100
MANFRED D . ENGELMANN
that during the spring and early summer before a seed crop was produced, these animals were carnivores. Even during the fall, the birds
continued to include insects in their diet along with seeds (personal
communication, F. C. Evans). McNab (1963) believed that the food
utilization data on the mouse (Odum et al., 1962) were too high. Golley
and Gentry (1964) recognized the fact that P. badius must have been
using foods other than seeds. Thus, these calculations on seed utilization demonstrate that studies on energetics require careful observations
on feeding habits, as well as careful measurements of metabolic and
reproductive processes.
It is of interest to coppare data from Wiegert’s very careful study
(1964) on the spittle bug Philaenus spumarius of “old fields” with the
data previously discussed from the salt marsh and the southern “old
field”. Wiegert also used the aforementioned sweep net and cage method
to measure the spittle bug population. He reported the confidence limits
for his samples. Respiration rates were measured on all stages of the
animals; the caloric values of these rates were determined; also feeding
habits and migratory patterns were studied. A great deal of Wiegert’s
study was devoted to the feeding efficiency of the various stages of the
spittle bug. He found that the immature individuals were 30% to 58%
efficient (average 36%) in assimilating nutriment from the plant sap,
while the adults were 71%-80y0 efficient (average 76%) on the same
material. However, when it came to listing the productivity for the
“old field”, Wiegert ran into difficulty. A population must be able to
produce a minimum number of new individuals, if it is to maintain itself
from year to year. The Philaenus population could not do this on the
“old field”. Wiegert calculated the intrinsic rate of natural increase of
this population on the “old field” to be -5.8 to -6.0. The continued
presence of the spittle bug population on the “old field” was supported
by immigration of new individuals from surrounding alfalfa fields. Thus,
the net production of P. spumarius ranged from 0.048 to 0.096 kcal/m2/
year and the gross productivity ranged from 0.582 to 1.167 kcal/m2/
year. Immigration contributed from 0.516 to 1.138 additional kcal
annually to each square meter of the “old field” in the form of immigrants which later failed to survive. In contrast, a sma€l alfalfa field
produced (net) 15.125 kcal/m2 of animals. Gross productivity amounted
to 38.565 kcal/m2/year. When the alfalfa in this field was cut, and the
quantity of primary producers drastically reduced, thereafter the spittle
bug adults moved out of these fields into other areas which could support them. Thus, although the “old field” was able to provide sustenance
for a certain number of adults, there was not energy sufficient to maintain the population on a permanent basis. Ninety to ninety-five per cent
of the adult animals dying on the “old field” were immigrants. Wiegert
MANFRED D . ENGELMANN
that during the spring and early summer before a seed crop was produced, these animals were carnivores. Even during the fall, the birds
continued to include insects in their diet along with seeds (personal
communication, F. C. Evans). McNab (1963) believed that the food
utilization data on the mouse (Odum et al., 1962) were too high. Golley
and Gentry (1964) recognized the fact that P. badius must have been
using foods other than seeds. Thus, these calculations on seed utilization demonstrate that studies on energetics require careful observations
on feeding habits, as well as careful measurements of metabolic and
reproductive processes.
It is of interest to coppare data from Wiegert’s very careful study
(1964) on the spittle bug Philaenus spumarius of “old fields” with the
data previously discussed from the salt marsh and the southern “old
field”. Wiegert also used the aforementioned sweep net and cage method
to measure the spittle bug population. He reported the confidence limits
for his samples. Respiration rates were measured on all stages of the
animals; the caloric values of these rates were determined; also feeding
habits and migratory patterns were studied. A great deal of Wiegert’s
study was devoted to the feeding efficiency of the various stages of the
spittle bug. He found that the immature individuals were 30% to 58%
efficient (average 36%) in assimilating nutriment from the plant sap,
while the adults were 71%-80y0 efficient (average 76%) on the same
material. However, when it came to listing the productivity for the
“old field”, Wiegert ran into difficulty. A population must be able to
produce a minimum number of new individuals, if it is to maintain itself
from year to year. The Philaenus population could not do this on the
“old field”. Wiegert calculated the intrinsic rate of natural increase of
this population on the “old field” to be -5.8 to -6.0. The continued
presence of the spittle bug population on the “old field” was supported
by immigration of new individuals from surrounding alfalfa fields. Thus,
the net production of P. spumarius ranged from 0.048 to 0.096 kcal/m2/
year and the gross productivity ranged from 0.582 to 1.167 kcal/m2/
year. Immigration contributed from 0.516 to 1.138 additional kcal
annually to each square meter of the “old field” in the form of immigrants which later failed to survive. In contrast, a sma€l alfalfa field
produced (net) 15.125 kcal/m2 of animals. Gross productivity amounted
to 38.565 kcal/m2/year. When the alfalfa in this field was cut, and the
quantity of primary producers drastically reduced, thereafter the spittle
bug adults moved out of these fields into other areas which could support them. Thus, although the “old field” was able to provide sustenance
for a certain number of adults, there was not energy sufficient to maintain the population on a permanent basis. Ninety to ninety-five per cent
of the adult animals dying on the “old field” were immigrants. Wiegert