L A K E n WE LLI N G T RI C L A D S
21
expressed earlier by Gurney (1916), Boycott (1936), Macan and Worthmgton (1951) and latterly reiterated by Macan (1963). However, the
point must also be made that the rate of dispersal of any species will be
characteristic of and determined by its particular ecology. All are agreed
that species may be absent from a suitable habitat simply through
chance. Moon (1957) has suggested that chance has played the most
important part in the distribution of Asellus in the north of England
but his argument has been refuted (Reynoldson, 1961b; Williams,
1962a). Another aspect which must be considered is that a species or a
group of species may be extending their range and any discernible pattern may be only a temporary phase determined largely by historical
rather than ecological events. In a sense, this is true of all organisms if
the time scale is of geological dimension, but here we are considering a
lesser scale, which can be termed “ecological” time. Some evidence of
what is happening in lnke-dwelling triclads can be obtained by comparing the faunas of the western islands and the extreme north of Scotland with those of the mainland of Britain and of Ireland. In producing
what may be regarded as a typical fauna, the data for those areas known
to contain all four species have been lumped together (i.e. the faunas of
lakes bhown in Fig. 3). Obviously, in any comparison of this averaged
fauna with that of another area, the proportion of lakes in the various
categories based on water chemistry will influence the data, as seen from
Table 11. To overcome this, lakes have been d:vided into two groups,
those with G7.5 mg/litre of calcium, which might be expected to support mostly Polycelis species, and those with > 7.5 mg/litre calcium
which might be expected to support the four species. Table IV compares
the percentage occurrence of the triclad species in a typical fauna and
the combined data for the similar faunas of Anglesey off the North
Wales coast and Islay off the west Scottish coast. Also shown are corresponding data for an area north of the Highland Fault. In the island
lakes with <7-5 mg/litre calcium it is apparent that Polycelis tenuis is
under-represented; the apparent over-representation of P. nigra is due
to the fewer low-calcium, high-altitude lakes in these islands. This trend
of scarcity of P. tenuis is made abundantly clear in lakes with >7.5 mg/
litre calcium. Dugesia and Dendrocoelum are also under-represented in
the higher calcium lakes of Anglesey and Islay, being altogether absent
in the former and represented by a single specimen found in Loch Finlaggin on Islay. This scarcity of species other than P. nigra was amply
confirmed from the examination of lakes and ponds on other, smaller
western islands. For example, in a total of seventeen lentic habitats
containing triclads on Skokholm, Bardsey, Isle of Man and Lismore
(Fig. l), most with 2 1 0 mg/litre calcium, all records referred to P. nigra.
When details of the six lakes, two on Anglesey, four on Islay, which
B
21
expressed earlier by Gurney (1916), Boycott (1936), Macan and Worthmgton (1951) and latterly reiterated by Macan (1963). However, the
point must also be made that the rate of dispersal of any species will be
characteristic of and determined by its particular ecology. All are agreed
that species may be absent from a suitable habitat simply through
chance. Moon (1957) has suggested that chance has played the most
important part in the distribution of Asellus in the north of England
but his argument has been refuted (Reynoldson, 1961b; Williams,
1962a). Another aspect which must be considered is that a species or a
group of species may be extending their range and any discernible pattern may be only a temporary phase determined largely by historical
rather than ecological events. In a sense, this is true of all organisms if
the time scale is of geological dimension, but here we are considering a
lesser scale, which can be termed “ecological” time. Some evidence of
what is happening in lnke-dwelling triclads can be obtained by comparing the faunas of the western islands and the extreme north of Scotland with those of the mainland of Britain and of Ireland. In producing
what may be regarded as a typical fauna, the data for those areas known
to contain all four species have been lumped together (i.e. the faunas of
lakes bhown in Fig. 3). Obviously, in any comparison of this averaged
fauna with that of another area, the proportion of lakes in the various
categories based on water chemistry will influence the data, as seen from
Table 11. To overcome this, lakes have been d:vided into two groups,
those with G7.5 mg/litre of calcium, which might be expected to support mostly Polycelis species, and those with > 7.5 mg/litre calcium
which might be expected to support the four species. Table IV compares
the percentage occurrence of the triclad species in a typical fauna and
the combined data for the similar faunas of Anglesey off the North
Wales coast and Islay off the west Scottish coast. Also shown are corresponding data for an area north of the Highland Fault. In the island
lakes with <7-5 mg/litre calcium it is apparent that Polycelis tenuis is
under-represented; the apparent over-representation of P. nigra is due
to the fewer low-calcium, high-altitude lakes in these islands. This trend
of scarcity of P. tenuis is made abundantly clear in lakes with >7.5 mg/
litre calcium. Dugesia and Dendrocoelum are also under-represented in
the higher calcium lakes of Anglesey and Islay, being altogether absent
in the former and represented by a single specimen found in Loch Finlaggin on Islay. This scarcity of species other than P. nigra was amply
confirmed from the examination of lakes and ponds on other, smaller
western islands. For example, in a total of seventeen lentic habitats
containing triclads on Skokholm, Bardsey, Isle of Man and Lismore
(Fig. l), most with 2 1 0 mg/litre calcium, all records referred to P. nigra.
When details of the six lakes, two on Anglesey, four on Islay, which
B