354 



COMMUNITY ECOLOGY: 



weather conditions. Relocations in response to 

 periodic climatic changes might cause significant 

 changes in a community if an entire species is re- 

 moved from the locality. 



ENVIRONMENTAL GRADIENTS 



If one considers a community as a fixed, static 

 entity rather than recognizing its true dynamic 

 nature, large to small changes in ecological factors 

 are better appreciated. Such changes or gradients 

 in environments are the rule in nature. Gradients 

 may be irregular or directional and each of these may 

 display sudden or gradual changes. For these rea- 

 sons, individual communities and their segments, 

 down to parts of individual species, display either a 

 regular distribution, owing to geographic zonation 

 of gradients, or an irregular distribution, owing 

 to a mosaic occurrence of gradients through space. 

 However, gradients exist in a hierarchy. First-order 

 gradients tend to reflect general climate and are ex- 

 pressed mostly as latitudinal or altitudinal zonation 

 of life. Second-order gradients normally relate to 

 local topography, being observed in somewhat re- 

 stricted effects of slope and exposure. Third-order 

 gradients exist within communities, and consist of 

 restricted expression of life in relation to very 

 localized variation in such things as soil or topogra- 

 phy. 



The various orders of gradients, and whether each 

 is irregular or directional, sudden or gradual, leads 

 to a basic framework of variation within any com- 

 munity. It is this basic framework that is modified 

 by daily, seasonal, and periodic directional and cyclic 

 changes. In addition, the changes — and hence, the 

 framework — are either functions of or are further 

 modified by activity, movements, extinction, and 

 evolution. 



NATURE OF VARIATIONS 



Variations operate as an intricate complex that 

 contributes to the dynamic nature of the community. 

 However, as was mentioned, this complex may be 

 examined from temporal and spatial points of view. 



TEMPORAL CHANGES 



Fluctuations in the composition and nature of 

 life through time can be studied in the species, com- 



munity or adjacent related communities (intercom- 

 munities). Species, community, and intercommunity 

 changes can be either noncyclic (not starting from 

 one condition and returning to it) or cyclic. Further- 

 more, noncyclic changes may prove either irregular 

 or directional. Cyclic changes, by definition, com- 

 plete a circle of events in which both start and finish 

 are represented by the same kind of life (Table 19.1). 



Species Changes. Species changes were discussed 

 in the chapter on fxjpulation ecology, but are men- 

 tioned here because they contribute to over-all com- 

 munity changes. Community changes, in turn, con- 

 tribute to intercommunity changes — and even these 

 are part of the grander scheme of things. 



Noncyclic irregular changes may involve additions, 

 subtractions, or replacements in a community. Ad- 

 ditions can come from immigration, subtractions 

 from emigrations, and replacement from substitution 

 for individuals lost through death of a noncata- 

 strophic nature. Because these changes are at the 

 species level, additions and subtractions must relate 

 to species already present in a given community. 

 Therefore, additions probably are of minor impor- 

 tance, contributing nothing that reproductive rate 

 might not produce; but subtractions might reflect 

 population pressure that causes emigration. Replace- 

 ment generally, is of significance because after indi- 

 vidual death, replacement usually is by a member 

 of a species other than the one that died — a member 

 of a species already present in the community, how- 

 ever. 



.Noncyclic directional changes in individuals, or even 

 entire species, can be due to any species' replacing 

 another as a result of competition. When the change 

 occurs within a species it might be due to chance or 

 to natural selection of a better-adapted individual. 

 When one species replaces another, it can be due to 

 competition. 



Cyclic changes in single creatures or entire species 

 are shown in periodicity and phasic cycles. Perwdicily 

 was discussed in terms of the annual occurrence of 

 particular stages in the life cycle of individuals or a 

 species. Phasic cycles start with the death of a member 

 of a species and end with the reestablishment of the 

 same species (Figure 19.2). For example, an in- 

 dividual may die. Then, decomposers of various 

 kinds may invade the organic remains and, during 

 decomposition, other organisms may follow a se- 

 quence in invading the remains. Finally, bare 

 ground is the consequence of complete decomposi- 



