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ECOLOGICAL FACTORS: 



alkali marsh 



Figure 17.15 Developmental stages in an alkali or saline lake. 



Alkali Marsh Stage. The margins of these lakes 

 often have two species of extremely tolerant plants, 

 glasswort [Salicomia) and bush picklewood {Allen- 

 rolfea) plants also found in coastal salt marshes of the 

 Pacific coast. 



Alkali Sink Stage. This community might actually 

 be a stable one (climax) and not developmental, be- 

 cause it is found in relatively uniform conditions of 

 salinity or alkalinity either on the margin of alkali 

 marshes or in the soils resulting from the drying of 

 temporary lakes. Common plants are saltbushes 

 (Alnplex) and greasewood {Sarcobatus vermiculatus) . 



Ponds and Temporary Ponds. These areas represent 

 late stages in the geological and ecological succes- 

 sion of a lake. Therefore, they each contain, some to 

 all of the stages presented under the discussion of 

 eutrophic lakes. 



Streams. .Streams are classified according to 

 geological age (young, mature, or old), and perma- 

 nence (temporary or permanent). Size also could be 

 indicated by referring to brooks, rivers, etc. Further 

 subdivision could be made according to succession 

 from open water (when present) to land. The stages 

 found in a stream could be like those presented in the 

 discussion of eutrophic lakes. 



LIGHT 



Solar radiation is essential for life as we know it, 

 but light is not a simple or uncomplicated factor in 

 an environment. It is indispensable for the basic 



food-making process of photosynthesis; at the same 

 time, its direct rays are lethal to many organisms. 

 Among plants, certain species require direct sunlight 

 throughout their life, others only during germination; 

 still others never can tolerate direct solar rays. Its 

 general importance to animals is shown by the fact 

 that most species have organs that perceive light, 

 although these perceptions might be simply of light 

 intensity rather than actual vision. However, the 

 importance of vision for locating food is indicated by 

 the fact that various organisms that might be preyed 

 upon are camouflaged. 



In spite of the critical nature of light, some or- 

 ganisms spend much (perhaps all) of their life in 

 eternal darkness. Cave life is a prime example of 

 this. However, there are only a few cave creatures — 

 a few bacteria, fungi, and animals such as crickets, 

 beetles, crayfish, fish, salamanders, and bats. Owing 

 to a unique radar-like sense, bats apparently are 

 the only animals that move freely into the extremely 

 dark areas of caves and back out into the open. The 

 permanent cave residents are remarkable for their 

 general lack of pigmentation and absence of func- 

 tional eyes. Apparently color is unnecessary to pro- 

 tect the animals from light and eyesight would be 

 relatively useless in caves. 



WAVELENGTH, INTENSITY, AND DURATION 



The wavelength, intensity, and duration of light 

 usually are aspects of greatest ecological significance. 

 Wavelengths are critical to animals because only 

 certain lengths are visible and many others are harm- 

 ful radiations. Plants also are harmed by some invis- 

 ible rays; however, parts of visible light are most ef- 

 ficient in stimulating photosynthesis. Ultraviolet 

 wavelengths are instrumental in killing many germs 

 and enabling many animals to form vitamin D. Also, 

 infrared light is of primary importance in producing 

 heat. 



Light intensity has a great many ecological as- 

 sociations. For example, it is directly related to the 

 amount of photosynthesis and transpiration in plants 

 and to the structure and function of developing and 

 adult plant organs. In the ocean and deep lakes in- 

 tensity of light regulates the concentration of micro- 

 scopic plankton near the surface. In dense forests 

 there are shade plants that can survive only in the 

 shade created by a tree canopy. These shade plants 

 are specialized to carry on adequate photosynthesis 



