Inter-tidal Zonation— BEVERIDGE and CHAPMAN 
197 
eluded that species requiring a wetting each 
day will not grow above the 9-foot level It 
will be seen too that the percentage of air 
exposure increases rapidly between 4- 2 and 
+ 3 feet. Further, there is a significant de- 
crease in the percentage of submergence be- 
tween 9 and 11 feet. 
An additional factor, that of tidal flow, 
which has been studied by Elmhirst (1933) 
and David (ms.), may exert a certain effect 
around E.(L).H.W.N. and E.(H).L.W.N. 
This effect is related to regular variations in 
rate of tidal flow during ebb and flood. Thus 
from either high or low water the tide runs 
7 per cent of its height in the first hour, 18 
per cent in the second hour, 25 per cent 
during each of the third and fourth hours, 18 
and 7 per cent during the fifth and sixth 
hours, respectively. 
RELATION OF SPECIES TO TIDE LEVELS 
A levelling survey was carried out to de- 
termine the upper and lower limits of the 
more important algal and animal species. 
Only the optimum range of a species was 
measured, stragglers being excluded, and 
wherever possible levels were obtained that 
would give a direct expression of the tidal 
influence. Additional readings were also 
taken in places where one of the environ- 
mental factors brought about a local change 
in the normal level, either raising the upper 
limit or depressing the lower limit. The re- 
sults of the levelling survey are incorporated 
in a series of diagrams for the different areas 
where the range (obtained by taking the 
mean of several levels) has been plotted 
against tide level 
In a few cases the lower limits of species 
growing well down on the shore are not 
known with certainty; this is because the sea 
is seldom calm at Piha and it is therefore dif- 
ficult to obtain levels at low water. In these 
cases observation has had to supplement act- 
ual readings. Thus the levels of species grow- 
ing in the vicinity of the tide pole were com- 
piled from observations made while watch- 
ing the tide pole. One may also say that up- 
(S.L.R. ), southwest of Lion Rock (S.W.L.R. ) . 
