L A K E DWELLING TRICLADS
49
Size of t r i c l a d population
300 1
I
I
I
I
I
.
I
1
5 0
100
150
200
2 5 0
300
350
400
4 5 0
Polycelis tenuis
I00
C--l-.-l
0-0 . . ..: . .... .,.... .. ,_..I. -.!...I I...D-. 1
. . :.:. ....
rn IlClII 1121131141 m
m
L o k e
FIG. 7. The species populations of lochs on Islay in ascending order of total population
to illustrate the influence of Polycelis nigm, P . lenuia, and P . felina upon one another.
(Reproduced with permission from Reynoldsoii, 1958a).
species. The low-calcium lakes in which P. tenuis is found occur in areas
(Northern Scotland, artificial tarns of the Lake District) where P. nigra
is absent.
Evidence of another sort was obtained when the P. tenuis population
in the College Pond was depleted by removing large numbers for experiments on the peritrich Urceolaria which is epizoic on this species
(Reynoldson, 1957); lesser numbers of P. nigra were removed. These removals are referred to subsequently as "predation". Changes in the
proportions of the three species in the pond are shown in Table XVI.
During 1950-52 large numbers of P. tenuis were removed and by 1952
the proportions had altered considerably. From 1952 to 1956 no
TABLE X V
Distribu,tion of Polycelis nigra aid P. tenuis in Lakes of Low Calcium
Content
Calcium mg/litre
<2*5
2.6-5.0
6-1-7.5
P . nigra alone
13
14
6
P . tenuia alone
6
6
2
Total lakes
19
22
13
P . nigra & P . tenuis
0
2
5