Inter-tidal Ecology at Narrow Neck Reef — Dellow 
373 
Fig. 12. Graph indicating the relationships be- 
tween the number of species, and of upper and 
lower limits at different heights on the shore. 
number of upper limits, and curve C the total 
number of species. There are two maxima in 
both A and B, one low down on the shore, 
and one higher up. The greatest number of 
both upper and lower limits occurs between 
+ 1 and -f-4 feet, i.e., between M.L.W.N. 
and M.L.W.S. (= M.L.W.M.). Curve A 
(lower limits) has a second pronounced max- 
imum between 7 and 10 feet, just below E. 
(L).H.W.N. Curve B (upper limits) rises 
to a peak between 10 and 13 feet, i.e., at 
M.H.W.S. There is a secondary maximum at 
E. ( H ) .L.W.N. 
In all cases but one (between 3 and 8 feet), 
the total number of species is less than the 
total number of limits at any one level. At 
both Wembury and Cardigan Bays, however, 
the total number of species at each level al- 
ways exceeded the total number of limits: 
that is, inter-tidal species in these localities 
have, in general, a wider vertical range than 
those at Narrow Neck. This difference may 
prove to be correlated with lower average 
minimum temperatures in Great Britain, as- 
sociated with higher latitudes. 
A further graph was constructed by plot- 
ting total limits minus total number of species 
against height on the shore (Fig. 13). Again 
there were two maxima— -at M.L.W.M. and 
M.H.W.M. — with a subsidiary maximum be- 
tween M.L.W.S. and E.L.W.S. 
It is possible that these levels may be of 
critical significance in determining vertical 
Fig. 13. Graph obtained by plotting the num- 
ber of species minus the number of limits against 
height on the shore. The two peaks indicate the 
positions of two critical levels (M.L.W.M. and 
M.H.W.M.). M.-E.L.W.S. (between 1 and 2 feet) 
may also be critical. 
zonation at Narrow Neck. Presumably, im- 
mediately above or below each critical level, 
there is a change in one or several of the 
factors comprising the external environment. 
This change must be of sufficient magnitude 
to restrict or inhibit optimum growth of the 
individuals concerned and involves a consid- 
eration of the nature of factors operating in 
the inter-tidal region. Investigations on the 
ecological factors are being continued, and it 
is hoped that further results will be published 
at a later date. 
SUMMARY 
An account is given of the plant and ani- 
mal communities at Narrow Neck Reef, 
Auckland, in relation to their levels on the 
shore and to the range of exposure annually 
undergone by each. Four leveling traverses 
were made in different parts of the area to 
illustrate the main patterns of zonation. 
The problem of critical levels is discussed. 
The following levels may be regarded as 
critical in determining zonation at Narrow 
Neck: 
NUMBER NUMBER 
OF OF 
TIDE LEVEL SPECIES LIMITS EXPOSURE 
per cent 
I. M.L.W.S.-E.L.W.S. 8 13 1 
II. M.L.W.M. 12 19 6 
III. M.H.W.M. 9 14 92 
