2
T. B . REYNOLDSON
one of the main conclusions to be drawn from these works is the paucity
of examples in which causative factors of distribution and abundance
have been satisfactorily demonstrated. The majority of important
studies relating to freshwater habitats such as those of Jewel1 (1935) on
sponges, Edmondson (1944) on rotifere, Boycott (1936) and Macan
(1950) on molluscs, Macan (1938, 1954) on corixids, Hynes (1954) on
(Jammarus, Bennike (1943) and Mann (1966) on leeches and Williams
(1962a, b) on Asellus are concerned mainly with descriptive aapects of
distribution and although in many cases a pattern related to some basic
feature of the environment has been demonstrated, how it operates remains to be determined. Although Williams’ preliminary studies (1963)
on Asellus are a notable exception, there has been little testing of hypotheses by experimentation either in the laboratory or the field. The
recent critical treatment of freshwater studies by Macan (1963) has
made this starkly clear. It is this situation which has evoked the pleas
of Varley (1967) and Cragg (1961) for more use of the field experiment,
and McFadden (1963) has gone so far as to doubt the value of field work
without it.
The fist stage of the work on lake-dwelling triclads has been largely
confined to the usual study of the distribution in the field of four main
species and an attempt to fit the results into the framework o€ the
physico-chemical properties of lakes as a beginning. This was based on
the examination of more than two hundred separate habitats in several
lakeland areas of northern Britain, Ireland and the western islands
(Fig. 1); limited work has also been done in Fennoscania. S h e any
pattern of distribution usually bears the stamp of historical events to
some variable degree, this aspect has also been considered. In fact, these
triclads show in relation to water chemistry a well-marked pattern
which is modified by their dispersal history.
As the second stage in the programme, hypotheses based on correlations obtained from field data have been tested by field and laboratory
experiments as part of an analysis t o determine the major factors involved. The planning of this stage has been adapted from Andrewartha
and Birch’s (1964,1960) concept of environmental components as modified by Andrewartha and Browning (1961) and further amended to
apply to freshwater habitats. Thus the influence of water chemistry
(especially calcium) and temperature on life-cycles have been studied
and tolerances determined. Food which is proving to be a key factor,
has been considered in some detail and attention has also been given to
predators, while competition especially between the different triclad
species themselves and at the intra-specific level, has been investigated.
Distribution and abundance have three main parameters, space, numbers and time. Detailed studies of the population biology of the four
T. B . REYNOLDSON
one of the main conclusions to be drawn from these works is the paucity
of examples in which causative factors of distribution and abundance
have been satisfactorily demonstrated. The majority of important
studies relating to freshwater habitats such as those of Jewel1 (1935) on
sponges, Edmondson (1944) on rotifere, Boycott (1936) and Macan
(1950) on molluscs, Macan (1938, 1954) on corixids, Hynes (1954) on
(Jammarus, Bennike (1943) and Mann (1966) on leeches and Williams
(1962a, b) on Asellus are concerned mainly with descriptive aapects of
distribution and although in many cases a pattern related to some basic
feature of the environment has been demonstrated, how it operates remains to be determined. Although Williams’ preliminary studies (1963)
on Asellus are a notable exception, there has been little testing of hypotheses by experimentation either in the laboratory or the field. The
recent critical treatment of freshwater studies by Macan (1963) has
made this starkly clear. It is this situation which has evoked the pleas
of Varley (1967) and Cragg (1961) for more use of the field experiment,
and McFadden (1963) has gone so far as to doubt the value of field work
without it.
The fist stage of the work on lake-dwelling triclads has been largely
confined to the usual study of the distribution in the field of four main
species and an attempt to fit the results into the framework o€ the
physico-chemical properties of lakes as a beginning. This was based on
the examination of more than two hundred separate habitats in several
lakeland areas of northern Britain, Ireland and the western islands
(Fig. 1); limited work has also been done in Fennoscania. S h e any
pattern of distribution usually bears the stamp of historical events to
some variable degree, this aspect has also been considered. In fact, these
triclads show in relation to water chemistry a well-marked pattern
which is modified by their dispersal history.
As the second stage in the programme, hypotheses based on correlations obtained from field data have been tested by field and laboratory
experiments as part of an analysis t o determine the major factors involved. The planning of this stage has been adapted from Andrewartha
and Birch’s (1964,1960) concept of environmental components as modified by Andrewartha and Browning (1961) and further amended to
apply to freshwater habitats. Thus the influence of water chemistry
(especially calcium) and temperature on life-cycles have been studied
and tolerances determined. Food which is proving to be a key factor,
has been considered in some detail and attention has also been given to
predators, while competition especially between the different triclad
species themselves and at the intra-specific level, has been investigated.
Distribution and abundance have three main parameters, space, numbers and time. Detailed studies of the population biology of the four