94
L. B. SLOBODRIN
VI. FIELD ESTIMATES OF EFFICIENCY
There are very few field studies which have any possibility of shedding relevant light on the value of ecological efficiency.
The steady state condition can be readily approximated in the
laboratory. In the field the existence of a steady state, in the limited
sense of identity of standing crop at the beginning and end of the
study may be empirically demonstrable but often the deviation from
a steady state is evaluated and calculations made accordingly. Various
correction factors and sampling errors are involved in field data. Uiiless
there is some apparent logical or biological impossibility involved, as
in Lindeman’s work, it seems best to accept field estimates at their
face value in the hope that there is no consistent trend to the errors
introduced.
Golley (1960) studied the energy budget of a selected group of
species on an old field in Michigan, in particular Microtus pennsylvanicus and Mustela brevicauda.
The magnitude of the Microtus values hinges on an assumed factor
which translates the number of animals trapped into population
density. Given this qualification, the energy budget was of the form of
Eq. ( 1 ) in which I was taken as the sum of growth and respiration,
that is faeces were assumed to be composed entirely of unassimilated
material.
Golley’s best estimate of respiratory loss as (respiration energy/
energy consumed) is 68% for Microtus and 93% for Mustela. The
estimates are based on fasting metabolism and may underestimate
respiratory losses under conditions of normal activity.
Faecal material constituted lO-lS% of the energy consumed by
Microtus and 10% by Mustela.
The Mustela energy budget is complete in that predation on Mustela
is essentially negligible and faecal and respiratory loss seem to balance
energy consumption. For Microtus, however, 1&22% of the energy
consumed remains to be accounted for. The energy loss associated with
body activity above the resting level must be subtracted from this
although the precise percentage cannot be determined.
Microtus is considered the primary food supply of Mustek. The
ratio of food consumed by Mustela. t o food consumed by Microtus, is
2%. Presumably, other predators also feed on Microtus, which would
somewhat increase the estimate of Microtus ecological efficiency. The
range of likely values for Microtus pennsylvanicus in an old field is,
therefore, 2-22y0.
Teal (1957) studied a small leafy pond. The chief plant-nutrient
source was apple leaves dropped in during autumn. Over forty species
L. B. SLOBODRIN
VI. FIELD ESTIMATES OF EFFICIENCY
There are very few field studies which have any possibility of shedding relevant light on the value of ecological efficiency.
The steady state condition can be readily approximated in the
laboratory. In the field the existence of a steady state, in the limited
sense of identity of standing crop at the beginning and end of the
study may be empirically demonstrable but often the deviation from
a steady state is evaluated and calculations made accordingly. Various
correction factors and sampling errors are involved in field data. Uiiless
there is some apparent logical or biological impossibility involved, as
in Lindeman’s work, it seems best to accept field estimates at their
face value in the hope that there is no consistent trend to the errors
introduced.
Golley (1960) studied the energy budget of a selected group of
species on an old field in Michigan, in particular Microtus pennsylvanicus and Mustela brevicauda.
The magnitude of the Microtus values hinges on an assumed factor
which translates the number of animals trapped into population
density. Given this qualification, the energy budget was of the form of
Eq. ( 1 ) in which I was taken as the sum of growth and respiration,
that is faeces were assumed to be composed entirely of unassimilated
material.
Golley’s best estimate of respiratory loss as (respiration energy/
energy consumed) is 68% for Microtus and 93% for Mustela. The
estimates are based on fasting metabolism and may underestimate
respiratory losses under conditions of normal activity.
Faecal material constituted lO-lS% of the energy consumed by
Microtus and 10% by Mustela.
The Mustela energy budget is complete in that predation on Mustela
is essentially negligible and faecal and respiratory loss seem to balance
energy consumption. For Microtus, however, 1&22% of the energy
consumed remains to be accounted for. The energy loss associated with
body activity above the resting level must be subtracted from this
although the precise percentage cannot be determined.
Microtus is considered the primary food supply of Mustek. The
ratio of food consumed by Mustela. t o food consumed by Microtus, is
2%. Presumably, other predators also feed on Microtus, which would
somewhat increase the estimate of Microtus ecological efficiency. The
range of likely values for Microtus pennsylvanicus in an old field is,
therefore, 2-22y0.
Teal (1957) studied a small leafy pond. The chief plant-nutrient
source was apple leaves dropped in during autumn. Over forty species