62
T. B. REYNOLDSON
food might be involved, this seems unlikely in view of their similar
biology; but Harper (1964) has made the point that biology is not always
an adequate basis on which to forecast ecological interactions.
This work started from a correlation established between the chemical condition of lakes and their triclad faunas. We now know that this is
a very indirect relation which nevertheless was detectable through a
food chain of at least three trophic levels, a fact which illustrates the
over-riding influence of the physico-chemical background of the environment. It follows that for a complete understanding of the distribution of triclads it will be necessary to follow the food chains back to the
primary producers. Again, looking beyond the hypothesis suggested
here which is now being tested experimentally in the laboratory and the
field, it will be ultimately necessary to consider the reasons why a particular species is successful or unsuccessful in the struggle for existence.
It is at this level of inter-action that other modifying factors of the
environment such as temperature and water chemistry may operate
differentially.
VII. CONCLUSIONS AND SUMMARY
The four main lake species of triclad, Polycelis nigru (Mull.), P . tenuis
Ijima, Dugesiu lugubris (0. Schmidt) and Dendrocoelum lacteum (Mull.)
all occupy the shallow littoral zone and feed on other organisms found
there. The feeding mechanisms of all are similar and there is wide overlap in the type and size of prey eaten by them and also by the young
and adults of the same species.
At the species level, triclad populations show restricted fluctuations in
numbers and live under conditions of food shortage for long periods of
the year. Numbers are regulated by intra-specific competition for food,
which stems mainly from two biological properties: high resistance to
death by starvation because of their ability to resorb and then regenerate
tissues, and a high degree of immunity from predators.
The triclad populations of lakes vary in size from complete absence to
very high numbers and there is a general, positive correlation with both
the calcium content and the total dissolved matter of the water. The
pattern of species distribution is also related to these factors. The relation is indirect and triclad abundance is determined primarily by the
amount of food available in the littoral zone, itself ultimately dependent
on the water-chemistry.
The distribution and abundance of the individual species of triclad
are determined primarily by inter-specific competition for food. The
outcome depends upon the pattern of distribution and abundance of
certain numerically important types of prey because, despite the high
degree of overlap, each triclad species feeds on a particular kind of prey
T. B. REYNOLDSON
food might be involved, this seems unlikely in view of their similar
biology; but Harper (1964) has made the point that biology is not always
an adequate basis on which to forecast ecological interactions.
This work started from a correlation established between the chemical condition of lakes and their triclad faunas. We now know that this is
a very indirect relation which nevertheless was detectable through a
food chain of at least three trophic levels, a fact which illustrates the
over-riding influence of the physico-chemical background of the environment. It follows that for a complete understanding of the distribution of triclads it will be necessary to follow the food chains back to the
primary producers. Again, looking beyond the hypothesis suggested
here which is now being tested experimentally in the laboratory and the
field, it will be ultimately necessary to consider the reasons why a particular species is successful or unsuccessful in the struggle for existence.
It is at this level of inter-action that other modifying factors of the
environment such as temperature and water chemistry may operate
differentially.
VII. CONCLUSIONS AND SUMMARY
The four main lake species of triclad, Polycelis nigru (Mull.), P . tenuis
Ijima, Dugesiu lugubris (0. Schmidt) and Dendrocoelum lacteum (Mull.)
all occupy the shallow littoral zone and feed on other organisms found
there. The feeding mechanisms of all are similar and there is wide overlap in the type and size of prey eaten by them and also by the young
and adults of the same species.
At the species level, triclad populations show restricted fluctuations in
numbers and live under conditions of food shortage for long periods of
the year. Numbers are regulated by intra-specific competition for food,
which stems mainly from two biological properties: high resistance to
death by starvation because of their ability to resorb and then regenerate
tissues, and a high degree of immunity from predators.
The triclad populations of lakes vary in size from complete absence to
very high numbers and there is a general, positive correlation with both
the calcium content and the total dissolved matter of the water. The
pattern of species distribution is also related to these factors. The relation is indirect and triclad abundance is determined primarily by the
amount of food available in the littoral zone, itself ultimately dependent
on the water-chemistry.
The distribution and abundance of the individual species of triclad
are determined primarily by inter-specific competition for food. The
outcome depends upon the pattern of distribution and abundance of
certain numerically important types of prey because, despite the high
degree of overlap, each triclad species feeds on a particular kind of prey