THE ENVIRONMENT 



315 



present. Strands are considered desert conditions 

 because there is a general lack of available moisture; 

 the moisture that is abundant is salty and not usable 

 to plants other than the strand-adapted, salt- 

 tolerant species. 



Immediately adjacent to the ocean there often is a 

 sand area devoid of plants and having a rather lim- 

 ited number of animals, often a restricted variety of 

 arthropods. 



The first site containing plants has the most severe 

 maritime influence. In some cases certain grasses 

 occur, but in many the plants are almost exclusively 

 succulents. These plants and the few animals with 

 them (again, mostly arthropods) are subject not only 

 to salt spray but to high winds which both dry and 

 sand blast the life and move the sand substrate. 

 Such groups of plants and animals, and any other 

 herbaceous communities that may follow, even 

 though slightly more remote from the ocean, are often 

 called coastal strand communities. 



Although herbaceous habitats may form several 

 coastal strand communities, the next stage (or stages) 

 of apparent real differences consists of shrubs. These 

 beach-dune communities may be closest to the sea if 

 the beaches are protected from maritime winds; how- 

 ever, where there is little protection, shrubs gradually 

 replace the coastal strand herbs. 



Further sequences of communities frequently fit 

 into land succession normal for the particular area. 

 In some cases the next stage may be an herb com- 

 munity; in others, a shrub or tree community. 



waters may possess various features to prevent re- 

 moval from their environment. Among these speciali- 

 zations are body shape (e.g., flattened or streamlined) 

 and attachment devices (e.g., sticky, hooks, or 

 growth to the substrate). Insufficient oxygen fre- 

 quently acts as a limiting factor. The oxygen con- 

 centration in any particular body of water often is 

 highest at low temperatures and salinities, and wind 

 and water movements are important in determining 

 the rate of oxygen intake by water. The salt content 

 of water influences organisms. Most life can exist 

 only within a relatively narrow range of water salin- 

 ity. If there is too much salt in the water, many 

 species lose their own body water; if there is too 

 little, they may take on too much water and burst. 

 Either efifect usually leads to death. 



Fresh-water environments are of two types, stand- 

 ing and moving waters. Standing waters tend to fill in 

 as a result of physical and biological developments 

 (succession) in a sequence of habitats. A possible 

 sequence is lake, pond, marsh, and swamp. Moving 

 waters usually show the result of physical develop- 

 ment from fast- to slow-moving water. This sequence 

 normally entails progression from brook to river, with 

 further history following that given in previous dis- 

 cussion of life cycles of streams (pp. 42-44). 



The ecology of lakes requires classification of 

 biological and physical subunits and then classifica- 

 tion of lakes themselves. The biological subunits are 

 treated as organism types and the physical subunits 

 as light zones and temperature layering. 



FRESH-WATER ENVIRONMENT 



Factors of most importance in fresh-water habitats 

 are temperature, suspended materials, current ve- 

 locity, oxygen-carbon dioxide concentration, and 

 salt content. Although temperature fluctuation in water 

 is much less than that on land, temperature fre- 

 quently is a primary limiting factor because the tol- 

 erance of aquatic life tends to be very low; many 

 aquatic plants and animals can exist only within 

 very narrow ranges of temperature. Suspended ma- 

 terials are related directly to light penetration. 

 Those waters with a great deal of suspended ma- 

 terials allow a minimum of light penetration and 

 may well have few organisms. The velocity of cur- 

 rents typically is directly associated with the dis- 

 tribution of most organisms and the adaptations of 

 certain animals. Animals inhabiting rapidly moving 



ORGANISM TYPES IN STANDING WATERS 



Fresh-water creatures usually are segregated ac- 

 cording to their general shape and appearance, or 

 life form, into benthos, periphylon, plankton, nekton, and 

 neuston. Benthos are bottom creatures that are 

 either resting upon, attached to, or living within the 

 underlying strata. Periphyton are attached to any 

 surface rising above the bottom. Plankton are float- 

 ing species whose movements are most dependent 

 upon water currents. They are either nonmotile or 

 very weakly motile, and are normally microscopic or 

 submicroscopic. Nekton are active swimmers that 

 are independent of water currents; usually they are 

 macroscopic. Neuston are all surface organisms. 



Benthos occur in a variety of bottom situations. In 

 general, dark and sandy bottoms have fewer species 

 (except for clams) than other areas. Typical benthic 



