56
T. B. REYNOLDSON
(Reynoldson and Young, 1963), is numerically dominant. This briclad
population is also exceptional in showing an increase since it was sampled
12 years ago (Reynoldson, 1958a) when a population of 88/h was recorded
compared with 340 in 1962. This was due to an increase in P. tenuis.
Apart from the poorest calcium lakes the increase in number of littoral
organisms is not marked, the increase in biomass being due to the
prevalence of larger species. A similar conclusion applies to the variety
of species but the key organisms alter; Plecoptera and Ephemeroptera
of poor lakes give way to gastropods and Asellus in richer lakes. Leeches
which probably compete with triclads for food, also dominate the richest
lakes especially in biomass, along with triclads. Apart from Loch Rushy
mentioned above, the variety of triclad species increases with standing
crop.
The conclusion which seems permissible from these results is that
triclad numbers are related to the potential food supply in the littoral
zone, and from what has been said earlier, are limited by this resource.
B. PZi? ZRazTzaZkz SPXCZZ#
Before considering complex causes of distribution one obvious explanation of the co-existence of the four species in productive lakes
must be dealt with, namely the possibility of vertical stratification of
micro-habitats. If each species had an optimum micro-habitat a t a
slightly different depth which still allowed mixing in the shallow littoral
zone, it might not be detected by the sampling method used. However,
apart from Chodorowski’s (1959,1960) work in rather different habitats
there is no indication of this from the literature (Humphries, 1936;
Berg, 1938). Proof that these four species are able to co-exist in the
shallow littoral zone of productive habitats is given by their occurrence
for upwards of 20 years in several small ponds around Bangor (N. Wales)
with depths no greater than 1-2 f t (25-50 cm). Zonation would also fail
to account for the reduction in species as lakes become less productive.
In Section V the major components of the triclad environment have
been examined and the only factor which seems to be eligible as a determinant of the observed field distribution is inter-specific competition.
The evidence of intra-specific competition for food, the overlap in range
of food among the four species, the larger population size of species in
isolation compared with mixed populations, the scarcity of predators
and the resistance to senescence all indicate with reasonable certainty
that food is the resource in short supply. This conclusion seems permissible despite the justified view of Mundie (1957) that the influence of
single factors tends to be overstressed when they are considered separately and despite the ample demonstration (e.g. Elton and Miller, 1954)
of the complex inter-action of organism and environment.
T. B. REYNOLDSON
(Reynoldson and Young, 1963), is numerically dominant. This briclad
population is also exceptional in showing an increase since it was sampled
12 years ago (Reynoldson, 1958a) when a population of 88/h was recorded
compared with 340 in 1962. This was due to an increase in P. tenuis.
Apart from the poorest calcium lakes the increase in number of littoral
organisms is not marked, the increase in biomass being due to the
prevalence of larger species. A similar conclusion applies to the variety
of species but the key organisms alter; Plecoptera and Ephemeroptera
of poor lakes give way to gastropods and Asellus in richer lakes. Leeches
which probably compete with triclads for food, also dominate the richest
lakes especially in biomass, along with triclads. Apart from Loch Rushy
mentioned above, the variety of triclad species increases with standing
crop.
The conclusion which seems permissible from these results is that
triclad numbers are related to the potential food supply in the littoral
zone, and from what has been said earlier, are limited by this resource.
B. PZi? ZRazTzaZkz SPXCZZ#
Before considering complex causes of distribution one obvious explanation of the co-existence of the four species in productive lakes
must be dealt with, namely the possibility of vertical stratification of
micro-habitats. If each species had an optimum micro-habitat a t a
slightly different depth which still allowed mixing in the shallow littoral
zone, it might not be detected by the sampling method used. However,
apart from Chodorowski’s (1959,1960) work in rather different habitats
there is no indication of this from the literature (Humphries, 1936;
Berg, 1938). Proof that these four species are able to co-exist in the
shallow littoral zone of productive habitats is given by their occurrence
for upwards of 20 years in several small ponds around Bangor (N. Wales)
with depths no greater than 1-2 f t (25-50 cm). Zonation would also fail
to account for the reduction in species as lakes become less productive.
In Section V the major components of the triclad environment have
been examined and the only factor which seems to be eligible as a determinant of the observed field distribution is inter-specific competition.
The evidence of intra-specific competition for food, the overlap in range
of food among the four species, the larger population size of species in
isolation compared with mixed populations, the scarcity of predators
and the resistance to senescence all indicate with reasonable certainty
that food is the resource in short supply. This conclusion seems permissible despite the justified view of Mundie (1957) that the influence of
single factors tends to be overstressed when they are considered separately and despite the ample demonstration (e.g. Elton and Miller, 1954)
of the complex inter-action of organism and environment.