LAKE DWELLING TRICLADS
63
to a greater extent than the others, i.e. each lhas a “food refuge”. Thus
Dug. lugubris alone feeds upon gastropod molluscs, Dend. lacteum
captures Asellus most efficiently and Polyeelis species take more
oligochae tes .
It is suggested that because these import$iit food organisms become
both less numerous and less varied in the littoral zone as lake productivity declines, the triclad species are forced to compete with increasing
seventy. As a result, lakes of low productivit:r (minerally poor) support
smaller numbers of triclads and fewer species than lakes of high productivity (minerally rich). Thus only P. nigru exists in the poorest lakes
while all four species co-exist in the richest lakes. The basis of coexistence of P. nigru and P. tenuis in all but the poorest lakes has not
been established, but it is probably due to a “food refuge” difficult to
detect because of the very similar nature of %heir food.
It is important to emphasize that competitive displacement depends
not only on the ecological properties of the organism but also on the
properties of the habitat. For example, in unproductive lakes all the
triclad species are effectively ecologically similar in their food, in productive lakes this is not so.
*
ACKNOWLEDGMENTS
I wish to acknowledge the help of Dr. J. 0. Young who was my
assistant for three productive years and also his permission to use the
data on littoral organisms collected jointly. The following people kindly
helped either by collecting and sending me triclads or making observations on their occurrence in the remoter places; Dr. M. Angel, Dr. R.
Carrick, Mr. R. C. Connolly, Dr. Llyr Gruffydd, Mr. H. G. Lloyd, Miss
S. MacLachlainn, Dr. A. Milne, Mr. P. Langton, Mr. R. J. Spittle, Mr. T.
Warwick and Mr. D. R. Williams. I have also to thank Mr. F. J. Mackereth for providing some of the chemical data.
I am grateful to Dr. A. G. Dahm and Dr. (3. B. Corbet who gave me
the benefit of their advice on the historical aspects of distribution and
to Dr. P. O’Donald who advised on the statistical analysis of Dr.
Mirolli’s data. The manuscript was read for me by Mr. L. H. Jackson
to whom I am grateful for several improvements.
The Department of Scientific and Industrial Research have generously provided money for an assistant during the last three years and
are continuing to do so. I am grateful to the Nature Conservancy and
the Royal Society who financed a major pan; of the travelling incurred
by the field work. I also wish to thank the Leverhulme Trust for the
grant which enabled me to study triclads in Sweden and N. Ireland.
Finally, I wish to thank Professor F. W. FCogers Brambell, F.R.s., for
his encouragement and support at all times,
63
to a greater extent than the others, i.e. each lhas a “food refuge”. Thus
Dug. lugubris alone feeds upon gastropod molluscs, Dend. lacteum
captures Asellus most efficiently and Polyeelis species take more
oligochae tes .
It is suggested that because these import$iit food organisms become
both less numerous and less varied in the littoral zone as lake productivity declines, the triclad species are forced to compete with increasing
seventy. As a result, lakes of low productivit:r (minerally poor) support
smaller numbers of triclads and fewer species than lakes of high productivity (minerally rich). Thus only P. nigru exists in the poorest lakes
while all four species co-exist in the richest lakes. The basis of coexistence of P. nigru and P. tenuis in all but the poorest lakes has not
been established, but it is probably due to a “food refuge” difficult to
detect because of the very similar nature of %heir food.
It is important to emphasize that competitive displacement depends
not only on the ecological properties of the organism but also on the
properties of the habitat. For example, in unproductive lakes all the
triclad species are effectively ecologically similar in their food, in productive lakes this is not so.
*
ACKNOWLEDGMENTS
I wish to acknowledge the help of Dr. J. 0. Young who was my
assistant for three productive years and also his permission to use the
data on littoral organisms collected jointly. The following people kindly
helped either by collecting and sending me triclads or making observations on their occurrence in the remoter places; Dr. M. Angel, Dr. R.
Carrick, Mr. R. C. Connolly, Dr. Llyr Gruffydd, Mr. H. G. Lloyd, Miss
S. MacLachlainn, Dr. A. Milne, Mr. P. Langton, Mr. R. J. Spittle, Mr. T.
Warwick and Mr. D. R. Williams. I have also to thank Mr. F. J. Mackereth for providing some of the chemical data.
I am grateful to Dr. A. G. Dahm and Dr. (3. B. Corbet who gave me
the benefit of their advice on the historical aspects of distribution and
to Dr. P. O’Donald who advised on the statistical analysis of Dr.
Mirolli’s data. The manuscript was read for me by Mr. L. H. Jackson
to whom I am grateful for several improvements.
The Department of Scientific and Industrial Research have generously provided money for an assistant during the last three years and
are continuing to do so. I am grateful to the Nature Conservancy and
the Royal Society who financed a major pan; of the travelling incurred
by the field work. I also wish to thank the Leverhulme Trust for the
grant which enabled me to study triclads in Sweden and N. Ireland.
Finally, I wish to thank Professor F. W. FCogers Brambell, F.R.s., for
his encouragement and support at all times,