54
T. B. REYNOLDSON
give rise to exceptions based on any single edaphic measurement as
emphasized by Larkin and Northcote (Zoc. cit.) and several examples of
this in regard to triclads have been discussed (p. 15).
The evidence already presented shows that the relation between water
chemistry and the distribution of triclad species is indirect, which is not
surprising for organisms at or near the apex of the food chain. These
would be more immediately dependent on the kinds and numbers of
their prey, and a closer, more direct relation between biomass and
variety of littoral organisms and triclads would be anticipated. It is
surprising that such comparative measurements of the littoral fauna are
scarce, although Tucker (1958) has shown that the variety and numbers
of littoral organisms are greater in calcium-rich, eutrophic ponds. Two
sets of such data have been obtained for strongly contrasted lakes, one
in North Wales, the other in the Loch Lomond area of Scotland. Both
represent crude measurements of the standing crop in the spring and
summer seasons respectively. Although Macfadyen ( 1963) has rightly
pointed out the limitations of standing crop as an indication of productivity, in this case it is not likely to be grossly misleading since only
macroscopic organisms were measured with life-cycles of comparable
length. Standing crop is also influenced by the amount of predation on
it. However, since predation is likely to be greater in minerally rich
lakes, any correction for this would further emphasize the contrast in
productivity between chemically poor and rich lakes.
The North Wales data refer to five lakes and represent the pooling of
independent estimates by nine students. They were obtained in two
ways; by removing all the animals from five stones of fist size and by
five-minute collections in the shallow littoral. Table XVIIIa shows
clearly the relation between calcium, triclad numbers, biomass and numbers of the total littoral fauna. The variety of the fauna in the chemically richer lakes is less striking, but key secondary production such as
Gammarus, Asellus and Lyrnnaea pereger was much greater in the minerally richer lakes. The Scottish data were obtained by Dr. J. 0. Young
and the author by making a fifteen-minute collection in the stony,
shallow littoral zone. The results (Table XVIIIb) show a clear relation
between the standing crop of the littoral organisms (excluding triclads)
expressed as biomass and the size of the triclad population. The regression
coefficient is highly significant (b = 17.5, P < 0.02 >0.005) but this
cannot be pressed since only twelve lakes were sampled. It is interesting
that this relation is stronger than that between calcium and the triclad
population. Rusky is an interesting lake in showing a much larger triclad
population than the low calcium value would suggest. But this is matched
by a comparably high littoral biomass, although Gummarus, a species
not regarded as potential food of planariid triclads unless damaged
T. B. REYNOLDSON
give rise to exceptions based on any single edaphic measurement as
emphasized by Larkin and Northcote (Zoc. cit.) and several examples of
this in regard to triclads have been discussed (p. 15).
The evidence already presented shows that the relation between water
chemistry and the distribution of triclad species is indirect, which is not
surprising for organisms at or near the apex of the food chain. These
would be more immediately dependent on the kinds and numbers of
their prey, and a closer, more direct relation between biomass and
variety of littoral organisms and triclads would be anticipated. It is
surprising that such comparative measurements of the littoral fauna are
scarce, although Tucker (1958) has shown that the variety and numbers
of littoral organisms are greater in calcium-rich, eutrophic ponds. Two
sets of such data have been obtained for strongly contrasted lakes, one
in North Wales, the other in the Loch Lomond area of Scotland. Both
represent crude measurements of the standing crop in the spring and
summer seasons respectively. Although Macfadyen ( 1963) has rightly
pointed out the limitations of standing crop as an indication of productivity, in this case it is not likely to be grossly misleading since only
macroscopic organisms were measured with life-cycles of comparable
length. Standing crop is also influenced by the amount of predation on
it. However, since predation is likely to be greater in minerally rich
lakes, any correction for this would further emphasize the contrast in
productivity between chemically poor and rich lakes.
The North Wales data refer to five lakes and represent the pooling of
independent estimates by nine students. They were obtained in two
ways; by removing all the animals from five stones of fist size and by
five-minute collections in the shallow littoral. Table XVIIIa shows
clearly the relation between calcium, triclad numbers, biomass and numbers of the total littoral fauna. The variety of the fauna in the chemically richer lakes is less striking, but key secondary production such as
Gammarus, Asellus and Lyrnnaea pereger was much greater in the minerally richer lakes. The Scottish data were obtained by Dr. J. 0. Young
and the author by making a fifteen-minute collection in the stony,
shallow littoral zone. The results (Table XVIIIb) show a clear relation
between the standing crop of the littoral organisms (excluding triclads)
expressed as biomass and the size of the triclad population. The regression
coefficient is highly significant (b = 17.5, P < 0.02 >0.005) but this
cannot be pressed since only twelve lakes were sampled. It is interesting
that this relation is stronger than that between calcium and the triclad
population. Rusky is an interesting lake in showing a much larger triclad
population than the low calcium value would suggest. But this is matched
by a comparably high littoral biomass, although Gummarus, a species
not regarded as potential food of planariid triclads unless damaged