3o8 MARINE ECOLOGY 



heights. On the whole, any one species usually occupies a very 

 definite vertical range and only occasionally is to be found outside 

 it, and then there is often some cause, such as the presence of a 

 rock pool, in which conditions for its existence are favourable. It is 

 not intended in this chapter to enter into any detailed discussion as 

 to the causes or factors controlling this zonation, an aspect which is 

 dealt with more fully in the last chapter (cf. p. 351). It is sufficient 

 here to point out that these zonations do exist and are characteristic 

 of a rocky shore. Furthermore, a glance at Table XII will show 

 that on the whole the zonation is remarkably similar around 

 most of the British Isles, and the same or very similar communi- 

 ties can be found at much the same level at the different localities. 

 The actual number of communities recognized depends upon two 

 factors : 



(a) The locality. It will be observed that the two Irish stations 

 have a much richer zonation, and this can probably be associated 

 with their position in relation to the Gulf Stream because this will 

 tend to produce a mixture of species from both cold and warm 

 waters. 



(b) The personal factor. Each investigator will tend to have a 

 somewhat different concept of what is represented by an algal 

 community, whilst the number of communities recognized will 

 also depend upon the time and thoroughness with which the shore 

 is examined. 



The terminology that has been employed has led to no little 

 confusion. Algal ecology, as such, commenced later than the 

 ecology of land vegetation. Some investigators have attempted to 

 apply the terms used in land ecology to algal ecology, whilst others 

 have considered that the conditions are sufficiently different to 

 make this appHcation impossible. Cotton (191 2), for example, 

 recognized five algal formations at Clare Island : 



(i) Rocky shore formation. 



(2) Sand and sandy mud formation. 



(3) Salt marsh formation. 



(4) River mouth formation. 



(5) Brackish bay formation. 



These were subdivided into associations, the rocky shore forma- 

 tion containing the associations of the exposed coast and the 



