.) ()
L A K E DWELLING T R I C L A D S
-.
resources, numbers of the same species, numbers of other species and
hazards. The component “resources” now includes the “food” component, and part of the component, “a place in which to live” of Andrewartha and Birch’s original scheme. While Browning’s arguments for
doing this have much to commend them, in the particular treatment of
the triclad environment adopted here, it seems more appropriate to use
the original classification. The main reason for doing so is that in the
quantitative work sampling was restricted to sheltered, stony shores.
This was an attempt to eliminate any differential effect of the resource
“substratum)) and to reduce certain “hazards” in the sense considered
by Browning. Under these circumstances five components are dealt
with; weather, a place to live, food, other organisms of the same species
and of different species.
A . W E A T H E R
This factor is from some aspects less complex in aquatic than in
terrestrial environments mainly because humidity is not involved, but
also because the extremes of temperature are less and change is not so
rapid. Indsed, temperature is the main variant for aquatic organisms
although rainfall may have indirect effects mainly by altering the water
level of lakes and the ionic composition of the lake water. Wind can
also be important but, as already indicated, variations in this factor
were reduced to a considerable extent. A detailed comparative study of
the tolerance of the four triclad species to temperature and its effect
upon the life-cycle has shown contrasts among them. The temperature
ranges for the complete life-cycle are shown in Table V and while this
factor has a differential influence, all the species would be able to breed
in British lakes if temperature were considered in isolation. The species
most affected by low temperature is Dugesia lugubris which is unable
to breed at 7.5” C and below. Water temperatures in our highest lakes
would seem to permit breeding of even this species (Reynoldson et al.,
1965). For example, it is able to breed in southern Finland and Leningrad (Luther, 1961) where winter temperatures are severe, although
summer temperatures may be higher than in Britain. While breeding
experiments suggest there is some parallel between altitude and the distribution of the Polycelis species afid Dug. lugubris which involves temperature, this factor cannot alone account for the pattern, as shown
clearly by the distribution of Dendrocoelum lacteum. This latter species
is the most active breeder at the lowest temperatures, yet in Britain it is
almost restricted to the warmer, more productive, lowland lakes. Temperature may prove to be a factor in distribution and abundance, but
only in conjunction with other more important factors. Although rainfall may cause some rapid changes in the chemistry of lake water,
B*
L A K E DWELLING T R I C L A D S
-.
resources, numbers of the same species, numbers of other species and
hazards. The component “resources” now includes the “food” component, and part of the component, “a place in which to live” of Andrewartha and Birch’s original scheme. While Browning’s arguments for
doing this have much to commend them, in the particular treatment of
the triclad environment adopted here, it seems more appropriate to use
the original classification. The main reason for doing so is that in the
quantitative work sampling was restricted to sheltered, stony shores.
This was an attempt to eliminate any differential effect of the resource
“substratum)) and to reduce certain “hazards” in the sense considered
by Browning. Under these circumstances five components are dealt
with; weather, a place to live, food, other organisms of the same species
and of different species.
A . W E A T H E R
This factor is from some aspects less complex in aquatic than in
terrestrial environments mainly because humidity is not involved, but
also because the extremes of temperature are less and change is not so
rapid. Indsed, temperature is the main variant for aquatic organisms
although rainfall may have indirect effects mainly by altering the water
level of lakes and the ionic composition of the lake water. Wind can
also be important but, as already indicated, variations in this factor
were reduced to a considerable extent. A detailed comparative study of
the tolerance of the four triclad species to temperature and its effect
upon the life-cycle has shown contrasts among them. The temperature
ranges for the complete life-cycle are shown in Table V and while this
factor has a differential influence, all the species would be able to breed
in British lakes if temperature were considered in isolation. The species
most affected by low temperature is Dugesia lugubris which is unable
to breed at 7.5” C and below. Water temperatures in our highest lakes
would seem to permit breeding of even this species (Reynoldson et al.,
1965). For example, it is able to breed in southern Finland and Leningrad (Luther, 1961) where winter temperatures are severe, although
summer temperatures may be higher than in Britain. While breeding
experiments suggest there is some parallel between altitude and the distribution of the Polycelis species afid Dug. lugubris which involves temperature, this factor cannot alone account for the pattern, as shown
clearly by the distribution of Dendrocoelum lacteum. This latter species
is the most active breeder at the lowest temperatures, yet in Britain it is
almost restricted to the warmer, more productive, lowland lakes. Temperature may prove to be a factor in distribution and abundance, but
only in conjunction with other more important factors. Although rainfall may cause some rapid changes in the chemistry of lake water,
B*