LAKE DWELLING TRICLADS
43
In Dendrocoelum lacteum the position is altered to some degree by the
fact that a considerable proportion of the population are annuals and
die after breeding (Yoiing and Iteynoldson, in press). As a result, when
the young are hatching a food crisis is postponed because adults are
dying a t the same time. This is shown by characteristic differences in
the population size-structure of Dendrocoelum and the planariid species. Also stemming from the contrast in longevity is both a higher
birth rate in Dendrocoelum (6-7 per adult compared with 1 in the planariid triclads), and a higher recruitment, 350% compared with 50-1000/.
This means that the amplitude of population fluctuations is several
times greater in Dendrocoelum during an annual cycle. It is also significant that Dendrocoelum is a more efficient predator than the planariid
species and seems to cause a detectable reduction in the population of
its main prey Asellus (p. 40) but other factors such as variation in severity
of inter-specific competition are also involved. Although the mechanism
of population regulation seems particularly evident in these triclads,
Maguire (1962) has suggested in the case of Cura ( = Curtesia) formanni
that some control on reproduction is exerted by ectocrine substances.
Thers are some puzzling features of his data which suggest that the
phenomenon he describes is more likely to be a physiological effect of
laboratory crowding in a lotic species and is not applicable to field
conditions.
From the viewpoint of distribution and abundance, the important
feature of intra-specific relationships is that serious food shortage occurs
periodically in all species and it is against such a background that interspecific relationships must be considered.
E. ORGANISNS OF DIFFERENT SPECIES
Here we are concerned with the effects of predators and parasites of
triclads and the competitors for resources, all of which theoretically are
capable of influencing the pattern of distribution and abundance. These
are treated under two heads.
1. Predators and Parasites
Predators may influence not only the abundance but also the distribution of their prey, but little is known of this factor among aquatic
invertebrates (Macan, 1963). The literature suggests that some animals
such as newts, sticklebacks and certain carnivorous insect larvae eat
triclads (Whitehead, 1922; Bolen, 1938), while Hyman (1951) maintains
that they are rarely eaten. The relative immunity of such soft-bodied
organisms is attributed to their unpleasant taste, Fnd Jennings (1957)
has produced experimental evidence of this. However, no substantial
observations have been reported on which to assess the extent of
43
In Dendrocoelum lacteum the position is altered to some degree by the
fact that a considerable proportion of the population are annuals and
die after breeding (Yoiing and Iteynoldson, in press). As a result, when
the young are hatching a food crisis is postponed because adults are
dying a t the same time. This is shown by characteristic differences in
the population size-structure of Dendrocoelum and the planariid species. Also stemming from the contrast in longevity is both a higher
birth rate in Dendrocoelum (6-7 per adult compared with 1 in the planariid triclads), and a higher recruitment, 350% compared with 50-1000/.
This means that the amplitude of population fluctuations is several
times greater in Dendrocoelum during an annual cycle. It is also significant that Dendrocoelum is a more efficient predator than the planariid
species and seems to cause a detectable reduction in the population of
its main prey Asellus (p. 40) but other factors such as variation in severity
of inter-specific competition are also involved. Although the mechanism
of population regulation seems particularly evident in these triclads,
Maguire (1962) has suggested in the case of Cura ( = Curtesia) formanni
that some control on reproduction is exerted by ectocrine substances.
Thers are some puzzling features of his data which suggest that the
phenomenon he describes is more likely to be a physiological effect of
laboratory crowding in a lotic species and is not applicable to field
conditions.
From the viewpoint of distribution and abundance, the important
feature of intra-specific relationships is that serious food shortage occurs
periodically in all species and it is against such a background that interspecific relationships must be considered.
E. ORGANISNS OF DIFFERENT SPECIES
Here we are concerned with the effects of predators and parasites of
triclads and the competitors for resources, all of which theoretically are
capable of influencing the pattern of distribution and abundance. These
are treated under two heads.
1. Predators and Parasites
Predators may influence not only the abundance but also the distribution of their prey, but little is known of this factor among aquatic
invertebrates (Macan, 1963). The literature suggests that some animals
such as newts, sticklebacks and certain carnivorous insect larvae eat
triclads (Whitehead, 1922; Bolen, 1938), while Hyman (1951) maintains
that they are rarely eaten. The relative immunity of such soft-bodied
organisms is attributed to their unpleasant taste, Fnd Jennings (1957)
has produced experimental evidence of this. However, no substantial
observations have been reported on which to assess the extent of