20
T. B . REYNOLDSON
proportion of lakes occupied. To summarize, a t low calcium levels Poly
celis nigrrr m:iint:iined the highest population, whilst ;It intermediate
levels and higher, I‘. ten& was nndoubtedly the dominant species
numcrically. 1)icyesirr lirguhris and 1)endroco~lum lacteuru only formed a
numerically important component when calcium exceeded 20 mg/litre.
This analysis is based upon numbers, but as adult Dugesia and Dendm
coelum arc several times larger than adult Polycelis, in terms of biomass
the former two species would form a larger part of the total population
and together would sl)proximately equal the biomass of the Polycelis
species in the highest calcium lakes. Also, whilst the total number of triclads reached a maximum a t 20-40 mg/litre of calcium, in terms of biomass a continuing upwtird trend would be shown throughout the entire
range (Fig. 6 ) .
iii. Conclusions and Summary. It has been showu that the standing
crop of triclads became larger as the calcium and T.D.M. concentrations
of lakes increased. I n lakes with calcium concentrations of 20 mg/litre
and upwards, all four species regularly coexisted, but as the concentration declined below 20 mg, Dugesia lugubris and Dendrocoelum lacteum
occurred in fewer lakes and in smaller numbers; below 5 mg they were
usually absent. Polycelis tenuis was the most abundant species from the
highest range down to 5 mg/litre of calcium and persisted in a few lakes
below 2 - 5 mg/litre. At 5 mg/!itre and lower, P. nigra was numerically
the dominant species.
An explanation of this natural pattern is the main purpose of this
account. The pattern differs from that of many other organisms in
showing the co-existence of several species a t one end of the series,
whilst as the other end is approached, all the species are gradually
eliminated. Most other natural distributions within a restricted habitat,
whether of vertebrates or invertebrates, terrestrial or aquatic, usually
depict only one of these situations (Andrewartha and Birch, 1954;
Hutchinson, 1961; Macan, 1963).
IV. HISTORICAL ASPECTS O F DISTRIBUTION A N D ABUNDANCE
In considering the factors which underlie any pattern of distribution,
it is necessary to ask the question, how far it reflects opport’unity for
dispersal and how far it is the outcome of interaction among the
physico-chemical and biotic factors of the environment,. This is especially pertinent for lake faunas where habitats tend to be more isolated
and temporary than most others so that dispersal may be relatively
more important. Talling (1951) with reference to ponds, reached the
general conclusion that physical barriers “do not modify Nature’s habitual abhorrence of the ecological vacuum”. His view is similar to that
T. B . REYNOLDSON
proportion of lakes occupied. To summarize, a t low calcium levels Poly
celis nigrrr m:iint:iined the highest population, whilst ;It intermediate
levels and higher, I‘. ten& was nndoubtedly the dominant species
numcrically. 1)icyesirr lirguhris and 1)endroco~lum lacteuru only formed a
numerically important component when calcium exceeded 20 mg/litre.
This analysis is based upon numbers, but as adult Dugesia and Dendm
coelum arc several times larger than adult Polycelis, in terms of biomass
the former two species would form a larger part of the total population
and together would sl)proximately equal the biomass of the Polycelis
species in the highest calcium lakes. Also, whilst the total number of triclads reached a maximum a t 20-40 mg/litre of calcium, in terms of biomass a continuing upwtird trend would be shown throughout the entire
range (Fig. 6 ) .
iii. Conclusions and Summary. It has been showu that the standing
crop of triclads became larger as the calcium and T.D.M. concentrations
of lakes increased. I n lakes with calcium concentrations of 20 mg/litre
and upwards, all four species regularly coexisted, but as the concentration declined below 20 mg, Dugesia lugubris and Dendrocoelum lacteum
occurred in fewer lakes and in smaller numbers; below 5 mg they were
usually absent. Polycelis tenuis was the most abundant species from the
highest range down to 5 mg/litre of calcium and persisted in a few lakes
below 2 - 5 mg/litre. At 5 mg/!itre and lower, P. nigra was numerically
the dominant species.
An explanation of this natural pattern is the main purpose of this
account. The pattern differs from that of many other organisms in
showing the co-existence of several species a t one end of the series,
whilst as the other end is approached, all the species are gradually
eliminated. Most other natural distributions within a restricted habitat,
whether of vertebrates or invertebrates, terrestrial or aquatic, usually
depict only one of these situations (Andrewartha and Birch, 1954;
Hutchinson, 1961; Macan, 1963).
IV. HISTORICAL ASPECTS O F DISTRIBUTION A N D ABUNDANCE
In considering the factors which underlie any pattern of distribution,
it is necessary to ask the question, how far it reflects opport’unity for
dispersal and how far it is the outcome of interaction among the
physico-chemical and biotic factors of the environment,. This is especially pertinent for lake faunas where habitats tend to be more isolated
and temporary than most others so that dispersal may be relatively
more important. Talling (1951) with reference to ponds, reached the
general conclusion that physical barriers “do not modify Nature’s habitual abhorrence of the ecological vacuum”. His view is similar to that