice, which live on the bodies of vertebrates-mainly birds and mammals-are normally considered parasites, but mosquitoes are not, even though they draw food from the same birds and mammals in a similar manner, because their interaction with their host lasts for only a short time. However, mosquitoes are closely associated ecologi-callywith the animals from which they draw blood. Mosquitoes also synchronize their diurnal and seasonal activities closely with those of their hosts, so that the interrelationship is very close.
Many fungi and some flowering plants, too, are parasitic on other plants, and a few are serious pests of crops.
Internal parasitism is generally marked by much more extreme specialization than external parasitism, as shown by the many protist and invertebrate parasites that infect humans. The more closely the life of the parasite is linked with that of its host, (he more its morphology and behavior are likely to have been modified during the course of its evolution. The same, of course, is true of symbiotic relationships of all sorts. Conditions within the body of another organism are different from those encountered outside and are apt to be much more constant in every way. Consequently, the structure of the parasite is often simplified, and unnecessary armaments and structures are lost as it evolves.
The ecosystem is the most complex level of biological organization. Communities are composed of the organisms present at a particular place, whereas ecosystems include the nonliving factors interacting with these organisms. In ecosystems there is a regulated transfer of energy and a controlled cycling of nutrients. The individual organisms and populations of organisms in an ecosystem act as parts of an integrated whole, adjust over time to their role in the ecosystem, and relate to one another in complex ways that we only partly understand. Despite their differences, all ecosystems regulate the flow of energy-ultimately derived from the sun-and the cycling of nutrients. The earth is a closed system with respect to the chemicals, but an open one in terms of energy. Collectively, the organisms that occur in ecosystems regulate [he capture and expenditure of that energy and the cycling of those chemicals. All organisms, including humans, depend on the ability of a few other organisms-plants, algae, and some bacteria in the case of carbon, and certain bacteria in the case of nitrogen, for example-for the basic components of life.
As distinct functional units, different kinds of ecosystems have more or less clearly recognizable boundaries, but they also intergrade into one another, sometimes almost imperceptibly; boundaries then become arbitrary. Ecosystems also change over time and slowly become modified into new ecosystems, whose characteristics come to differ increasingly from those that preceded them. Thus the com-ilex ecosystems of the tropical rain forests have changed gradually in adapting to the articular conditions of temperature, seasonably, and soil that typify these places. Fhe ecosystems of the tundra have developed in a similar way, but in relation to the Unerent environmental conditions of the far north. As the climate changes in a given ilace, the ecosystems that are present there change along with it, as do the individual populations within these ecosystems. By this process, the overall characteristics of the populations gradually adjust to the new conditions. Not all ecosystems are natural; we nay also speak of an ecosystem in an aquarium or in a cultivated field.