Solving the Enigma of Microtine "Cycles"

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Abstract
A frequently expressed opinion is that despite massive efforts the multi-annual cycles in density exhibited by many microtine rodents remain mysterious; moreover, understanding awaits completion of one or a few clever experiments or elegant mathematical models that will discredit all but the one true hypothesis. I contend in this brief, personal essay that, on the contrary, we are actually quite close to an adequate understanding of microtine cycles. This optimistic perspective requires that we adopt a multi-factorial model of vole demography, a position that allows us to comprehend how many extrinsic and intrinsic factors act synergistically and sequentially to produce the density changes we observe. I argue that this approach is not only supported by a modern systems view of reality, but is consistent with the extensive data base accumulated over a 60 year period. A multi-factorial perspective is illustrated by data for Microtus californicus, a well-studied species that shows considerable geographic and temporal variation in demographic pattern. At least eight key factors (four extrinsic and four intrinsic) are required to explain the multi-annual cycles in this species. The resulting model is complex, but not chaotic or non-testable.