L A K E DWELLING TRICLADS
51
lake oiily 6 acres (2.5 hect.) in area with a relatively large population
of P. nigra (72/h). In January 1956, 2 500 Y. tenu,is were introduced
into this lake and collections of triclads have been made from it at
intervals since. Up to 1063 no P. tenuis had been recovered and it
appears as if this species has been unable to establish itself. It may
be that the numbers of P . tenuis introduced were too few in relation to
Y. nigra to enable it to succeed in this harsh habitat, or, less likely, it
has not had sufficient time to build its numbers high enough to be
detected. The second experiment of this sort was made in a small springfed pond with a restricted fauna (p. 61) 4 m2 in area in the College
grounds. Baiting over 8 days showed that this small pond initially
supported about 5 000 P. tenuis. Of these, 1 500 were returned together
with 400 P . nigra. Subsequent sampling showed that P. nigra failed to
establish itself under these initially favourable conditions; only a few
were found 2 years after introduction and these have disappeared since.
A similar but smaller introduction of Dugesia lugubris was successful
and the implications of this are considered later (p. 61).
The final evidence of competition between these two species relates
to reproduction. Taylor and Reynoldson (1962) showed that in a habitat
containing only P . nigra this species was able to achieve two peaks of
breeding, in early spring and late summer. I n a habitat also containing
P. tenuis and Dug. lugubris the second peak did not occur.
Evidence of competition for food between triclad species belonging
to different genera is not so striking, and this may have several causes.
Competition is probably not so intense as between congeneric species
because the overlap in food is less (p. 38) and it is therefore not so
clearly demonstrable. There are no productive lakes which lack Polycelis and yet contain either Dugesia or Dendrocoelum so that any numerical restriction due to the presence of Polycelis cannot be measured.
There are lakes containing Polycelis with and without the other two
species, but these do not provide such a useful comparison since Polycelis is so dominant numerically whenever it occurs. Also, the fewer
lakes containing Dugesia and Dendrocoelum make the comparisons less
valid. However, the considerable degree of overlap in food between the
species of different genera (p. 38) and the shortage of this resource
(p. 40) would be expected to lead to competition at the inter-generic
and inter-family levels, and there js some evidence of this. In presenting
such evidence it has been necessary to lump together the data for
Dendrocoelum and Duqesia since they are so few taken separately.
Table XVIIa compares the increase in total number of Polycelis species
with that of Dugesia plus Dendrocoelum over the full range of lakes; and
it is clear that the increase in size of the Polycelis population is halted at
the point where the numbers of Dugesia plus Dendrocoelum become
51
lake oiily 6 acres (2.5 hect.) in area with a relatively large population
of P. nigra (72/h). In January 1956, 2 500 Y. tenu,is were introduced
into this lake and collections of triclads have been made from it at
intervals since. Up to 1063 no P. tenuis had been recovered and it
appears as if this species has been unable to establish itself. It may
be that the numbers of P . tenuis introduced were too few in relation to
Y. nigra to enable it to succeed in this harsh habitat, or, less likely, it
has not had sufficient time to build its numbers high enough to be
detected. The second experiment of this sort was made in a small springfed pond with a restricted fauna (p. 61) 4 m2 in area in the College
grounds. Baiting over 8 days showed that this small pond initially
supported about 5 000 P. tenuis. Of these, 1 500 were returned together
with 400 P . nigra. Subsequent sampling showed that P. nigra failed to
establish itself under these initially favourable conditions; only a few
were found 2 years after introduction and these have disappeared since.
A similar but smaller introduction of Dugesia lugubris was successful
and the implications of this are considered later (p. 61).
The final evidence of competition between these two species relates
to reproduction. Taylor and Reynoldson (1962) showed that in a habitat
containing only P . nigra this species was able to achieve two peaks of
breeding, in early spring and late summer. I n a habitat also containing
P. tenuis and Dug. lugubris the second peak did not occur.
Evidence of competition for food between triclad species belonging
to different genera is not so striking, and this may have several causes.
Competition is probably not so intense as between congeneric species
because the overlap in food is less (p. 38) and it is therefore not so
clearly demonstrable. There are no productive lakes which lack Polycelis and yet contain either Dugesia or Dendrocoelum so that any numerical restriction due to the presence of Polycelis cannot be measured.
There are lakes containing Polycelis with and without the other two
species, but these do not provide such a useful comparison since Polycelis is so dominant numerically whenever it occurs. Also, the fewer
lakes containing Dugesia and Dendrocoelum make the comparisons less
valid. However, the considerable degree of overlap in food between the
species of different genera (p. 38) and the shortage of this resource
(p. 40) would be expected to lead to competition at the inter-generic
and inter-family levels, and there js some evidence of this. In presenting
such evidence it has been necessary to lump together the data for
Dendrocoelum and Duqesia since they are so few taken separately.
Table XVIIa compares the increase in total number of Polycelis species
with that of Dugesia plus Dendrocoelum over the full range of lakes; and
it is clear that the increase in size of the Polycelis population is halted at
the point where the numbers of Dugesia plus Dendrocoelum become