RELATIONS AMONG SPECIES 



357 



ies, and each individual within a species, possesses an 

 ecological amplitude that determines its environ- 

 mental distribution. However, ecological amplitudes 

 vary, especially among different species. For this 

 reason, one also finds particular environments in 

 which species can exist together, and as one goes fur- 

 ther from this environment, a gradual dropping out 

 of one species after another. 



The precise horizontal and vertical space in- 

 habited by organisms to form a large, individual 

 community is a function of many environmental 

 factors. Each environmental factor normally en- 

 compasses a hierarchy of gradients in which each 

 small gradient contributes to the nature of the next 

 higher gradient. Each level of gradients, and es- 

 pecially the largest level, is characterized by either 

 a continuous or discontinuous change in the particu- 

 lar factor. Therefore, organisms are subjected to 

 both individual and collective ecological factors and 

 the gradations of each and all. Certain individual 

 factors or combinations of factors will prove limit- 

 ing to each species; if further intolerable situations 

 are present, the particular species cannot exist. On 

 the other hand, where organisms occur, the species 

 are in dynamic equilibrium with their physical en- 

 vironment and the other organisms composing their 

 ecosystem. Owing to these various physical and 

 biotic relationships, species and communities dis- 

 play less diversity than do their habitat and the 

 environment in general. 



So far we have assumed rather fixed environmental 

 conditions, yet the complexity of even static habi- 

 tats is difficult to comprehend. Soon, the topic of 

 biogeographical dynamics will briefly outline what 

 takes place when environments shift in relation to 

 geomorphic cycles. 



to be approached. In fact, the basic workings of 

 most habitats are likely to lead to eventual stability 

 (see discussion of succession). This stability, or 

 equilibrium, is not rigid or fixed; actually it is 

 dynamic, like the swinging of a pendulum about a 

 central point in its fluctuation. 



Equilibrium is best conceived in terms of a balance 

 of nature. Although individual populations, and even 

 entire communities, fluctuate, there are mechanisms 

 that regulate or control departure from average con- 

 ditions (Figure 19.4). For example, if a species be- 

 comes overabundant it is more likely to be a source of 

 food for more animals, to have increased parasitism 

 and disease, and to enter into greater competition 

 with its own kind. In other words, an overabundant 

 species is reduced by density-dependent factors of its 

 environment. On the other hand, a species with re- 

 duced numbers is affected only slightly by density- 

 dependent factors and tends to increase in numbers. 



density independent 



EQUILIBRIUM 



density dependent 



Figure 19.4 The two major groups of ecological factors contributing 

 to community equilibrium. 



Equilibrium is not simply the result of all ecological 

 factors affecting species in direct relation to the num- 

 ber of individuals per unit area, because some factors 

 are independent of species density. These latter 

 ecological factors, which can include most physical 

 aspects of an environment, are termed density inde- 

 pendent factors. 



TENDENCY FOR EQUILIBRIUM 



It is easy to overemphasize the changes and con- 

 ditions for change that are present in any commu- 

 nity. All communities normally progress to a 

 condition of equilibrium under the set of environ- 

 mental conditions present at a given time. Under 

 uniform conditions (which probably do not exist in 

 nature), change regularly is directional and toward 

 greater complexity in the sequence of communities 

 occupying a particular site. However, the more 

 complex the replacing communities become, the 

 greater is the tendency for equilibrium conditions 



COMMUNITY INFLUENCE 



In addition to modifying one another, communi- 

 ties often modify their physical environment. 



COMMUNITY INTERRELATIONS 



The influence of one community upon another is 

 directly related to the size and stability of each and 

 to the distance between the communities. In general, 

 the larger a community the more influence it is likely 

 to have upon others. Proximity is important in inter- 



