364 



COMMUNITY ECOLOGY: 



Stage can invade. From this point, further succession 

 might approximate xerarch succession beyond the 

 herb stage. 



Xerarch and hydrarch succession have many 

 similarities, the most striking being that in both kinds 

 of succession one stage creates the conditions neces- 

 sary for the next. Also, whether primary succession 

 starts upon a rock or within water, the later serai 

 stages, from herb stage onward, can be much the 

 same. 



In moister and colder areas, hydrarch succession 

 usually shows a notable deviation. Such environ- 

 ments might have one or more of the early hydrarch 

 stages prior to an invasion of sphagnum moss (or 

 perhaps some other aquatic plant), but even the pri- 

 mary stage might be sphagnum. However, when 

 present this moss develops into a dense growth, called 

 a peat bog, that can support the weight of a man. 

 The peat bog stage often is invaded next by a shrub 

 stage, but the invader sometimes is a tree stage. In 

 the event of a shrub stage, a tree stage usually follows. 

 Finally, the tree stage is followed by the climax (see 

 Figure 17.14, p. 321). 



BIOGEOGRAPHICAL DYNAMICS 



The places occupied by populations and communi- 

 ties rarely are static or fixed, and even when they are, 

 such conditions are temporary. Always there are 

 tendencies toward range expansion or restriction. 

 Expansion is the normal consequence of a reproduc- 

 tive rate that produces more individuals that can be 

 supported by the carrying capacity of an ecosystem. 

 The excess of individuals leads to population pres- 

 sure, a form of struggle for existence. Population 

 pressure, in turn, regularly leads to dispersal of the 

 young. In many organisms, eggs or comparable 

 stages are structurally adapted for passive dispersal, a 

 form of emigration, to areas other than the site of 

 birth. In other organisms, behavioral adaptations as 

 related to population pressure appear to lead to 

 emigration in the form of active dispersal of the 

 young. 



Area contraction is due to unfavorable environ- 

 ments. During unfavorable time periods, individual 

 populations of a species are reduced in size. Faced 

 with minimal environmental conditions and such 

 things as catastrophies, remnants of decimated 

 populations may emigrate to sites of optimum condi- 

 tions that remain for the species. Moreover, while 



these unfavorable conditions remain, the species will 

 not normally leave these optimum environments. 



Such factors contribute to ever-changing areas, a 

 subject summarized by the concept of biogeographi- 

 cal dynamics. However, fuller appreciation of the ' 

 dynamics of distribution can be gained only when 

 other factors are examined (Figure 19.9). For ex- 

 ample, dispersal adaptations are the manner or 

 means of increasing a species range. However, dis- 

 persal adaptations alone account neither for sizes 

 of communities, for sites of community origin, for the 

 types of communities, nor for the organization of 

 communities. Understanding of such things necessi- 

 tates reconsideration of such matters as highway vs. 

 barriers, geomorphic cycles, ecological amplitude, 

 interorganism relationships, and evolution. These 

 operate and function in a process much the same as 

 biotic succession. Here, the process is called bio- 

 geographical succession. 



Each stage of biogeographical succession involves 

 success and failure of organisms. Success demands 

 passage through certain steps or stages. These steps 

 are population pressure owing to an excess of individuals, 

 dispersal (emigration) across highways to a particular 

 area, and ecesis. (Ecesis really is three steps: first, 

 becoming an occupant of an area; second, increasing 

 population size; and third, becoming integrated with 



ecesis ecesis 



changed area 



Figure 19.9 The factors affecting the biogeographical dynamics of a 

 populotion. 



