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CHARLES J. EREBS AND JUDITH H. MYERS
much different than those recorded for the complete plant. Whether
lemmings can select plants or plant parts of high nutrient content,
even when average nutrient content is low, is unknown. However,
Thompson’s (1965) test of the selectivity of Microtus pennsylvanicus
indicated that they did not seem to recognize the quality of the food
plant.
To summarize, we must conclude that, contrary to the statement of
Schultz (1969, p. 86), the biology of the lemming is a very important
element for understanding the tundra ecosystem. Evidence confirms
Pieper’s (1964) hypothesis that lemming grazing actually increases
the nutrients available to plants, and that synchronous cycles between
lemming density and nutrient content in plants are the result of the
effect of lemming density on the availability of nutrients to the plants.
Thus lemmings affect the tundra plants but the plants do not seem to
determine lemming density changes. Data published thus far seem to
contradict the nutritional threshold hypothesis.
What other experiments might be done to test the food hypothesis?
Schultz (1969) has begun experiments by fertilizing six-acre plots in
the tundra. There are two problems with this experimental design.
As well as changing the quality and quantity of food, the amount of
cover is also altered dramatically by fertilization. This confounds any
interpretations unless cover can be changed on other areas without
changing the food supply. Secondly, six acres is a very small part of
the tundra. Immigration to the optimal habitat may be great, thereby
complicating the understanding of the demography of lemmings in the
fertilized area.
Studies using islands or enclosed populations could be very revealing.
Can the tundra, like a southern Indiana grassland, support more
animals than it presently does? Can an increasing lemming population
be established on habitat from which a peak population has just been
removed? One thing is clear: measuring lemming density alone is
not sufficient. We must have detailed information about birth, death,
and growth rates, or any environmental measurements are useless.
Furthermore, we need a greater understanding of how the microtine
actually interacts with its environment. We still have little idea of
what the nutrient requirements of microtines are, or how selective
they are in choosing the quality of their food. The nutrient levels
reported thus far are values for complete plants and the nutrients have
only been measured during the summer.
Nutrient cycling in the tundra is exaggerated by the thin layer
of soil above the permafrost and the slow rates of decomposition.
Therefore, the interactions between the lemmings and their environment are likely to be quite different from those of the Microtus in
CHARLES J. EREBS AND JUDITH H. MYERS
much different than those recorded for the complete plant. Whether
lemmings can select plants or plant parts of high nutrient content,
even when average nutrient content is low, is unknown. However,
Thompson’s (1965) test of the selectivity of Microtus pennsylvanicus
indicated that they did not seem to recognize the quality of the food
plant.
To summarize, we must conclude that, contrary to the statement of
Schultz (1969, p. 86), the biology of the lemming is a very important
element for understanding the tundra ecosystem. Evidence confirms
Pieper’s (1964) hypothesis that lemming grazing actually increases
the nutrients available to plants, and that synchronous cycles between
lemming density and nutrient content in plants are the result of the
effect of lemming density on the availability of nutrients to the plants.
Thus lemmings affect the tundra plants but the plants do not seem to
determine lemming density changes. Data published thus far seem to
contradict the nutritional threshold hypothesis.
What other experiments might be done to test the food hypothesis?
Schultz (1969) has begun experiments by fertilizing six-acre plots in
the tundra. There are two problems with this experimental design.
As well as changing the quality and quantity of food, the amount of
cover is also altered dramatically by fertilization. This confounds any
interpretations unless cover can be changed on other areas without
changing the food supply. Secondly, six acres is a very small part of
the tundra. Immigration to the optimal habitat may be great, thereby
complicating the understanding of the demography of lemmings in the
fertilized area.
Studies using islands or enclosed populations could be very revealing.
Can the tundra, like a southern Indiana grassland, support more
animals than it presently does? Can an increasing lemming population
be established on habitat from which a peak population has just been
removed? One thing is clear: measuring lemming density alone is
not sufficient. We must have detailed information about birth, death,
and growth rates, or any environmental measurements are useless.
Furthermore, we need a greater understanding of how the microtine
actually interacts with its environment. We still have little idea of
what the nutrient requirements of microtines are, or how selective
they are in choosing the quality of their food. The nutrient levels
reported thus far are values for complete plants and the nutrients have
only been measured during the summer.
Nutrient cycling in the tundra is exaggerated by the thin layer
of soil above the permafrost and the slow rates of decomposition.
Therefore, the interactions between the lemmings and their environment are likely to be quite different from those of the Microtus in