28
T . B . REYNOLDSON
(Efford, 1962), suggest that they have been influenced by the same historical events and physiographical barriers, and they provide evidence of
survival of freshwater organisms in Britain during the Ice age, in contradiction to the views of Ullyott (1936) and Thienemann (1 950) who argued
that Britain was re-colonized from the Continent via land connections
after the last glacial period. The difficulties of accepting this have been
outlined (p. 24) but the absence of Dug. gonocephala from Britain needs
further consideration. It would appear that this species either never
reached Britain at any time or failed to survive glaciation. Beauchamp
and Ullyott (1932) have suggested that it is competitively superior to
C. alpina and P. felina only when summer temperatures exceed 16" C.
It is possible therefore that it was eliminated during a glacial period and
never succeeded in re-colonizing, but until we know more about the
basis of the supposed competition such an argument cannot be taken
further.
This historical approach, although unavoidably speculative, supports
the view that the presenbday pattern of distribution in those areas containing all four species is the outcome of varied ecological attributes and
represents a stable condition measured in ecological time. The fact that
Dugesia and Dendrocoelum only occupy 70% of the most favourable
habitats on the mainland (Table 11) is evidence that even here triclads
are still extending their range and conceivably the pattern might alter.
However, in N. Ireland where there are no important physiographic
barriers, triclads seem to have dispersed more completely and Dugesia
has for example occupied all the fully favourable habitats without in
other ways altering the pattern. Admittedly this is based on few data,
and more extensive sampling especially in Southern Ireland would be
instructive. Further support for the reality of the pattern can be derived from triclad distribution in a large lake such as Windermere. Here
there is a range of habitats from relatively sheltered, productive bays
lined by deciduous trees with a correspondingly richer fauna supporting
all four triclad species, to less productive, more exposed places supporting mainly Polycelis species. Thus we see the same pattern in a, single
lake, produced by ecological and not historical events.
V. DETERMINANTS OF DISTRIBUTION A N D ABUNDANCE
All environmental factors which influence the survival and reproduction of the individual are involved in any explanation of distribution
and abundance, they differ only in degree. Andrewartha and Birch
(1954) produced a classification of such factors which although criticized provides an orderly approach to the problem and their initial ideas
have been further discussed and amended by Browning (1962). This
author has classified the environment into five components: wen ther,
T . B . REYNOLDSON
(Efford, 1962), suggest that they have been influenced by the same historical events and physiographical barriers, and they provide evidence of
survival of freshwater organisms in Britain during the Ice age, in contradiction to the views of Ullyott (1936) and Thienemann (1 950) who argued
that Britain was re-colonized from the Continent via land connections
after the last glacial period. The difficulties of accepting this have been
outlined (p. 24) but the absence of Dug. gonocephala from Britain needs
further consideration. It would appear that this species either never
reached Britain at any time or failed to survive glaciation. Beauchamp
and Ullyott (1932) have suggested that it is competitively superior to
C. alpina and P. felina only when summer temperatures exceed 16" C.
It is possible therefore that it was eliminated during a glacial period and
never succeeded in re-colonizing, but until we know more about the
basis of the supposed competition such an argument cannot be taken
further.
This historical approach, although unavoidably speculative, supports
the view that the presenbday pattern of distribution in those areas containing all four species is the outcome of varied ecological attributes and
represents a stable condition measured in ecological time. The fact that
Dugesia and Dendrocoelum only occupy 70% of the most favourable
habitats on the mainland (Table 11) is evidence that even here triclads
are still extending their range and conceivably the pattern might alter.
However, in N. Ireland where there are no important physiographic
barriers, triclads seem to have dispersed more completely and Dugesia
has for example occupied all the fully favourable habitats without in
other ways altering the pattern. Admittedly this is based on few data,
and more extensive sampling especially in Southern Ireland would be
instructive. Further support for the reality of the pattern can be derived from triclad distribution in a large lake such as Windermere. Here
there is a range of habitats from relatively sheltered, productive bays
lined by deciduous trees with a correspondingly richer fauna supporting
all four triclad species, to less productive, more exposed places supporting mainly Polycelis species. Thus we see the same pattern in a, single
lake, produced by ecological and not historical events.
V. DETERMINANTS OF DISTRIBUTION A N D ABUNDANCE
All environmental factors which influence the survival and reproduction of the individual are involved in any explanation of distribution
and abundance, they differ only in degree. Andrewartha and Birch
(1954) produced a classification of such factors which although criticized provides an orderly approach to the problem and their initial ideas
have been further discussed and amended by Browning (1962). This
author has classified the environment into five components: wen ther,