372 
PACIFIC SCIENCE, Vol. IV, October, 1950 
feet, but both Chamaesipho columna 
and Elminius modestus occur in the 
Encrusting Coralline association, 2 feet 
below. 
2. Cranwell and Moore (1938) list Li- 
china pygmaea and Melaraphe oliveri 
as the chief dominants in the "supra- 
littoral” or "splash zone" at Narrow 
Neck. However, Lichina does not exist 
as a dominant here, and it has been 
shown from the leveling survey that 
Melaraphe does not normally occur 
above E.H.W.S. 
3. The same association-complex is to be 
found with only slight local modifica- 
tions farther north on Takapuna and 
Milford Reefs where the substrate is 
volcanic basalt. 
CRITICAL LEVELS 
Inasmuch as the inter-tidal region may be 
divided into a number of zones, each char- 
acterized by certain species which are absent 
or insignificant in other zones, it may be as- 
sumed that some levels are more important 
than others in restricting the upward or down- 
ward extension of a species. Colman ( 1933 ) , 
David (1943), Chapman (1943), and Evans 
(1947) have investigated the problem of 
critical levels 5 in Great Britain, and Doty 
(1946) has made similar studies on the Pa- 
cific coast of North America. The fact that 
David found only one such level at Aberyst- 
wyth identical with one of Colman’s levels 
at Wembury Bay, namely E.(L) .H.W.N., 
indicates that each locality should be treated 
on its own merits. Both the number and the 
position of critical levels appear to vary from 
coast to coast. 
From an examination of Figure 7, it is ap- 
parent that there are certain relationships be- 
tween the total number of species, the num- 
5 A critical level may be defined as a level at 
which a relatively great number of species reach 
the upper or lower limit of their vertical ranges. 
ber of upper and lower limits, and the level 
on the shore. For example, six species have 
their lower limits and eight species have their 
upper limits of vertical distribution between 
the levels of 9 and 12 feet, making a total 
of 14 limits, while only nine species occur 
between those levels. Between 5 and 8 feet, 
on the other hand, there is a total of eight 
limits, with the occurrence of nine species. 
The concept of critical levels is based on the 
assumption that the fewer the number of 
species and the greater the number of upper 
and lower limits at a certain level, the more 
critical it will be in limiting the vertical range 
of a species or community. 
The relationship between height on the 
shore, number of species, and number of up- 
per and lower limits of species was investi- 
gated from data obtained in the leveling sur- 
vey. Colmans (1933) method for finding 
critical levels was followed. By taking from 
Figure 7 the number of species with limits 
between — 1 and +2 feet, 0 and +3 feet, 
+ 1 and +4 feet, and so on, three curves 
were constructed (Table 2 and Fig. 12). 
TABLE 2 
Relationship of Height on Shore 
and Species 
DIFFER- 
ENCE 
BETWEEN 
TOTAL 
SPECIES 
FEET ABOVE 
OR BELOW 
LOWER 
UPPER 
TOTAL 
TOTAL 
AND 
TOTAL 
A.H.I 
!.D. 
LIMITS 
LIMITS 
LIMITS 
SPECIES 
LIMITS 
+ 12- 
+ 15 
0 
1 
1 
1 
0 
11- 
14 
0 
3 
3 
3 
0 
10- 
13 
1 
9 
10 
9 
1 
9- 
12 
6 
8 
14 
9 
5 
8- 
11 
6 
7 
13 
10 
3 
7- 
10 
7 
3 
10 
10 
0 
6- 
9 
5 
4 
9 
9 
0 
5- 
8 
3 
5 
8 
9 
■1 
4- 
7 
4 
3 
7 
8 
1 
3- 
6 
6 
8 
14 
12 
2 
2- 
5 
10 
7 
17 
13 
4 
+ 1- 
+4 
11 
8 
19 
13 
6 
0- 
+3 
9 
4 
13 
8 
5 
— 1- 
+2 
5 
3 
8 
6 
2 
In Figure 12, curve A represents the num- 
ber of lower limits at each level, curve B the 
