LAKE DWELLING TRICLADS
53
substantial, i.e. within the calcium range of 20-40 mg/l. This suggests that
the latter two species are curtailing further increase in Polycelis numbers. Table XVIIb compares the sizes of Polycelis populations in the
presence and absence of Dugesia plus Dendrocoelum and it is apparent
that in all cases Polycelis occurs in smaller numbers when the other
species are present. Unfortunately, the few lakes providing data of this
kind do not permit statistical analysis to measure significance but the
results are in the right direction in each case. The only other evidence
of competition relates to a comparison of fecundity in the laboratory
and in the field for the planariid species. Both P . nigra and P . tenuis
show fecundity in the field amounting to about 5 % of the laboratory
value, but in Dugesia the corresponding value is about 2%. Such a
contrast is probably explained by the fact that Dugesia is the last to
breed and does so under more severe conditions of food shortage following
breeding in the Polycelis species.
It seems reasonable to conclude that these triclad populations live
under a condition of severe food shortage and that competition for this
resource occurs not only at the intra-specific level but also at the interspecific and inter-generic levels. This could play a decisive role in determining their distribution and abundance.
VI. THE EXPLANATION OF DISTRIBUTION AND
ABUNDANCE - A HYPOTHESIS
The four triclad species considered here show many similarities in
their ecology such as habitat, feeding mechanism and life-cycle, so that
it is convenient to consider the total tricled population as a unit initially
and then to treat the individual species and their inter-actions.
A. THE TOTAL TRICLAD POPULATION
A relation has been demonstrated between the size of the triclad
standing crop in lakes and both the calcium and total dissolved matter
of the water. Similar relations based on more qualitative data have been
suggested for other littoral organisms such as gastropods (Boycott,
1936; Macan, 1950), leeches (Bennike, 1943; Mann, 1955), Asellus
(Reynoldson, 1961b; Williams, 1962a) and tubificids (Kennedy, 1964).
However, the phenomenon is even more widespread, extending to other
lake communities. Thus bacteria and productivity have been correlated
(Hayes, 1961), while Rawson (1960) has demonstrated the importance
of the dissolved mineral content of lake water in relation to productivity.
Larkin and Northcote (1 958) have shown that significant correlations
exist between this chemical feature and the amount,of plankton, biomass of benthic animals and fish. While these trends are unmistakable,
nevertheless the individual features of a lake may be over-riding and
0
53
substantial, i.e. within the calcium range of 20-40 mg/l. This suggests that
the latter two species are curtailing further increase in Polycelis numbers. Table XVIIb compares the sizes of Polycelis populations in the
presence and absence of Dugesia plus Dendrocoelum and it is apparent
that in all cases Polycelis occurs in smaller numbers when the other
species are present. Unfortunately, the few lakes providing data of this
kind do not permit statistical analysis to measure significance but the
results are in the right direction in each case. The only other evidence
of competition relates to a comparison of fecundity in the laboratory
and in the field for the planariid species. Both P . nigra and P . tenuis
show fecundity in the field amounting to about 5 % of the laboratory
value, but in Dugesia the corresponding value is about 2%. Such a
contrast is probably explained by the fact that Dugesia is the last to
breed and does so under more severe conditions of food shortage following
breeding in the Polycelis species.
It seems reasonable to conclude that these triclad populations live
under a condition of severe food shortage and that competition for this
resource occurs not only at the intra-specific level but also at the interspecific and inter-generic levels. This could play a decisive role in determining their distribution and abundance.
VI. THE EXPLANATION OF DISTRIBUTION AND
ABUNDANCE - A HYPOTHESIS
The four triclad species considered here show many similarities in
their ecology such as habitat, feeding mechanism and life-cycle, so that
it is convenient to consider the total tricled population as a unit initially
and then to treat the individual species and their inter-actions.
A. THE TOTAL TRICLAD POPULATION
A relation has been demonstrated between the size of the triclad
standing crop in lakes and both the calcium and total dissolved matter
of the water. Similar relations based on more qualitative data have been
suggested for other littoral organisms such as gastropods (Boycott,
1936; Macan, 1950), leeches (Bennike, 1943; Mann, 1955), Asellus
(Reynoldson, 1961b; Williams, 1962a) and tubificids (Kennedy, 1964).
However, the phenomenon is even more widespread, extending to other
lake communities. Thus bacteria and productivity have been correlated
(Hayes, 1961), while Rawson (1960) has demonstrated the importance
of the dissolved mineral content of lake water in relation to productivity.
Larkin and Northcote (1 958) have shown that significant correlations
exist between this chemical feature and the amount,of plankton, biomass of benthic animals and fish. While these trends are unmistakable,
nevertheless the individual features of a lake may be over-riding and
0