plant community with reference to the series of changes that 

 have been, and are, taking place. In addition to sociological 

 analysis, a complete description of vegetation must include dis- 

 cussion of the dynamics within and between communities, with 

 particular reference to their position as a successional stage or a 

 climax. 



ENVIRONMENTAL GRADIENTS AND HABITAT PATTERNS 



The habitat is the place occupied by a population or a com- 

 munity in which a particular combination of environmental 

 conditions is present, such as the foothill slopes where ponderosa 

 pines are growing, or a marshy lakeshore covered with sedges and 

 associated species. The environmental conditions within one kind 

 of habitat exhibit variation from spot to spot, either irregularly 

 and suddenly, or gradually, as may be seen in going from a lake- 

 shore inland. The extent of a habitat is delimited by the ecological 

 amplitude of one or more of the species under consideration; for 

 example, the habitat where Pinus ponderosa is dominant comprises 

 the rocky foothills in many of the western states, varying consider- 

 ably in altitude and latitude, with fairly wide ranges in precipi- 

 tation, temperature, and other factors, each of which may change 

 at a different rate than the others. For example, the temperature 

 conditions may vary appreciably in an altitudinal range of 2000 

 feet, while the soil texture and humus content remain about the 

 same. Major and minor habitats occur, the former marked by 

 dominants in the vegetation, the latter by minor species associated 

 with the dominants but requiring special conditions for their 

 growth, such as mosses and lichens in openings among shrubs in 

 the Subarctic, or between bunches of grass in prairie. 



Since climatic, topographic, edaphic (soil), and biotic condi- 

 tions vary to a greater or lesser degree within a landscape (see 

 Figures 1-1 and 5-1), numerous habitats and plant communities 

 are formed and become manifest in a mosaic or zonation of 

 vegetation. The changes in vegetation are often more abrupt than 

 those in habitat conditions, because when the ecological ampli- 

 tude or requirements of one or more species, especially dominants. 



Habitat Patterns, CHanges, and Clinmax • 137 



