Inter-tidal Zonation at Rangitoto Island, New Zealand. 
(Studies in Inter-tidal Zonation 4.)^ 
J. A. Carnahan 2 
INTRODUCTION 
This study was undertaken to elucidate, if 
possible, the causative agents determining 
the distribution of inter-tidal plants and ani- 
mals on the shores of Rangitoto Island. The 
work comprised an attempt to classify the 
inter-tidal communities, followed by an ex- 
amination of tidal and other environmental 
factors. 
I wish to thank Professor V. J. Chapman for 
his advice and criticism. My thanks are also 
due to Miss R. F. de Berg, Mr. V. W. 
Lindauer, Miss L. B, Moore, and Mr. J. E. 
Morton for assistance with identifications; to 
Mr. P. Carnahan for help with the instrument 
levelling; and to Mr, C. Harvey for preparing 
the graphs for reproduction. Finally, the work 
on Rangitoto was facilitated by the co- 
operation given by the Naval Department 
and by the staff at Rangitoto Naval Base. 
DESCRIPTION OF LOCALITY 
Rangitoto Island is situated in the south- 
west corner of the Hauraki Gulf, New Zea- 
land, off the entrance to Auckland Harbour 
(Fig. 1). It is a roughly circular island, with a 
diameter of 3 miles. The western and southern 
coasts are about 3 miles from the mainland, 
while the adjoining island of Motutapu pro- 
tects its eastern shores. Only the northern 
^This paper contains the substance of a thesis 
presented at Auckland University College for the 
degree of M.Sc., 1948. 
^Junior Lecturer, Department of Botany, Massey 
Agricultural College, Palmerston North, New Zealand. 
Manuscript received August 2, 1949. 
coast is exposed, but even this faces a gulf, 
not the open sea. 
Rangitoto is a volcano which has been 
active in recent times. The greater part of the 
island consists of flow basalt and exhibits the 
characteristic low conical shape of such vol- 
canoes. This portion is probably more or less 
contemporaneous with the Pleistocene basalt 
volcanoes which are of frequent occurrence 
in the Auckland metropolitan area. The final 
cinder cone within the original crater may 
have ceased activity within historic times. 
The principal substratum is the basic lava 
which comprises the island volcano. Upon 
sheltered shores, this occurs as solid flows or 
as broken scoria (Fig. 2). On fairly exposed 
shores, the flows are somewhat broken and 
the scoria is waterwashed. On the more ex- 
posed northern coasts, the heavy surf raised 
by northeast winds has produced difling and 
boulder beaches. 
Mud occurs as harbour silt beyond the 
termination of flows and scorias. It also ap- 
pears wherever it is possible for sedimentation 
to take place. It is conspicuous in protected 
bays and inlets and on scoria shores. Minor, 
localised substrata are developed from ‘’na- 
tural brick” (sedimentary material baked by 
contact with hot lava), sand, and shell. 
TERMINOLOGY 
The writer follows Chapman (1946) and 
Beveridge and Chapman (1950) in applying 
terrestrial plant ecological terminology to 
inter- tidal communities of plants and animals. 
This is based on the assumption of the cli~ 
[ 35 } 
