Inter-tidal Zonation — - CARNAHAN 
The object of the work classed under this 
section was the determination of these varia- 
bles, partly from detectable variations in the 
vertical levelling and partly in more empirical 
ways. 
The factors sought may be classified rough- 
ly into two groups: the modifying factors, evi- 
denced by variations in vertical distribution; 
and the presence-or-absence factors, as mani- 
fested by differences in transect composition. 
Modifying Factors. Most of the observed 
variations in vertical distribution took the 
form of raised upper limits. There were few 
cases of lowering of lower limits. The 
general basis of the factors inducing the for- 
mer may be the prevention of normal desic- 
cation, and of the latter, the lessening of 
normal submergence. The upper limit of 
Hormosira hanksii is raised by as much as 1.0 
foot by prevention of run-off on fairly flat 
scoria shores and in mangrove swamps. Simi- 
larly, seepage induced by the pervious nature 
of the scoria substratum results in the occur- 
rence of Zostera nana, Aeodes nitidissima, the 
kelps, and species of Pterocladia in the lower 
littoral at much higher levels than those de- 
termined by the tidal factors. 
At exposed points, the upper limits of 
some species at the upper boundary of the 
littoral may be raised to considerable heights 
through the influence of spray, while aeration 
may lower the lower limits of some species 
at the lower boundary of the littoral. Mela- 
rhaphe oliveri, whose normal upper limit is 
E.H.W.S.T., is found on the northern coasts 
on top of the cliffs, 10.0 feet or more above 
theoretical E.H.W.S.T. In exposed places, 
Chamaesipho columna may be found dominant 
down to E.L.W.S.T., more than 2 feet below 
its lower limit, on less exposed shores. This 
may be due to wave-induced aeration or to 
increased light because of the clearer water of 
the outer shores. 
The micro-climates, provided by the topog- 
raphy of the scoria or by actual vegetation, 
present another case of the raising of upper 
45 
l 
limits by the reduction of exposure-induced 
desiccation. Within clefts in the upper littoral, 
where the relative humidity was determined 
by rough wet-and-dry bulb tests to be about 
25 per cent higher than in the open, the upper 
limits of Catenella nipae and Caloglossa lepri- 
eurii are raised by 0.7 foot. Under Hormosira 
hanksii thalli, similar tests show the humidity 
to be about 30 per cent higher than in the 
open. In the Hormosira consociation, the 
subordinate Corallina officinalis occurs up to 
3.0 feet above its normal upper limit, and 
Microdictyon sp. nearly as much. 
In other cases, direct protection from inso- 
lation appears responsible for raising the 
upper limits of species, e.g., Codium adhaerens 
(raised 1.5 feet) and Ecklonia radiata var. 
richardiana (0.75 foot). 
Presence-or-absence Factors. Of the fac- 
tors controlling the presence or absence of 
species at one point, the one best exhibited 
at Rangitoto is relative physical exposure. 
The geography of the island provides a wide 
range of conditions. It would appear that 
physical exposure inhibits the establishment 
of some species, while sheltered conditions, 
and especially the encouragement of sedi- 
mentation, act against other species. 
Xiphophora chondrophylla var. minus, Carpo- 
phyllum plumosum, Pterocladia lucida, P. capil- 
lacea, Glossophora kunthii, and Chamaesipho 
hrunnea occur only under conditions of ex- 
posure, while Hormosira hanksii occurs only in 
shelter. Cystophora torulosa, Scytothamnus aus- 
tralis, and Splachnidium rugosum are dominant 
only in exposed places. Conversely, Saxostrea 
glomerata thins out with exposure. Most of 
the other dominants, however, appear in- 
different to the degree of physical exposure. 
Corallina officinalis is reduced to its basal por- 
tion by exposure. 
The nature of the substratum affects the 
presence or absence of species. The sedimen- 
tation induced by shelter results in fasciations 
and serai communities. In the case of the pre- 
dominant rocky shores, the variable nature of 
