426 
PACIFIC SCIENCE, VoL XVII, October 1963 
taken at random distances along transverse and 
longitudinal transects. Some 100 sq m quadrats 
were established during this study. 
RESULTS 
During the decline, the Mytilus edulis popu- 
lation at depths of 2 m dropped from an average 
of about 1,100 to less than 10 individuals per 
0.1 m 2 . Even more drastic reduction eventually 
occurred at greater depths (Table 1). And as 
Mytilus declined at specific sites, populations of 
the red alga Corallina chilensis, which are stated 
in terms of percentage of rock surface covered 
in Table 1, increased on the same sites, particu- 
larly above depths of 4 m. There was, however, 
a considerable lag between Mytilus depletion 
and the spread of Corallina. This lag permitted 
rapid population increases of a few invertebrates. 
But, as will be discussed later, some of these 
increases were cut back after the encroachment 
of Corallina got underway. In turn these popula- 
tion shifts triggered other reciprocal changes 
among less noticeable species, especially among 
platyhelminths, nemerteans, mollusks, annelids, 
and crustaceans (Table 2). Fortunately sufficient 
samples had been taken prior to, or early in, 
the Mytilus decline to establish a basis of com- 
parison with subsequent unforeseen events. The 
samples of November, 1958 (Table 1), reveal 
( 1 ) that most of the Mytilus population lived 
on horizontal surfaces at depths between 2 and 
4 m, ( 2 ) that its density decreased sharply with 
increasing depth to the 7-m level, and (3) that 
it was absent from this point downward. These 
relationships persisted unchanged as late as Jan- 
uary 15, 1959. Soon thereafter, however, it was 
noted that Mytilus shells were beginning to ac- 
cumulate in unusual numbers on the shoreward 
bottom of the reef. Unfortunately an impending 
trip to Europe precluded additional sampling 
for several months. But by August, 1959, the 
Mytilus population at the 2-m level (samples 
were taken only a few meters horizontally from 
the predecline quadrats) had dropped about 
80%. And, as can be seen in Table 1, by Febru- 
ary, I960, considered here to be early post- 
decline, Mytilus had almost disappeared from 
the entire reef. Apparently at depths below 2 
m the initial rate of decline was not as rapid, 
but complete disappearance eventually occurred. 
The later occurrence of small individuals at 
depths of 8 and 9 m is probably explained by 
dislodgment of small clumps from the reef-top. 
As seen in Table 1 , Corallina increased markedly 
during this time, and came to form a uniform 
mat, some 6 cm thick, over much of the reef’s 
upper surface. This trend continued until at the 
culmination of the study it covered 65% of the 
shallowest part of the reef with a mat some 9 
cm thick. 
Meanwhile significant changes occurred in 
molluscan populations. Whereas an average of 
25 molluscan species per plot (among a total of 
some 35 species on all plots) occurred with 
Mytilus (at 2-m depths) prior to the decline, 
this average dropped to 11 by September, 1959 
TABLE 1 
Changes in Population Densities, with Depth and Time, of Mytilus and Corallina 
(Dashes indicate absence of data) 
DEPTH 
(m) 
Mytilus edulis: IND /0.1 M 2 * 
BY DATE 
Corallina chilensis: % COVERf 
BY DATE 
11/58 
9/59 
10/59 
2/60 
11/58 
9/59 
10/59 
2/60 
2 
1,086 
176 
95 
5 
5 
25 
50 
65 
4 
180 
90 
0 
0 
5 
10 
15 
35 
6 
95 
50 
0 
0 
- 
5 
5 
10 
7 
7 
20 
0 
0 
5 
- 
- 
0 
8 
0 
0 
7 
0 
- 
5 
5 
- 
9 
0 
7 
0 
0 
0 
0 
0 
- 
* Individuals per quadrat, 
f Per cent of quadrat surface covered by plant. 
