140 
PACIFIC SCIENCE, Vol. XVII, April 1963 
Distribution of Species Populations along 
Environmental Gradient 
The distribution pattern of the populations 
of Sargassum may not be understood when only 
the depth of water is taken into consideration as 
an environmental factor. The effect of the wave 
action on it must be considered at the same 
time. The survey stations are classed in three 
groups according to the degree of wave action, 
as shown above. Average degree of cover of 
each species is plotted against depth. For five 
major species, the curves of average degree of 
cover were superimposed to obtain the graph 
represented by Figure 7. The curve for each in- 
dividual species overlaps the others and the 
peaks of the different curves occur at different 
depths. For the places moderately exposed to 
wave action (Fig. 7, lower graph), the curve 
for S. hemiphyllum has its peak at a depth of 
1 m below low watermark, that of S. ringgold- 
ianum comes second, and next in order are S. 
piluliferum, S. patens, and S. serrati folium, from 
the shallowest to the deepest. In places subjected 
to stronger wave action (Fig. 7, upper graph), 
the order of appearance is the same as above, 
but the peaks of the curves for degree of cover 
occur deeper, except in the case of S. ringgold- 
ianum. 
The method of gradient analysis developed 
% 
Fig. 6 . Variation of average vegetational cover in 
relation to depth of water. 
Average of whole quadrat data 
x X Sheltered 
O O Moderate wave action 
A A Exposed 
Fig. 7. Change of cover degree in relation to depth 
of water. (Abbreviation of species names as in Fig. 5.) 
by Whittaker (1956) is introduced here to 
make the distribution pattern of these Sargas- 
sum species more clear. The depth of water and 
exposure to wave action are considered as the 
principal environmental gradients. The popula- 
tion nomograms for six major species con- 
structed on this basis are shown in Figure 8. S. 
serratifolium, S. piluliferum, and 5". hemiphyl- 
lum have their peaks at the stations exposed 
to stronger wave action, but their peaks oc- 
curred at different depths, in the order given, 
with S. hemiphyllum at the shallower end of the 
series. The species which occurred more abun- 
dantly at the places subject to more moderate 
wave action are S. ringgoldianum and S. fulvel- 
lum. The former grows more abundantly at 
places of 2 m depths where the bottom is hard 
and wave action is moderate. It has a tendency 
to grow in shallower places with both increas- 
ing and decreasing strength of wave action. 
This bimodal distribution pattern is peculiar 
to this species only. Only young individuals of 
S. fulvellum, an annual species, were found 
when the investigation was carried out, and the 
degree of cover of this species was very low. 
Nevertheless a population nomogram of dis- 
tinctive pattern was obtained. S. patens and S. 
tortile grow abundantly on bottoms more shel- 
tered from wave action. However, the popula- 
tion nomogram for one species differs more or 
less from the others in the position of maximum 
value, even if the distribution patterns are 
similar to each other. Generally speaking, most 
species of Sargassum grow in deeper places as 
the strength of the wave action increases. 
