ECOLOGICAL FACTORS 351 



(9) Biota. On a rocky coast this is largely concerned with over- 

 shadowing and the degree of epiphytism which may often reach 

 such proportions that the host plant is torn away with its massive 

 burden because of the resistance offered to the water. There may 

 also be animals, usually molluscs, which feed on the seaweeds, and 

 these can be present in such quantity as to reduce the number of 

 plants considerably or even to keep the area bare. An example of this 

 is the behaviour of Hydrohia Ulvae on Norfolk marshes where it is 

 present in such abundance that certain areas are kept more or less 

 clear of Monostroma and Ulva. In addition to the molluscs there is 

 the further problem of the phanerogamic vegetation on the salt 

 marshes. In certain cases this may provide additional shade or 

 lower the surface evaporation so that algae can grow at higher 

 levels than they would do on the open marsh, e.g. Catenella repens 

 around bushes of Suaeda fruticosa on the Norfolk marshes. The 

 density of the phanerogamic vegetation, e.g. swards of Puccinellia 

 7naritima or Spartina patens, may prevent any real algal vegetation 

 from developing. This can be seen on many west coast marshes of 

 England and also on the marshes of New England. 



(10) Light. Measurements show that the incident light is cut 

 down very considerably at even a depth of i m., and hence algae 

 living near low-water mark will be existing under very different 

 light conditions to those near high-water mark. This factor is said 

 by many workers to be of great importance in determining vertical 

 range, but it is of course very difficult to disentangle its effect from 

 that of the other factors. In heavily silt-laden waters this factor will 

 probably assume even greater dimensions. 



Although all these factors may be operating continually through- 

 out the year, it must not be forgotten that only one factor operating 

 at the critical period in the life history of a single species may be of 

 even more importance. Johnson and Skutch (1932) have stressed 

 this point, and they maintained that a maximum water loss during 

 the most active growing period may be of paramount importance in 

 determining the presence or absence of some species. 



With this general introduction we may now turn to consider 

 studies dealing more specifically with zonation on a rocky shore. 

 Baker (1909, 19 10) carried out numerous field observations on the 

 algal zones found around the Isle of Wight, and also conducted 

 experiments in which the four principal fucoids were grown in jars 



