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CHARLES J. KREBS AND JUDITH H. MYERS
single mechanism hypothesis is criticized simply because observations
have been made in which a number of factors have been shown to
influence populations. This is not a valid criticism of our search for a
single underlying mechanism for microtine cycles. Many environmental
factors affect the average density level of rodent populations, and we
recognize this as a second problem different from the question of what
prevents unlimited increase (Chitty, 1960). Other environmental
factors will affect population density in a sporadic manner, and not
play a necessary role in every cycle. In any single study of one species
over one fluctuation, the different roles of these factors may be impossible to disentangle. The whole concept of necessary and sufficient
conditions precludes generalizations from single observations.
Most of the research we will summarize here is concerned with the
question of what prevents unlimited increase (although often phrased as
its converse, what causes population declines).
B. W H A T C A U S E S T H E C Y C L I C P E R I O D I C I T Y ?
This question is logically secondary to the first question, and yet
many biologists have been concerned with the three to four year
periodicity. The cause of the particular periodicity will very much
depend on the driving force behind the population fluctuations. The
periodicity might be imposed by the physical or biotic environment, or
it might be internally generated as some function of generation time
of the rodents.
c. W H A T P R O D U C E S S Y N C H R O N Y ?
Vole or lemming populations over thousands of square miles may
reach peak numbers in the same year, and this is another aspect of
population cycles that must somehow be explained. Again, this problem
is logically secondary to the first problem, and only when we can explain
the fluctuations will we be able to study the causes of synchrony.
D. W H A T D E T E R M I N E S T H E A M P L I T U D E O F T H E C Y C L E ?
Some years of peak numbers are much higher than others, and the
same is true of the low points of the cycle. Variations in amplitude have
been especially noted in the Norwegian lemming in which “lemming
years” occur at irregular intervals. Very few attempts have been made
to determine why the amplitude is larger in some years or why it
varies from one habitat to another.
CHARLES J. KREBS AND JUDITH H. MYERS
single mechanism hypothesis is criticized simply because observations
have been made in which a number of factors have been shown to
influence populations. This is not a valid criticism of our search for a
single underlying mechanism for microtine cycles. Many environmental
factors affect the average density level of rodent populations, and we
recognize this as a second problem different from the question of what
prevents unlimited increase (Chitty, 1960). Other environmental
factors will affect population density in a sporadic manner, and not
play a necessary role in every cycle. In any single study of one species
over one fluctuation, the different roles of these factors may be impossible to disentangle. The whole concept of necessary and sufficient
conditions precludes generalizations from single observations.
Most of the research we will summarize here is concerned with the
question of what prevents unlimited increase (although often phrased as
its converse, what causes population declines).
B. W H A T C A U S E S T H E C Y C L I C P E R I O D I C I T Y ?
This question is logically secondary to the first question, and yet
many biologists have been concerned with the three to four year
periodicity. The cause of the particular periodicity will very much
depend on the driving force behind the population fluctuations. The
periodicity might be imposed by the physical or biotic environment, or
it might be internally generated as some function of generation time
of the rodents.
c. W H A T P R O D U C E S S Y N C H R O N Y ?
Vole or lemming populations over thousands of square miles may
reach peak numbers in the same year, and this is another aspect of
population cycles that must somehow be explained. Again, this problem
is logically secondary to the first problem, and only when we can explain
the fluctuations will we be able to study the causes of synchrony.
D. W H A T D E T E R M I N E S T H E A M P L I T U D E O F T H E C Y C L E ?
Some years of peak numbers are much higher than others, and the
same is true of the low points of the cycle. Variations in amplitude have
been especially noted in the Norwegian lemming in which “lemming
years” occur at irregular intervals. Very few attempts have been made
to determine why the amplitude is larger in some years or why it
varies from one habitat to another.