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COLLEGE ZOOLOGY 



Ecologic pyramid 



The food chains in the web of Hfe con- 

 tribute to biological pyramids which may 

 not be evident to the casual observer, but 

 which exist throughout nature. We will 

 consider here only the pyramid of biomass, 

 which gives a picture of the food chain re- 

 lations. To simplify this pyramid, we will 

 consider only alfalfa, calves, and a boy. 

 This also shows the important relationship 

 between plants and animals. 



Since all energy comes from sunlight, only 

 the green plants can convert this energy 

 into a form that can be utilized by animals. 

 Studies have shown that it takes 20 million 

 alfalfa plants to produce 17,850 pounds of 

 alfalfa, to produce 4V2 yearlings, weighing 

 2250 pounds. These 4^2 yearlings are re- 

 quired to feed one boy to the age of 12, 

 weighing 105 pounds (Fig. 446). This pyra- 

 mid of biomass shows graphically that the 

 actual protoplasmic mass, as well as num- 

 bers, becomes less toward the peak. Thus, 

 to produce a given amount of human proto- 

 plasm, a great deal more calf protoplasm is 

 required; and to produce a unit of calf 

 protoplasm, a much greater weight of alfalfa 

 is required. 



Much is being written today about the 

 "population problem." Will human popula- 

 tions exceed their food supply? It is easy 

 to see that if the amount of biomass in 

 the system, represented by the ecologic 

 pyramid, is changed at any level, all the 

 other levels would have to adjust to the 

 changed conditions. How large a population 

 the world can support depends on how 

 much we can increase the food supply for 

 man. It can readily be seen that the ecologic 

 pyramid has socio-economic implications for 

 human society. 



Productivity 



The productivity of an ecosystem is the 

 amount of available food material of all 

 types that it produces over a given period 

 of time. Primary productivity refers to the 



food produced by producer organisms which 

 are chiefly green plants. Secondary produc- 

 tivity is the amount produced by consumer 

 organisms, principally animals. Animals are 

 not merely consumers in relation to pro- 

 ductivity; even if they are not consumed 

 in turn by larger animals they return a cer- 

 tain amount of available energy to the eco- 

 system in the form of feces and their own 

 dead bodies. 



Interactions between species 



Species may be treated as populations and, 

 as such, are subdivisions of ecosystems. Two 

 species can interact in various ways. If two 

 or more species contend in rivalry for the 

 same niche, it is called competition; only 

 one species occurs in an ecologic niche in a 

 habitat. If a larger species regulady con- 

 sumes the smaller ones as food, the phe- 

 nomenon is called predation. This involves 

 the maintenance of a balance in the num- 

 bers of both species; the disappearance of 

 the prey would result in the same fate for 

 the predator. A similar balance exists be- 

 tween parasite and host species. Competi- 

 tion, predation, and parasitism are often 

 called negative interactions between species. 

 Examples of positive interactions are found 

 in commensalism and mutualism which are 

 considered under the general heading of 

 symbiosis. 



SYMBIOSIS 



When two different species of organisms 

 live together, the relationship is called sym- 

 biosis. Mutualism is sometimes erroneously 

 called "symbiosis." Symbiosis is best used as 

 a collective term to include mutualism, com- 

 mensalism, and parasitism. The three terms 

 -—mutualism, commensalism, and parasitism 

 —are usually employed for the main types of 

 associations that exist between organisms of 

 different species. However, there is no very 

 sharp line of demarcation between them. 



