CHAPTER XIV 



ECOLOGICAL FACTORS, GEOGRAPHICAL 

 DISTRIBUTION, LIFE FORM 



ECOLOGICAL FACTORS 



Studies of the conditions controlling the distribution of algae on 

 various rocky and salt-marsh coasts has shown that although the 

 habitats are very different, nevertheless the controlling factors are 

 very similar. They may be summarized briefly as follows : 



(i) The nature of the coast, whether exposed or sheltered. This 

 applies only to rocky shores because salt marshes always develop in 

 sheltered areas. 



(2) Tidal rise. This factor varies considerably from place to 

 place, but on a rocky coast the height of the rise controls the width 

 of the bands, the smaller the tidal rise the narrower will be the algal 

 zones. This factor will also operate on salt marshes, but owing to 

 the great horizontal extent of the belts it is only by accurate levelling 

 that the effect of the factor becomes evident. 



(3) Submergence and exposure operating through the daily ebb 

 and flow of the tide. In many cases it is probable that this factor 

 acts indirectly because it plays a considerable part in determining 

 salinity, moisture content and water loss from the algae. There 

 will be, however, certain species, especially the more delicate 

 Rhodophyceae, which require to be immersed every day or which 

 can only tolerate a few hours' exposure to drying. 



(4) Non-tidal exposure, or the number of consecutive days 

 during which no tide covers the area, is a factor which principally 

 operates during the periods of neap tides. On the salt marshes it 

 may assume considerable importance, especially on the higher 

 marshes, and in such habitats it is noticed that the principal algae 

 are either Cyanophyceae or marsh fucoids, both of which are 

 protected against desiccation by their histological structure or by 

 the presence of a mucilaginous envelope. Very few Chlorophyceae 

 appear capable of withstanding long periods during which they are 

 not covered by the tide, although they may be found in salt pans on 

 high marshes where the presence of the water enables them to 

 exist. Unfortunately this factor has never been studied on a rocky 



