L A K E DWELLING TRICLADS
47
1961). Within the range of population size observed in this work, living
space does not appear to be critical. Although it is shared by a large
number of other organisms such as Cammarus, leeches of several species,
oligochaetes, molluscs and various insect larvae, the requirements are
usually not specific and living space as represented by the undersurface
of non-bedded stones does not usually restrict the size of the littoral
fauna of lakes. In this connection Macan and Mackereth (1957) have discussed the possibility of jostling in overcrowded conditions causing
Cammarus to be exposed to current and predation in streams but this
does not occur in triclads. It has been shown (p. 40) that the food of triclads is in short supply especially during the 2-3 months after breeding,
and it is reasonable to assume that triclads are sensitive to any inroads
which may be made upon this resource. Depletion may result from the
feeding activities of a wide range of animals, from those closely related
to those only related in the sense that both are aquatic animals. For
example, Asellus may be eaten by several species of fish, molluscs by
leeches, oligochaetes by insect larvae and so on. The effects of such predation will be several and complex. For example, reductions in food
supply will lower the overall triclad population or a specific part of it,
depending on the type of food. It will also increase the intensity of any
inter- and intra-specific competition in triclads and its timing. But the
evidence, admittedly meagre, suggested that timing ;vas largely the result of intra-specific activities (Reynoldson, 1964). Generally it can be
assumed that the more distantly related the predators using a food resource, the less intense and prolonged the ensuing competition. This
follows because the more contrasted the ecology, usually but not always
a function of taxonomic relationship, the less likely are peak demands
to coincide and the more likely the availability of alternative food.
Therefore the main pressure from inter-specific competition is likely to
come from other triclad species. However, occasionally unrelated species
may exert an influence sufficiently powerful to restrict distribution
(Harper, 1964) but we know insufficient about this aspect of triclad
ecology to indicate its prevalence. Leeches, because of their abundance
in the littoral zone (Mann, 1955) and because they show appreciable
overlap in food with triclada (Mann, 1962), are likely to be the most
important component of the fauna in this respect and should be the tirst
to be studied.
Reliable evidence of inter-specific competition among triclads, almost
certainly for food, in view of the demonstrated shortage (p. 40) and
overlap (p. 38), has been obtained from several sources. It is most
clearly demonstrated between Polycelis nigra and P . tenuis by comparing the size of the P. nigra populations in Anglesey where the species
occurs mostly alone, with that of the mainland where it occurs with
47
1961). Within the range of population size observed in this work, living
space does not appear to be critical. Although it is shared by a large
number of other organisms such as Cammarus, leeches of several species,
oligochaetes, molluscs and various insect larvae, the requirements are
usually not specific and living space as represented by the undersurface
of non-bedded stones does not usually restrict the size of the littoral
fauna of lakes. In this connection Macan and Mackereth (1957) have discussed the possibility of jostling in overcrowded conditions causing
Cammarus to be exposed to current and predation in streams but this
does not occur in triclads. It has been shown (p. 40) that the food of triclads is in short supply especially during the 2-3 months after breeding,
and it is reasonable to assume that triclads are sensitive to any inroads
which may be made upon this resource. Depletion may result from the
feeding activities of a wide range of animals, from those closely related
to those only related in the sense that both are aquatic animals. For
example, Asellus may be eaten by several species of fish, molluscs by
leeches, oligochaetes by insect larvae and so on. The effects of such predation will be several and complex. For example, reductions in food
supply will lower the overall triclad population or a specific part of it,
depending on the type of food. It will also increase the intensity of any
inter- and intra-specific competition in triclads and its timing. But the
evidence, admittedly meagre, suggested that timing ;vas largely the result of intra-specific activities (Reynoldson, 1964). Generally it can be
assumed that the more distantly related the predators using a food resource, the less intense and prolonged the ensuing competition. This
follows because the more contrasted the ecology, usually but not always
a function of taxonomic relationship, the less likely are peak demands
to coincide and the more likely the availability of alternative food.
Therefore the main pressure from inter-specific competition is likely to
come from other triclad species. However, occasionally unrelated species
may exert an influence sufficiently powerful to restrict distribution
(Harper, 1964) but we know insufficient about this aspect of triclad
ecology to indicate its prevalence. Leeches, because of their abundance
in the littoral zone (Mann, 1955) and because they show appreciable
overlap in food with triclada (Mann, 1962), are likely to be the most
important component of the fauna in this respect and should be the tirst
to be studied.
Reliable evidence of inter-specific competition among triclads, almost
certainly for food, in view of the demonstrated shortage (p. 40) and
overlap (p. 38), has been obtained from several sources. It is most
clearly demonstrated between Polycelis nigra and P . tenuis by comparing the size of the P. nigra populations in Anglesey where the species
occurs mostly alone, with that of the mainland where it occurs with