334
CHARLES J. KREBS AND JUDITH H. MYERS
concentrates nutrients in organic material so that they are unavailable
the next year to the growing plants. After the peak year, nutrient
levels in the forage are low-too low to permit adequate reproduction
by the lemmings. In the next two years greater amounts of nutrients
are released and by the fourth year both the nutrient levels in the forage
and the densities of lemmings have recovered. The building block of
this hypothesis is that at some times the nutritional levels (particularly
phosphorus and calcium) are below the thresholds necessary for
reproduction by the lemmings.
Let us investigate this hypothesis by looking at the data on which
it is based (Fig. 35). The two years of peak lemming density have
corresponding peak phosphorus levels. But what is contrary to the
hypothesis is that phosphorus levels are the same in the years before
and after peak population densities. That is, the same levels of phosphorus give rise to increasing lemming population densities as are
supposed to be limiting to the reproduction of the lemming in the year
following the peak. Therefore, although data indicate synchronous
cycles of nutrients and lemmings, there is no indication of nutrient
levels falling below those which can give riee to increasing lemming
densities.
To determine if nutrients are related to reproduction in lemmings
we have plotted the proportion of females pregnant and the percentage
phosphorus in the forage for July and August of the three years for
which these data are available (Fig. 36). As is typical of cycling
microtines (see page 296), the breeding season in the year of peak
densities is shortened so that in August of 1960 when the nutrient
levels are high, the proportion of pregnant females is low. There is no
indication from these data that phosphorus deficiency limits reproduction in lemmings.
Calcium is another nutrient which is stressed by Schultz (1969)
as being important, and in this case it is the necessity of threshold
levels of calcium for adequate lactation by the lemmings which is
thought to be important. Mullen (1968) in his study of lemmings at
Barrow, Alaska monitored the onset and rate of mammary gland
development in pregnant females. He found that in 1960, when Pieper's
(1964) data show high concentrations of calcium and other nutrients in
the forage, the development of the mammary glands began later and
proceeded more slowly during pregnancy than in 1962 when calcium
concentration was low. So there is no indication of calcium limiting
lactation, at least as measured by mammary gland development.
One of the characteristics which seems almost always to be associated
with increasing lemming populations is that the lemmings breed in the
winter preceding the peak. From this observation one would predict
'
CHARLES J. KREBS AND JUDITH H. MYERS
concentrates nutrients in organic material so that they are unavailable
the next year to the growing plants. After the peak year, nutrient
levels in the forage are low-too low to permit adequate reproduction
by the lemmings. In the next two years greater amounts of nutrients
are released and by the fourth year both the nutrient levels in the forage
and the densities of lemmings have recovered. The building block of
this hypothesis is that at some times the nutritional levels (particularly
phosphorus and calcium) are below the thresholds necessary for
reproduction by the lemmings.
Let us investigate this hypothesis by looking at the data on which
it is based (Fig. 35). The two years of peak lemming density have
corresponding peak phosphorus levels. But what is contrary to the
hypothesis is that phosphorus levels are the same in the years before
and after peak population densities. That is, the same levels of phosphorus give rise to increasing lemming population densities as are
supposed to be limiting to the reproduction of the lemming in the year
following the peak. Therefore, although data indicate synchronous
cycles of nutrients and lemmings, there is no indication of nutrient
levels falling below those which can give riee to increasing lemming
densities.
To determine if nutrients are related to reproduction in lemmings
we have plotted the proportion of females pregnant and the percentage
phosphorus in the forage for July and August of the three years for
which these data are available (Fig. 36). As is typical of cycling
microtines (see page 296), the breeding season in the year of peak
densities is shortened so that in August of 1960 when the nutrient
levels are high, the proportion of pregnant females is low. There is no
indication from these data that phosphorus deficiency limits reproduction in lemmings.
Calcium is another nutrient which is stressed by Schultz (1969)
as being important, and in this case it is the necessity of threshold
levels of calcium for adequate lactation by the lemmings which is
thought to be important. Mullen (1968) in his study of lemmings at
Barrow, Alaska monitored the onset and rate of mammary gland
development in pregnant females. He found that in 1960, when Pieper's
(1964) data show high concentrations of calcium and other nutrients in
the forage, the development of the mammary glands began later and
proceeded more slowly during pregnancy than in 1962 when calcium
concentration was low. So there is no indication of calcium limiting
lactation, at least as measured by mammary gland development.
One of the characteristics which seems almost always to be associated
with increasing lemming populations is that the lemmings breed in the
winter preceding the peak. From this observation one would predict
'