Population Dynamics — PEQUEGNAT 
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TABLE 2 
Data Gathered from 0.1 m 2 Quadrats at 2-m Level Before, During, and 
After Major Part of Mytilus Decline 
NO. SPECIES IN PHYLA (Average) 
PHYLA 
Predecline 
Decline 
Postdecline 
Porifera 
2.5 
1.5 
2 
Coelenterata 
3 
3 
3 
Platyhelminthes 
2 
3 
4 
Endoprocta 
1 
0.5 
0 
Ectoprocta 
5 
3.5 
6 
Sipunculoidea 
1 
1 
1 
Nemertea 
1 
1 
3 
Mollusca 
25 
11 
25 
Annelida 
12 
7 
21 
Arthropoda (Crustacea) 
8 
6.5 
11 
Echinodermata 
4 
2.5 
5 
Chordata (Ascidiacea) 
1 
1 
1 
TOTAL ANIMAL SPECIES 
66 
48.5 
82 
Numerable Species 
56 
42 
72 
Motile Species 
36 
28 
47 
TOTAL ANIMAL INDIVIDUALS/0.1 M 2 
1759 
1754 
841 
(Table 2). Then, as the result of the encroach- 
ment of different molluscan species, it increased 
again to an average of 25 by February, I960. 
Four types of population shifts were exhibited 
by molluscan species during this time: (1) the 
numbers of one species originally present in- 
creased, (2) several species declined but per- 
sisted, (3) others declined in numbers and 
eventually disappeared, and (4) previously ab- 
sent species invaded the Corallina mat. Chama 
pellucida is the only species that increased and 
retained its population gain, although, as would 
be expected, the new individuals were very 
small. Twelve of the original 35 species suffered 
severe population drops, but were still present 
in February, I960. Illustrative of these, in addi- 
tion to Mytilus edulis, are Hiatella arctica , which 
dropped from an average of 135 to 10 indi- 
viduals / 0.1 m 2 , and Pterorytis nuttalli, which 
dropped from an average of 16 to 2 / 0.1 m 2 . 
Twenty species of mollusks that were originally 
present disappeared completely. Among the 
more prominent of these are Mytilus calif ornia- 
nus, Modiolus capox, Tegula ligulata, Amphissa 
hicolor, and Gians carpenteri . Among the 15 
invading species are Anomia peruviana, Chlamys 
hastatus, Flabellina iodinea, Doriopsilla fulva, 
and Erato vitellina. The following three species 
were unaffected by the other changes: Crepipa- 
tella lingulata, Pholadidea penita, and P. ovoidea. 
Similar changes took place among the poly- 
chaetes. Whereas an average of 12 species oc- 
curred on Mytilus quadrats (2-m depth) prior 
to the decline, this dropped to 7 in October, 
1959, and then rose sharply to 21 by February, 
I960 (Table 2). The greatest increase was ex- 
hibited by Nereis grubei, which shot from an 
early average of 8 to 96 individuals / 0.1 m 2 
by October, 1959. It persisted at or above 80 / 
0.1 m 2 into February, I960. The populations of 
Anaitides mucosa and Platynereis sp. more than 
doubled in this time. Arabella iricolor declined 
at first, but recovered its earlier population size 
by February, I960. On the other hand, Eunice 
rubra, Typosyllis pulchra, Polycirrus sp., and 
Polydora sp. declined sharply and eventually dis- 
appeared. Eleven species of polychaetes, not 
previously present, invaded the Corallina mats. 
Among the more interesting of these are Lum- 
brinereis zonata, Anaitides sp., and Nainereis 
dendritica. None of the invading species came 
to be represented by many individuals. 
Among the crustaceans, Balanus tintinnabu- 
lum displayed a marked population surge from 
