organisms of 

 various colocated 

 species 



e.g., behavioral and bio- 

 chemical aspects of plant- 

 herbivore interactions; 

 competition, predation. 

 "Community ecology" 



munities, and the 

 nonliving portion 

 of earth it 

 occupies 



Ecosystem A geographically As an ecosystem (e.g., a 

 delimited assemblage lake) is the smallest 

 of coadapted com- relatively self-contained 



ecological system, it is the 

 logical unit for system 

 dynamics, and study of per- 

 turbations to ecosystem 

 function and changes in one 

 system which affect another. 

 Major terrestrial ecosystems 

 (e.g., grasslands of western 

 North America) are called 

 "biomes." Their study 

 constitutes "Ecosystem, 

 ecology. " 



Biosphere All organisms and The all-inclusive system 

 all parts of the is often too large to 



earth they occupy examine. Biogeochemical 

 and modify including studies of the migration 

 oceans, soils, and and distribution of 

 atmosphere biogenic elements may 



require a biosphere 



system. 



2. Energy Flow and 



Cycling of Matter 



in Ecosystems 



An organism, unlike a non-living natural entity, requires for its 

 maintenance a continuous input of energy and continuous exchange of 

 molecules with its environment. The plants, animals, and bacteria 

 which live together in an ecosystem all ultimately derive the energy 

 they require from the solar radiation that impinges on the surface of 

 the system. The mjn-iad species of an ecosystem can be grouped into a 

 small number of trophic categories on the basis of the manner in which 

 each derives its energy -producers, consumers (first-, second-, third- 

 order) and decomposers. 



The preponderant amount of available chemical energy in an 

 ecosystem is in the organic matter of the producer level. Succeeding 

 levels of herbivores and carnivores in a food web have approximately 

 one-seventh as much living matter as the preceding level. Respiration 

 losses from each level, and the inevitable losses in transformation of 

 compounds removed from the preceding level, cause this sharp 

 decrease in energy content as the trophic pyramid is ascended. Since 

 the size of individuals tends to increase as the pyramid is ascended, 

 population size drops markedly. 



Organisms which decompose organic material are relatively 

 inefficient in utilizing the energy that they release for body 

 construction, yet they dissipate a large fraction of the chemical energy 



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