352 



COMMUNITY ECOLOGY: 



exists in some appearance of uniformitv, in the oc- 

 cupation of space, in the hmits of a boundar\. and in 

 some permanence or durability through time. 



THE COMMUNITY, A CONTINUUM? 



Although many communities appear as distinct 

 units, or stamh. having precise boundaries, manv 

 merge gradually into others. This is a natural conse- 

 quence of physical and biotic factors' following gradi- 

 ents; and because the important factors of tempera- 

 ture and moisture normally change through space, 

 habitats may assume the appearance of a spectrum. 

 There may be distinct communities, but one almost 

 imperceptibly changes into another, resulting in an 

 over-all arrangement called a continuum. 



When a continuum exists any segregation of com- 

 munities becomes somewhat arbitrary. For practical 

 purposes, one approaches the problem by attempting 

 to define pure entities and to treat the zone of transi- 

 tion between any two pure units as a special edge 

 effect, called an ecotone. Because these edges have 

 peculiar ecological properties that cause them to be 

 vers' important habitats, they cannot be ignored. .-M- 

 though ecotones can be perplexing, especially when 

 thev are extremely broad, and one might wish he 

 could avoid them because they hinder any tidy 

 scheme of community classification, ecotones gen- 

 erally are no more difficult to recognize than so- 

 called "pure" communities. .Actually, communities 

 in general probably are no more difficult — in some re- 

 spects, they are easier — to classify than individual 

 groups of organisms. 



COMMUNITY VARIATIONS 



Like populations, and because each is composed of 

 many populations, communities display variations in 

 their living components. For practical purposes, 

 these variations can be classified as lanporal or spatial. 



Temporal and spatial variations are the conse- 

 quence of chance, ecological amplitude, competition, 

 and the environment. Moreover, each of these major 

 factors leading to variation is subject to change and is 

 intimately related to other factors; thus, temporal and 

 spatial variations really are not distinct from one 

 another. 



SOURCES OF VARIATION 



.Although the sources of variation are chance, eco- 

 logical amplitude, competition, and the environment. 

 it is convenient to deal with these sources singly and 

 collectively under certain topics (Figure 19.1). 

 These topics are activity, movement, and mviron- 

 mentat gradients. Other topics of importance — 

 individual adaptation, evolution, extinction, chance, 

 and geological changes, especially in reference to 

 highways and environments — have already been 

 treated thoroughly enough for present discussion. 

 However, they will be reconsidered, along with the 

 present subject, in relation to biogeographical dy- 

 namics (pp. 364-371). 



ACTIVITY 



Few plants or animals are active to the same extent 

 or are present throughout ever\- day and season of the 

 year. In green plants there is a daily fluctuation 

 cycle. During the day the plants carrv on photo- 

 synthesis and respiration; at night, photosynthesis 

 ceases and respiration continues. In animals the 

 daily cycle usually consists of a period of sleep and 

 one of activity. Actually four periods are recognized 

 as possible: roughly, dawn hours, daylight hours, 

 evening hours, and night hours. In relation to these 

 periods certain animals are active while others are 

 inactive. 



Seasonal variations in activity normally have 

 more influence upon community composition and 

 structure than do daily cycles. In plants there are 

 definite parts of the year for germination, vegeta- 

 tive growth, flowering, fruiting, and seed maturity. 

 Many species flower during the spring, but the fact 

 that others flower in the summer, fall, or even the 

 winter causes conspicuous seasonal variations in 

 community structure. Therefore, when viewed at any 

 one time the many plant species display many to all 

 f)ossible phases of growth and reproduction. In ani- 

 mals, also, most to all stages of life cycles are repre- 

 sented by the various species independently, each 

 may be carrying on courtship, bearing young, or 

 functioning in some other process. In addition to ac- 

 tive organisms, h>oth plant and animal species often 

 have an inactive period or periods of the year. Plant 

 dormancy displays the loss of leaves and reduction 

 of functions in perennial plants and the presence of 



