50
T. B . REYNOLDSON
TABLE X V I
Changes in the Proportions of Triclad Species du,e to Artijcial
Predation on Polycelis tenuis
Percentage of total population
Sample
Polycelia
Dugeaia
size
tenuia
P. nigra
lugubris
_ _ _ ~ _ _ ~
. - -. - _. .
__ -
Before predation
After predation
Recovery period
(1948-1952)
(19521954
1955
1956 early
1956 middle
1956 late
1957
> 500
400
630
440
400
420
720
400
63
10
17
30
41
57
70
55
13
33
35
38
28
15
14
15
25
67
47
32
31
28
16
30
“predation” occurred and the species gradually re-adjusted to the initial
proportions. Unfortunately it was not possible to measure changes in
absolute numbers, so that interpretation must be cautious. It could be
argued that the data show nothing more than the recovery of the
P. tenuis population without reference to the other species. However,
this cannot be so because from 1952 to 1955 both species of Polycelis
increased: if it was simply a matter of proportional change due to recovery of P. tenuis, P. nigra would decline persistently. The apparent
more rapid response to “predation” on P. tenuis by Dugesia may be
explained partly as an artefact due to the use of proportions and partly
due to the potentially higher fecundity of Dugesia (Reynoldson, 1961a).
Such changes are most satisfactorily explained by the hypothesis that
P. nigra initially increased its numbers when competition was less severe
as a result of the removal of P. tenuis. When P. tenuis was allowed
respite from “predation” after 1952, both species increased at first, but
ultimately P. tenuis was able to do so more effectively. The original
proportions were reached 4 years after “predation” on P. tenuis ceased.
Dugesia reacted in a similar manner. Apart from providing evidence of
competition these data are of particular interest since they indicate, for a
particular habitat, the period of time necessary for equilibrium to be reestablished among the three species.
Further circumstantial evidence of competition was obtained by introducing species into natural habitats. Llyn Teryn is a high altitude
T. B . REYNOLDSON
TABLE X V I
Changes in the Proportions of Triclad Species du,e to Artijcial
Predation on Polycelis tenuis
Percentage of total population
Sample
Polycelia
Dugeaia
size
tenuia
P. nigra
lugubris
_ _ _ ~ _ _ ~
. - -. - _. .
__ -
Before predation
After predation
Recovery period
(1948-1952)
(19521954
1955
1956 early
1956 middle
1956 late
1957
> 500
400
630
440
400
420
720
400
63
10
17
30
41
57
70
55
13
33
35
38
28
15
14
15
25
67
47
32
31
28
16
30
“predation” occurred and the species gradually re-adjusted to the initial
proportions. Unfortunately it was not possible to measure changes in
absolute numbers, so that interpretation must be cautious. It could be
argued that the data show nothing more than the recovery of the
P. tenuis population without reference to the other species. However,
this cannot be so because from 1952 to 1955 both species of Polycelis
increased: if it was simply a matter of proportional change due to recovery of P. tenuis, P. nigra would decline persistently. The apparent
more rapid response to “predation” on P. tenuis by Dugesia may be
explained partly as an artefact due to the use of proportions and partly
due to the potentially higher fecundity of Dugesia (Reynoldson, 1961a).
Such changes are most satisfactorily explained by the hypothesis that
P. nigra initially increased its numbers when competition was less severe
as a result of the removal of P. tenuis. When P. tenuis was allowed
respite from “predation” after 1952, both species increased at first, but
ultimately P. tenuis was able to do so more effectively. The original
proportions were reached 4 years after “predation” on P. tenuis ceased.
Dugesia reacted in a similar manner. Apart from providing evidence of
competition these data are of particular interest since they indicate, for a
particular habitat, the period of time necessary for equilibrium to be reestablished among the three species.
Further circumstantial evidence of competition was obtained by introducing species into natural habitats. Llyn Teryn is a high altitude