Chapter XV 



ECOLOGICAL FACTORS 



Any attempt to understand and interpret the zonation to be ob- 

 served on a rocky coast or on a salt marsh must be based upon an 

 appreciation of the different ecological factors and the part that 

 they play. It may be expected that more than one factor will nor- 

 mally exert a controlling influence, but at one time of the year or at 

 a certain phase in the life history of an organism one single factor 

 may become paramount. It has also become increasingly evident 

 that a proper appreciation of some of the factors cannot be divorced 

 from a study of the physiology of the algae. Certain aspects of algal 

 physiology must therefore be discussed in this connexion rather 

 than in the chapter on physiology. 



The essential factors can broadly be divided into four groups, 

 (i) physiographic (dynamic according to some writers), (2) physical, 

 (3) chemical, (4) biological. Similarly, at any given point on the 

 coast, one can conveniently attempt to segregate the factors into 

 those that are (i) causal, i.e. responsible, so far as one can see, for 

 the basic zonation to be observed, (2) presence or absence, i.e. 

 determine whether a species shall be present or not, such as the in- 

 fluence of fresh water or a change in the type of rock, but not deter- 

 mining the level at which the species grows, (3) modificatory, i.e. 

 causing a variation in the level normally occupied, e.g. eflect of 

 spray, of cracks, of shade, etc. 



A. Physiographic factors 



The dominant factor here is the tide, though it operates in- 

 directly in a nimiber of ways : 



(a) Degree of shelter or exposure to the waves 



This is primarily a presence or absence factor, some species, 

 Durvillea, Postelsia, possessing holdfasts that enable them to with- 

 stand strong wave action. Many examples are to be found in the 

 literature illustrating change of zonation with varying degrees of 

 exposure. It must be supposed that even the sporeUngs possess this 



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