LAKE DWELLING TRICLADS
57
Competition in the narrower sense (Birch, 1957) is often regarded as
including two facets, “exploitation” and “interference” (Brian, 1956;
Park, 1962). There was no evidence from field or laboratory observations that “interference” is important in these triclads. Since the “exploitation” type of competition enters largely into this discussion some
comment on definitions is necessary. Klomp (1961) has recently discussed competition from both the historical and definitive viewpoints,
and my reading of his paper is that Birch’s (loc. cit.) and Milne’s (1961)
definitions emphasizing a resource in short supply and Klomp’s clear
enunciation of the role of a common regulatory factor are all acceptable
since they reduce to the same essential. Pontin’s (1963) reluctance to
accept Milne’s definition is misleading since he was dealing with the result
of a process, Milne with the cause. DeBach and Sundby (1963) have
suggested that competition occurs whenever a resource is used by two
or more organisms, even when it is abundant. This is a matter of definition, but their view that competitive exclusion (or displacement, as they
prefer) can take place when all resources are abundant, is misleading
(cp. Solomon, 1957). Their supporting experiments suggest that “local”
food shortages (Andrewartha and Browning, 1961) developed; in addition, their technique appeared to incorporate an artificial densitydependent factor. Deternination of distribution (and abundance) by
inter-specific competition leads on to this concept of competitive displacement about which a great deal has been written since Darwin. At
one extreme we have the improbable (Gilbert et al., 1952; Klomp, 1961)
or absurd (Pontin, 1963) situation of different species with identical
ecology; such species, if they existed, could not be differentially influenced by any ecological process. At the other, species which as a
result of their evolutionary history are so different ecologically that
they never compete. Between these extremes are all grades of severity
of competition and outcome which may for convenience be reduced to
three main categories. There are those situations in which one species
will always eliminate another in a particular environment, those in which
co-existence is usual, and those where intermittent co-existence is common. Although a mathematical approach to this problem with special
reference to Tribolium populations has produced the same three cabgories (Park et al., 1964), the underlying causes may be dissimilar since
co-existence in the field is usually associated with special circumstances
as indicated below (cp. Pontin, 1961). It is interesting that Park et al.
(loc. cit.) were unable to establish conditions for co-existence of T . castaneum and T . confusum in their experiments although a few populations
came near to this. Examples from nature of elimination are usually
inferred from patterns of distribution in space or time, but as Park (1948)
and more recently Klomp (1961) and Hairston (1964) have pointed out,
’
57
Competition in the narrower sense (Birch, 1957) is often regarded as
including two facets, “exploitation” and “interference” (Brian, 1956;
Park, 1962). There was no evidence from field or laboratory observations that “interference” is important in these triclads. Since the “exploitation” type of competition enters largely into this discussion some
comment on definitions is necessary. Klomp (1961) has recently discussed competition from both the historical and definitive viewpoints,
and my reading of his paper is that Birch’s (loc. cit.) and Milne’s (1961)
definitions emphasizing a resource in short supply and Klomp’s clear
enunciation of the role of a common regulatory factor are all acceptable
since they reduce to the same essential. Pontin’s (1963) reluctance to
accept Milne’s definition is misleading since he was dealing with the result
of a process, Milne with the cause. DeBach and Sundby (1963) have
suggested that competition occurs whenever a resource is used by two
or more organisms, even when it is abundant. This is a matter of definition, but their view that competitive exclusion (or displacement, as they
prefer) can take place when all resources are abundant, is misleading
(cp. Solomon, 1957). Their supporting experiments suggest that “local”
food shortages (Andrewartha and Browning, 1961) developed; in addition, their technique appeared to incorporate an artificial densitydependent factor. Deternination of distribution (and abundance) by
inter-specific competition leads on to this concept of competitive displacement about which a great deal has been written since Darwin. At
one extreme we have the improbable (Gilbert et al., 1952; Klomp, 1961)
or absurd (Pontin, 1963) situation of different species with identical
ecology; such species, if they existed, could not be differentially influenced by any ecological process. At the other, species which as a
result of their evolutionary history are so different ecologically that
they never compete. Between these extremes are all grades of severity
of competition and outcome which may for convenience be reduced to
three main categories. There are those situations in which one species
will always eliminate another in a particular environment, those in which
co-existence is usual, and those where intermittent co-existence is common. Although a mathematical approach to this problem with special
reference to Tribolium populations has produced the same three cabgories (Park et al., 1964), the underlying causes may be dissimilar since
co-existence in the field is usually associated with special circumstances
as indicated below (cp. Pontin, 1961). It is interesting that Park et al.
(loc. cit.) were unable to establish conditions for co-existence of T . castaneum and T . confusum in their experiments although a few populations
came near to this. Examples from nature of elimination are usually
inferred from patterns of distribution in space or time, but as Park (1948)
and more recently Klomp (1961) and Hairston (1964) have pointed out,
’