16
M. E. SOLOMON
explanation of the phenomenon in terms of the lucerne fleas’ habit of
feeding upon the dead bodies of their own species, which exert a toxic
effect (Wallace, 1962).
The marked fluctuations observed by Wallace were out of phase with
each other; when the density on some patches was high, on some of
the nearby patches it was low or intermediate in level. This might easily
have been regarded as local variation of little general significance and
sampling replicated until the effects were ((ironed out)’. Wallace’s
findings show that the fluctuations of very localized sub-populations
can be most instructive, and that vital information may be lost if they
are regarded simply as a nuisance.
B. DETECTING AND ASSESSING REGULATION
One could consider, on the one hand, means of detecting only the
presence of regulation in the dynamics of populations, and, on the
other hand, means of assessing its extent or effectiveness. But since the
first operation nearly always involves something of the second, it is
convenient to deal with both together.
1. B y the Tendency of Density to Return towards a Mean
This is the most direct way of detecting and measuring regulation,
the nearest approach to observing the process itself. It is often made
difficult by the fact that the actual “target” level of density, towards
which regulation tends to direct a population, itself changes continually
under the direct and indirect influence of various environmental conditions. Nevertheless, as we shall see, it is sometimes practicable.
Obviously, if we are to observe the return of density towards a
recognizable (‘target” level or a mean density, there must first be a
displacement of density from this level. Such a displacement may
be either natural or artificial.
a. After naturally imposed changes. A simple example of this method
is given by Klomp (1962), using the annual estimates of the density of
pine looper larvae (Bupalus piniariw) shown in Fig. 5. Klomp gives
a value of 20 as his estimate of a mean density for the period, and I
have added SL horizontal line to the diagram at this level. The argument
is that in a highly regulated population the trend from one point to
the next should be upwards ( + ) if the former point is below the mean,
and downwards ( - ) if it is above the mean. From the graph it can be
seen that the expected and actual trend from each point is as follows:
Point
1
2
3
4
5 6 7
8 9 1 0
Expected trend
- - + + - - - + + +
Observed trend
+ - + + + - - + + +
M. E. SOLOMON
explanation of the phenomenon in terms of the lucerne fleas’ habit of
feeding upon the dead bodies of their own species, which exert a toxic
effect (Wallace, 1962).
The marked fluctuations observed by Wallace were out of phase with
each other; when the density on some patches was high, on some of
the nearby patches it was low or intermediate in level. This might easily
have been regarded as local variation of little general significance and
sampling replicated until the effects were ((ironed out)’. Wallace’s
findings show that the fluctuations of very localized sub-populations
can be most instructive, and that vital information may be lost if they
are regarded simply as a nuisance.
B. DETECTING AND ASSESSING REGULATION
One could consider, on the one hand, means of detecting only the
presence of regulation in the dynamics of populations, and, on the
other hand, means of assessing its extent or effectiveness. But since the
first operation nearly always involves something of the second, it is
convenient to deal with both together.
1. B y the Tendency of Density to Return towards a Mean
This is the most direct way of detecting and measuring regulation,
the nearest approach to observing the process itself. It is often made
difficult by the fact that the actual “target” level of density, towards
which regulation tends to direct a population, itself changes continually
under the direct and indirect influence of various environmental conditions. Nevertheless, as we shall see, it is sometimes practicable.
Obviously, if we are to observe the return of density towards a
recognizable (‘target” level or a mean density, there must first be a
displacement of density from this level. Such a displacement may
be either natural or artificial.
a. After naturally imposed changes. A simple example of this method
is given by Klomp (1962), using the annual estimates of the density of
pine looper larvae (Bupalus piniariw) shown in Fig. 5. Klomp gives
a value of 20 as his estimate of a mean density for the period, and I
have added SL horizontal line to the diagram at this level. The argument
is that in a highly regulated population the trend from one point to
the next should be upwards ( + ) if the former point is below the mean,
and downwards ( - ) if it is above the mean. From the graph it can be
seen that the expected and actual trend from each point is as follows:
Point
1
2
3
4
5 6 7
8 9 1 0
Expected trend
- - + + - - - + + +
Observed trend
+ - + + + - - + + +