LAKE DWELLING TRICLADS
61
paralleled by the presence of snails. The importance of this type of prey
was nicely illustrated by a simple field experiment. Previously (p. 51)
reference was made to the failure of Polycelis nigra to establish itself in
a small P . tenuis habitat ‘which contained largely Asellus meridianus,
Hydrobia ( = Potamopyrgus) jenkinsi and Luwi,briculus variegatus. When
the comparable experiment was done with Dugesia, although the initial
numbers introduced were smaller than in the #case of P . nigra, 250 compared with 400, it had no difficulty in establishing itself and this is
attributed to the presence of Hydrobia in lslrge numbers. The broad
relationship suggested between the distribution of gastropods and
Dugesia is also supported by the work of Boycott (1936), Macan (1950),
and Hunter (1957) on snail distribution, but it is worth mentioning at
this point that the rapid spread of Hydrolrict in Britain (Hunter and
Warwick, 1957) may well influence the futurls distribution of Dugesia,
because this snail is more tolerant of unproductive habitats than the
common species Lymnaea pereger. This may make less productive
habitats available to Dugesia. The overall picture presented here suggests
that as lakes become less productive the total hriclad population declines
and ultimately disappears because of food shortage. The parallel restriction of species, ultimately to Polycelis wigra, is due to the lesser numbers
and variety of prey so that “food refuges” become smaller and eventually
disappear, forcing the triclads to compete for the same food resource
to an increasing degree.
An explanation of the co-existence of Polycelis nigra and P. tenuis is
more difficult although evidence of competition between them is more
obvious, both features resulting from the c1oc:e taxonomic relationship.
The elimination of P . tenuis a t low calcium levels and the reduction or
elimination of P. nigra at high levels is almost certainly due to competition for food; their co-existence in lakes of intermediate type suggests
incomplete overlap of food. This was not demfonstrated convincingly by
the study of feeding, but it is to be expected because of close taxonomic
relationship that any difference might be slight and difficult to detect.
In this connection it is interesting to note that P. nigra fed more extensively than P. tenuis on Plecoptern (Reynoldson and Young, 1963),
a group characteristic of unproductive lakes, but this needs further
testing. A second point is that lakes containing only P. nigra seem to
have a rather smaller population than those containing P. tenuis in
addition (Table XIVa), suggesting that the two species together are
able to tap a wider food source. A third point is that co-existence seems
to be more difficult in the most, productive lakes, for example in Shropshire P. nigra was not recorded in 5/12 meres. Here the faunas are likely
to depart farthest from those of unproductive lakes and therefore would
suit P. tenuis more. Although some aspect of their ecology other than
C*
61
paralleled by the presence of snails. The importance of this type of prey
was nicely illustrated by a simple field experiment. Previously (p. 51)
reference was made to the failure of Polycelis nigra to establish itself in
a small P . tenuis habitat ‘which contained largely Asellus meridianus,
Hydrobia ( = Potamopyrgus) jenkinsi and Luwi,briculus variegatus. When
the comparable experiment was done with Dugesia, although the initial
numbers introduced were smaller than in the #case of P . nigra, 250 compared with 400, it had no difficulty in establishing itself and this is
attributed to the presence of Hydrobia in lslrge numbers. The broad
relationship suggested between the distribution of gastropods and
Dugesia is also supported by the work of Boycott (1936), Macan (1950),
and Hunter (1957) on snail distribution, but it is worth mentioning at
this point that the rapid spread of Hydrolrict in Britain (Hunter and
Warwick, 1957) may well influence the futurls distribution of Dugesia,
because this snail is more tolerant of unproductive habitats than the
common species Lymnaea pereger. This may make less productive
habitats available to Dugesia. The overall picture presented here suggests
that as lakes become less productive the total hriclad population declines
and ultimately disappears because of food shortage. The parallel restriction of species, ultimately to Polycelis wigra, is due to the lesser numbers
and variety of prey so that “food refuges” become smaller and eventually
disappear, forcing the triclads to compete for the same food resource
to an increasing degree.
An explanation of the co-existence of Polycelis nigra and P. tenuis is
more difficult although evidence of competition between them is more
obvious, both features resulting from the c1oc:e taxonomic relationship.
The elimination of P . tenuis a t low calcium levels and the reduction or
elimination of P. nigra at high levels is almost certainly due to competition for food; their co-existence in lakes of intermediate type suggests
incomplete overlap of food. This was not demfonstrated convincingly by
the study of feeding, but it is to be expected because of close taxonomic
relationship that any difference might be slight and difficult to detect.
In this connection it is interesting to note that P. nigra fed more extensively than P. tenuis on Plecoptern (Reynoldson and Young, 1963),
a group characteristic of unproductive lakes, but this needs further
testing. A second point is that lakes containing only P. nigra seem to
have a rather smaller population than those containing P. tenuis in
addition (Table XIVa), suggesting that the two species together are
able to tap a wider food source. A third point is that co-existence seems
to be more difficult in the most, productive lakes, for example in Shropshire P. nigra was not recorded in 5/12 meres. Here the faunas are likely
to depart farthest from those of unproductive lakes and therefore would
suit P. tenuis more. Although some aspect of their ecology other than
C*