HYPOTHESIS OF ANIMAL EXCLUSION 



321 



plankton production overshadowing the influence of all other forms. In areas where 

 there are not one or two quite outstanding herbivorous animals, and where vertical 

 migration may not be so produced, the two principles of grazing and exclusion, which 

 it would appear must be in progress in all areas, may be very much more difficult to 

 disentangle. In our area Salpa fusiformis is another very abundant herbivorous animal, 



EUPHAUSIA SUPERB A 



40.000 



EUPHAUSIA SUPERBA 



30000 - 



20.000 



10,000 



Fig. 169. Showing the numbers of Euphausia superba taken in N 100 H nets at deep-water and shallow- 

 water stations, each series being arranged from left to right in order of decreasing phosphate values which 

 are also shown as curves. This figure should be compared with Fig. 145 showing increasing phytoplankton 

 with decreasing phosphate values. The night stations are marked "N". 



which may well play an important part with Euphausia superba in controlling phyto- 

 plankton production. The two species had a very similar distribution in our survey. 

 If the phytoplankton under particularly favourable conditions of growth should out- 

 step the grazing activities of Euphausia superba and Salpa, then these vertically mi- 

 grating animals may be expected to undergo the same gradual process of exclusion, the 

 mechanism of which is discussed in a later section. 



If a small herbivorous animal becomes exceedingly abundant then it too might 

 play its part in the control of the phytoplankton. On p. 84 it was suggested that the 

 poverty of phytoplankton at Sts. WS 41 and WS 42, accompanied by a reduced phos- 

 phate content, which would have led one to expect a rich phytoplankton production, 



