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ECOLOGICAL FACTORS: 



nomenon; rarely are two organisms found in the same 

 habitat that are not in competition for some of the 

 food or space. Exploitation is of two types, predalwn 

 and parasitism. In predation the one benefited, the 

 predator, usually destroys and often consumes the one 

 harmed, the prey. Also, predator and prey tend to be 

 separated from one another a good deal of the time. 

 In parasitism the one benefited, the parasite, often 

 destroys the one harmed, the host, but the parasite 

 often lives most to all of its life upon or within its 

 host. However, during its life cycle, a parasite may 

 live with many hosts, often various intermediate hosts 

 and the final, or definitive, host of the adult parasite. 

 Also, not all parasites absolutely require a parasitic 

 relationship. Many of these facultative parasites can 

 obtain their nutrients in another way. A large num- 

 ber of facultative parasites live and feed upon dead 

 plants and animals; such parasites are saprophytic. On 

 the other hand, many parasites are so specialized to 

 that way of life that they cannot obtain nutrients in 

 any other manner. These dependent forms are called 

 obligate parasites. 



The symbiotic phenomena in which only benefit 

 occurs are commensalism, cooperation, and mu- 

 tualism. These are called positive interactions and 

 are presented in their probable evolutionary sequence. 

 Commensalism is the simplest and is especially com- 

 mon in the sea where many active organisms benefit 

 from gaining shelter upon or within sedentary forms. 

 Cooperation is a little more complex. An example 

 occurs in the marine masking crab that benefits from 

 the camouflage provided by creatures the crab tends 

 upon its back. The creatures in turn gain from the 

 crab's keeping them in well-oxygenated situations. 

 The classical example of mutualism is found in cer- 

 tain extremely close associations of algae and fungi 

 that are of mutual benefit. The algae form food and 

 the fungi collect moisture. These organisms are so 

 intimately related that they are recognized by a dis- 

 tinct name, lichens. 



The harmful, or negative interactions, in their 

 probable order of evolutionary origin are competition, 

 amensalism, and exploitation. Competition may or 

 may not be a serious factor in interspecific relation- 

 ships, but it is of low symbiotic development because 

 the individuals frequently have only moderate con- 

 tact with one another. Amensalism usually features 

 more contact and is more definite. An example is the 

 relationship between the bread mold {Pemcillium) and 

 many bacteria, the mold secretes the drug penicillin 



which is well-known for its bacteria-killing properties; 

 however, the bacteria are not known to affect the 

 mold. Exploitation should require no further elabora- 

 tion. Also, most of us are familiar with the parasitic 

 activities of tapeworms, fleas, and other animals and 

 with the relationships between carnivorous (meat- 

 eating) animals and the prey they eat. 



INTERORGANISM RELATIONSHIPS 



Another way of describing symbiosis is in terms 

 of the effects of plants upon plants, plants upon ani- 

 mals, animals upon plants, and animals upon animals. 



An important example of plants influencing plants 

 is the phenomenon of layering. Layering of trees, 

 shrubs, and other growth forms in a single habitat 

 can indicate either competition or dependence. Com- 

 petition exists when a tree is shading a shrub that 

 requires open sunlight and because of this competi- 

 tion (in this case probably really amensalism) the tree 

 may, in a sense, kill the shrub. This also would be 

 an example of why the temporary, developmental 

 communities are replaced by one another and finally 

 by the permanent, stable climax. On the other hand, 

 a particular shrub may require some shading by trees. 

 Because bright sunlight would kill the shrub, the 

 shrub is a commensal upon the tree. 



Three additional interplant relationships are found 

 in special kinds of plants: saprophytes, epiphytes, and 

 parasites. Saprophytes are those plants that break 

 down the complex products of dead green plants. For 

 example, many green plants form basic foods, carbo- 

 hydrates, fats, and proteins, and living tissues, mostly 

 proteins. Saprophytic bacteria decompose the basic 

 foods and tissues into carbon dioxide, water, and cer- 

 tain nitrogen compounds (ammonia, nitrates, and 

 nitrites). Epiphytes are plants that grow upon other 

 plants, man-made objects, and even some animals. 

 Among such plants are lichens, mosses, ferns, and 

 orchids. When true epiphytes grow upon living crea- 

 tures, the epiphytes are commensals. Parasites are 

 distinguished from epiphytes because only parasites 

 feed upon living creatures. Examples of plant para- 

 sites can be found throughout the phyla of the plant 

 kingdom. 



Plants in general have a pronounced influence upon 

 animals. All animals rely upon plants as a basic 

 source of food because only green plants can form 

 food from inorganic materials. Many animals utilize 

 plants as a means of shelter; some plants are animal 



