58
T. B. REYNOLDSON
this is not conclusive and needs to be supported by experiment. Displacement by competition has been reasonably established for the special
case of sessile animals (e.g. Connell, 1961) and plants (e.g. de Wit, 1960),
and recently in the parasitic insect genus Aphytis (DeBach and Sundby,
1963). But the frequent reference in ecological texts to competitive
displacement in stream-dwelling triclads (Beauchamp and Ullyott, 1932)
a study unsupported by experiments, is symptomatic of the paucity
of sound evidence on this matter, especially for invertebrates. In theory,
co-existence depends on one of several possible circumstances. There may
be seasonal reversal of competitive superiority for the resource in short
supply (Hutchinson, 1948, 1957) which Klomp (1961) has shown to demand density-dependent reaction; overlap in resource may be temporary as suggested by Lack (1946) for birds of prey, or it may be
incomplete e.g. Kohn (1959) on gastropods. In the latter case Andrewartha and Birch (1960) have pointed out the frequent need in laboratory
experiments to provide a special niche (space resource) for one of the
species if co-existence is to occur, although Utida (1957) was able to
achieve this without such manipulation. Intermittent co-existence may
occur when the average population level of one species at equilibrium
is so low that periodically elimination occurs, for example during a “low”
in the course of natural fluctuation; an aspect often overlooked in this
context. Here, dispersal will re-establish the less successful species from
time to time (cp. Skellam, 1951). Comparable examples illustrating
these several categories were discussed for plants by Harper et al. (1961).
With this short summary of competition and co-existence as a background we can now consider the special case of lake-dwelling triclads,
bearing in mind that any hypothesis must explain exclusion a t one end
of the habitat series and co-existence at the other.
Study of the food of the four triclad species has shown that while
there is considerable overlap in prey organisms nevertheless species
representative of the three genera show some distinct contrasts. Thus
Dendrocoelum lacteum is a more active hunter than the others and is
able to capture Asellus so that in any competition which may develop
for this prey it will be at an advantage. Dugesia lugubris alone feeds on
gastropods to a considerable extent and this type of prey therefore provides a “food refuge” for it. Polycelis sp. tend to feed more on oligochaetes than the other species but they do not appear to have any
special “food refuge” and indeed may not have been under any selective
pressure to develop one since they are the most successful triclads. Thus
each of these triclads would be expected to co-exist in habitats which
provided sufficiently large populations of the more specific prey organisms. Correspondingly, if one or more of these “food refuges” are absent or
too small then competition might be expected to proceed to elimination.
T. B. REYNOLDSON
this is not conclusive and needs to be supported by experiment. Displacement by competition has been reasonably established for the special
case of sessile animals (e.g. Connell, 1961) and plants (e.g. de Wit, 1960),
and recently in the parasitic insect genus Aphytis (DeBach and Sundby,
1963). But the frequent reference in ecological texts to competitive
displacement in stream-dwelling triclads (Beauchamp and Ullyott, 1932)
a study unsupported by experiments, is symptomatic of the paucity
of sound evidence on this matter, especially for invertebrates. In theory,
co-existence depends on one of several possible circumstances. There may
be seasonal reversal of competitive superiority for the resource in short
supply (Hutchinson, 1948, 1957) which Klomp (1961) has shown to demand density-dependent reaction; overlap in resource may be temporary as suggested by Lack (1946) for birds of prey, or it may be
incomplete e.g. Kohn (1959) on gastropods. In the latter case Andrewartha and Birch (1960) have pointed out the frequent need in laboratory
experiments to provide a special niche (space resource) for one of the
species if co-existence is to occur, although Utida (1957) was able to
achieve this without such manipulation. Intermittent co-existence may
occur when the average population level of one species at equilibrium
is so low that periodically elimination occurs, for example during a “low”
in the course of natural fluctuation; an aspect often overlooked in this
context. Here, dispersal will re-establish the less successful species from
time to time (cp. Skellam, 1951). Comparable examples illustrating
these several categories were discussed for plants by Harper et al. (1961).
With this short summary of competition and co-existence as a background we can now consider the special case of lake-dwelling triclads,
bearing in mind that any hypothesis must explain exclusion a t one end
of the habitat series and co-existence at the other.
Study of the food of the four triclad species has shown that while
there is considerable overlap in prey organisms nevertheless species
representative of the three genera show some distinct contrasts. Thus
Dendrocoelum lacteum is a more active hunter than the others and is
able to capture Asellus so that in any competition which may develop
for this prey it will be at an advantage. Dugesia lugubris alone feeds on
gastropods to a considerable extent and this type of prey therefore provides a “food refuge” for it. Polycelis sp. tend to feed more on oligochaetes than the other species but they do not appear to have any
special “food refuge” and indeed may not have been under any selective
pressure to develop one since they are the most successful triclads. Thus
each of these triclads would be expected to co-exist in habitats which
provided sufficiently large populations of the more specific prey organisms. Correspondingly, if one or more of these “food refuges” are absent or
too small then competition might be expected to proceed to elimination.