632 



sedimentation gradually turns a lake into a 

 bog. 



Limiting factors of importance in fresh- 

 water environments are temperature, trans- 

 parency of the water, extent of water cur- 

 rents, and concentrations of oxygen and cer- 

 tain inorganic salts. All of these are subject 

 to rapid change as when the evaporation of 

 the water in a small pond increases the salt 

 content of the water. Seasonal changes are 

 also crucial in many cases. 



Marine environment 



Like the fresh waters, the seas are also 

 aquatic environments. They differ from the 

 former, however, in many aspects of Geo- 

 logic interest. Because of their size, depth, 

 and continuity, the ocean environments are 

 more stable than those of the fresh waters. 



The marine environments may also be 

 broken down into smaller subdivisions. The 

 relatively shallow waters of the continental 

 shelves are called the neritic zone, and this 

 may be further divided into zones related 

 to tidal activity. The deep waters beyond the 

 continental shelves constitute the oceanic 

 region. Its subdivisions are vertical rather 

 than horizontal. The euphotic zone is the 

 upper portion of the oceanic region in which 

 effective photosynthesis takes place. Because 

 of this the euphotic zone is often called the 

 "producing" region. The regions of the 

 oceans, beyond the depth where light can 

 penetrate, are called the bathyal zones 

 when they occur on continental slopes, and 

 they are called abyssal zones elsewhere. It 

 should be noted that the fresh-water en- 

 vironments can also be subdivided in this 

 manner, but such subdivisions are far less 

 stable. Thus, one of the principal differences 

 in approach between fresh-water and marine 

 ecology is that geographic divisions have 

 more weight in the former, while zonation 

 is more important in the latter. 



The limiting factors in the marine en- 

 vironments are generally the same as those 

 of the fresh water, but their relative impor- 

 tance often differs. This is especially true of 



COLLEGE ZOOLOGY 



light, for there are few bodies of fresh water 

 in which some light cannot penetrate to the 

 bottom. Thus light is less of a limiting factor 

 for the fresh-water environments. Salt con- 

 centration at a given place varies less in 

 marine environments and has less signifi- 

 cance as a limiting factor. 



Terrestrial environment 



The environments of the land are the 

 most variable. They can be contrasted with 

 the aquatic environments on the basis of a 

 number of problems they offer to plant and 

 animal populations. 



On the land, moisture is always a limiting 

 factor. Air, as contrasted with water, per- 

 mits rapid temperature variation, but offers 

 little variation in oxygen and carbon dioxide 

 content. Also, air lacks the supporting abil- 

 ity of water and thus imposes a necessity for 

 strong skeletal structure and special means 

 of locomotion upon land animals. Land 

 areas are not continuous, and this presents 

 geographic barriers not generally present in 

 aquatic, especially marine, environments. 

 Finally, the nature of the substrate (soil) is 

 of greater importance in land environments. 

 To summarize: climate and substrate are 

 the two groups of physical factors most im- 

 portant to land ecology. 



In subdividing terrestrial environments, 

 the most useful concept is that of the biome, 

 the largest conveniently recognized com- 

 munity unit. A biome is a unit resulting 

 from the interaction of a regional climate, 

 regional biota (animal and plant life), and 

 substrate. Examples are grasslands, tundras, 

 coniferous forests, deciduous forests, and 

 deserts. 



Symbiotic environment 



There are many examples in nature of 

 two species habitually living in a more or 

 less close relationship with one another, 

 other than that of predation. All of these 

 may be called symbiosis, and each species 

 forms a more or less important part of the 

 environment of the other. Since the en- 



