Population Dynamics— -PEQUEGN AT 
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TABLE 4 
Relative Population Densities of Large, Motile Species Before and 
During the Decline of Mytilus Populations 
SPECIES 
INDIVIDUALS /M 2 * 
Predecline 
During Decline 
depth (m) f 
depth (m)f 
2-7 
7.1-11.2 
2-7 
7.1-11.2 
Pisaster giganteus 
33 
1.7 
1.2 
1.5 
Strongylocentrotus purpuratus 
3.7 
0.5 
1.6 
1.0 
Strongylocentrotus franciscanus 
0.5 
5.8 
1.1 
4.8 
Parastichopus parvimensis 
0.1 
0.6 
0 
0.5 
* Average number of individuals per square meter. 
f Reef is divided roughly into upper (2-7 m depth) and lower parts to show some redistribution of individuals of some 
species with depth. 
appeared only after Corallina became predomi- 
nant; (3) others maintained their populations 
at previous levels, though most underwent tem- 
porary fluctuations; and (4) still others de- 
creased and, in some instances, disappeared. Most 
of the major population shifts occurred where 
Mytilus was previously dominant, but a few 
significant shifts were noted below the 4-m line. 
The degree of these effects may provide some 
measure of the closeness of relationship of in- 
dividual species to the mussel (and to Corallina ), 
both in positive and negative (inhibitory) ways. 
Our knowledge of these relationships will be 
increased significantly when careful studies are 
made of the feeding habits of the many species 
involved. 
Some species increased in numbers simply 
because space became available and they had the 
reproductive capacity to spread out. Balanus 
tintinnahulum exemplifies this; its larvae were 
available to settle prior to the encroachment of 
Corallina. It is apparent, also, that Corallina is 
unable to invade an established Mytilus bed, 
but when the latter is gone the alga moves in 
after a few weeks’ lag. Other species increased 
after this alga had become established, apparently 
because it provided a more suitable milieu. Quite 
possibly some species increased as a result of the 
withdrawal of predators not adapted to living 
in algal mats. The marked decrease in disaster 
giganteus resulted from an abrupt loss of food. 
The shift of remaining individuals to the lower 
part of the reef is probably related to the pres- 
ence there of Chama pellucida, an alternate food 
source. The anemone Anthopleura sp. dropped 
from an average of 40 individuals to 17/0.1 m 2 
because of a reduction- of substratum rather than 
of food. Prior to the decline most anemone in- 
dividuals lived on mussel valves, with a few 
living in the rock niches. The number increased 
on the rock as the mussel declined, but the total 
available space was not sufficient to sustain the 
previous populations. The reduction of Para- 
stichopus parvimensis may be accounted for by 
the greater production of detritus by Mytilus 
than Corallina, but this is purely conjectural. 
The different reactions of the sea urchins Stron- 
gylocentrotus purpuratus and S. franciscanus to 
the Mytilus decline indicate that broad generali- 
zations are not appropriate. Whereas 5'. purpura- 
tus suffered a severe decline, the other species 
was unaffected. It is perhaps significant that S. 
purpuratus existed primarily on the upper half 
of the reef, whereas S. franciscanus held forth 
on the lower half. The withdrawal of S. pur- 
puratus may have resulted from the reduction 
of niches provided by the Mytilus beds and the 
fact that Corallina produced a very compact 
cover over the rocky surface. 
Some interesting relationships between the 
numbers of species and individuals occurred 
during this disruptive change in community 
structure. These can be ascertained from the last 
four line items of Table 2. Note, for example, 
