332
CHARLES J. KREBS AND JUDITH 11. MYERS
Microtus montanus. At the time of the fertilizer addition the Microtus
population on the experimental area was at a lower density than the
control population, but while the control population declined through
the summer, the experimental population maintained its lower density
and declined during the next winter. The fertilizer did not increase
the amount of protein in one of the favored food plants of the voles,
Carex sp., and there was no relation between the protein content of
Carex and the density of voles supported, in various meadows studied by
Hoffmann. Thus, no relation between quality of food and vole density
could be shown and the addition of fertilizer, while possibly delaying
the vole decline for a few weeks, did not prevent it.
A six-acre fertilized plot with three to four times the net primary
productivity of a control plot was established in the tundra by Schultz
(1969) and followed from 1961 to 1965. Protein, phosphorus and calcium
levels in the vegetation were raised four to five times by this experimental treatment. Only during one winter (1963), following a year of
relatively high lemming numbers, was the lemming population higher
on the fertilized plot and these animals were rapidly taken by predators
d b r the spring melt-off. A higher lemming density was not reported
for the “improved” habitat in years of high lemming density and so this
experiment does not provide evidence that the quantity or quality of
tundra vegetation is a limiting factor to lemming populations.
Both fertilization of the grassland and supplemental feeding were
tested by Krebs and DeLong (1965) as techniques for preventing
the decline of Microtus californicus. While the animals on the experimental area demonstrated high growth rates, good recruitment, and
increasing densities for the first five months of the experiment, all
of these factors suddenly began to decline. The control population
continued to increase to high numbers while the population with
supplemental food declined to a very sparse density. This experiment
was criticized by Batzli and Pitelka (1971) for the failure to take into
account the quality of the natural food which was available to the
voles, the nutritive quality of the supplemental food, or the possible
effect of feeding stations on the social structure of the population.
Finally, biological assay of the amount of food available to populations of short-tailed voles, Microtus agrestis and bank voles, Clethrionomys glareolus, showed that just previous to the population decline
there was sufficient food to maintain the populations for a year (Chitty
et al., 1968). A Microtus population given supplemental oats and
carrots did not increase in density and body growth rates remained
low, typical of declining vole populations. An interesting observation
in this study was that it was impossible to predict either from the body
weight or the condition of voles kept in 1 1-ft2 enclosures when they had
CHARLES J. KREBS AND JUDITH 11. MYERS
Microtus montanus. At the time of the fertilizer addition the Microtus
population on the experimental area was at a lower density than the
control population, but while the control population declined through
the summer, the experimental population maintained its lower density
and declined during the next winter. The fertilizer did not increase
the amount of protein in one of the favored food plants of the voles,
Carex sp., and there was no relation between the protein content of
Carex and the density of voles supported, in various meadows studied by
Hoffmann. Thus, no relation between quality of food and vole density
could be shown and the addition of fertilizer, while possibly delaying
the vole decline for a few weeks, did not prevent it.
A six-acre fertilized plot with three to four times the net primary
productivity of a control plot was established in the tundra by Schultz
(1969) and followed from 1961 to 1965. Protein, phosphorus and calcium
levels in the vegetation were raised four to five times by this experimental treatment. Only during one winter (1963), following a year of
relatively high lemming numbers, was the lemming population higher
on the fertilized plot and these animals were rapidly taken by predators
d b r the spring melt-off. A higher lemming density was not reported
for the “improved” habitat in years of high lemming density and so this
experiment does not provide evidence that the quantity or quality of
tundra vegetation is a limiting factor to lemming populations.
Both fertilization of the grassland and supplemental feeding were
tested by Krebs and DeLong (1965) as techniques for preventing
the decline of Microtus californicus. While the animals on the experimental area demonstrated high growth rates, good recruitment, and
increasing densities for the first five months of the experiment, all
of these factors suddenly began to decline. The control population
continued to increase to high numbers while the population with
supplemental food declined to a very sparse density. This experiment
was criticized by Batzli and Pitelka (1971) for the failure to take into
account the quality of the natural food which was available to the
voles, the nutritive quality of the supplemental food, or the possible
effect of feeding stations on the social structure of the population.
Finally, biological assay of the amount of food available to populations of short-tailed voles, Microtus agrestis and bank voles, Clethrionomys glareolus, showed that just previous to the population decline
there was sufficient food to maintain the populations for a year (Chitty
et al., 1968). A Microtus population given supplemental oats and
carrots did not increase in density and body growth rates remained
low, typical of declining vole populations. An interesting observation
in this study was that it was impossible to predict either from the body
weight or the condition of voles kept in 1 1-ft2 enclosures when they had