42
T. B . REYNOLDSON
describes as the scramble type. It is perhaps significant that as a group
they appear to be inefficient predators and suffer from a shortage of
food, absolute rather than relative (Andrewartha and Browning, 1961;
Birch, 1962). Recently, MacArthur (1961) and Pimentel (1961, 1963)
have emphasized the fact that predators can only feed on the “interest”
and not the “capital” of their prey populations if they are to survive.
The planariid triclads seem to be a good example of this, indeed their
special properties make it imperative that some safety valve should be
built into their biology. They have avoided over-exploitation of their
prey by the evolution of a behaviour pattern which restricts their feeding largely to damaged organisms or to those behaving abnormally
(p. 38). This may have evolved by the genetic-feedback mechanism now
being explored by Pimentel, provided some degree of population isolation occurs; aquatic,populations are probably isolated to a much greater
extent than terrestrial populations. On the other hand, one would expect
considerable selective pressure in favour of increased predatory efficiency. In my view, this mechanism which Pimentel assumes to have
evolved by natural selection and which is supported by experimental
evidence (Pimentel et al., 1963), sets the limits within which a population normally oscillates, i.e. it functions as a coarse adjustment, while
the fine adjustment is achieved by a proximate (Baker, 1938; Lack,
1954) density governing factor (sensu Nicholson, 1954). Orians (1962)
and Bakker (1964) have discussed the inter-relations of ultimate (evolutionary) and proximate (ecological) factors. In the case of triclads the
proximate factor is intra-specific competition for food. This can be
clearly seen in tricleds by comparing rates of reproduction under natural
conditions and under adjusted field conditions (Table XII). A genetic
mechanism has too great a lag period, especially in those species with a
long generation time, to operate speedily enough. Lack (1954) believed
clutch size, an indication of birth rate, to be adjusted directly to food
supply by natural selection, but as MacArthur (1961) pointed out, even
in birds the problem is often more complex than this. If a population
is to survive, the birth rate must also be adjusted to take into account
the hazards the species has to meet, and the apportionment of nutriment between the needs of the parent for survival and the production
of young (Fisher, 1930; Birch, 1960) will vary among the different species for this reason. For example, triclads have few natural hazards and
when food becomes scarce after limited breeding they soon divert it
from reproduction to maintenance of the individual; blowflies have to
face more hazards among which is dispersal of their food and when this
resource is short they convert as much food as possible to reproduction
and sacrifice size of the individual to some extent; internal parasites
with extreme hazards to meet are virtually egg-laying machines.
T. B . REYNOLDSON
describes as the scramble type. It is perhaps significant that as a group
they appear to be inefficient predators and suffer from a shortage of
food, absolute rather than relative (Andrewartha and Browning, 1961;
Birch, 1962). Recently, MacArthur (1961) and Pimentel (1961, 1963)
have emphasized the fact that predators can only feed on the “interest”
and not the “capital” of their prey populations if they are to survive.
The planariid triclads seem to be a good example of this, indeed their
special properties make it imperative that some safety valve should be
built into their biology. They have avoided over-exploitation of their
prey by the evolution of a behaviour pattern which restricts their feeding largely to damaged organisms or to those behaving abnormally
(p. 38). This may have evolved by the genetic-feedback mechanism now
being explored by Pimentel, provided some degree of population isolation occurs; aquatic,populations are probably isolated to a much greater
extent than terrestrial populations. On the other hand, one would expect
considerable selective pressure in favour of increased predatory efficiency. In my view, this mechanism which Pimentel assumes to have
evolved by natural selection and which is supported by experimental
evidence (Pimentel et al., 1963), sets the limits within which a population normally oscillates, i.e. it functions as a coarse adjustment, while
the fine adjustment is achieved by a proximate (Baker, 1938; Lack,
1954) density governing factor (sensu Nicholson, 1954). Orians (1962)
and Bakker (1964) have discussed the inter-relations of ultimate (evolutionary) and proximate (ecological) factors. In the case of triclads the
proximate factor is intra-specific competition for food. This can be
clearly seen in tricleds by comparing rates of reproduction under natural
conditions and under adjusted field conditions (Table XII). A genetic
mechanism has too great a lag period, especially in those species with a
long generation time, to operate speedily enough. Lack (1954) believed
clutch size, an indication of birth rate, to be adjusted directly to food
supply by natural selection, but as MacArthur (1961) pointed out, even
in birds the problem is often more complex than this. If a population
is to survive, the birth rate must also be adjusted to take into account
the hazards the species has to meet, and the apportionment of nutriment between the needs of the parent for survival and the production
of young (Fisher, 1930; Birch, 1960) will vary among the different species for this reason. For example, triclads have few natural hazards and
when food becomes scarce after limited breeding they soon divert it
from reproduction to maintenance of the individual; blowflies have to
face more hazards among which is dispersal of their food and when this
resource is short they convert as much food as possible to reproduction
and sacrifice size of the individual to some extent; internal parasites
with extreme hazards to meet are virtually egg-laying machines.