Demographic strategies in fluctuating populations of small rodents

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Abstract
A graphic model for individual selection determined by the logarithmic growth rates, dN i /N i ·dt, is developed for studying optimal demographic strategies at different phases of microtine cycles. In a density-independent situation (no crowding) selection leads to maximization of total life-time reproductive output (or equivalently, the Malthusian parameter, m) at the expense of competitive (contest type) abilities. In a density-dependent situation (crowding), selection leads to maximization of contest type competitive abilities at the expense of reproductive output. These two modes of selective pressure are called r- and α-selection. r-selection is presumed to occur during the increase phase of a cycle. As the habitat becomes crowded, α-selection takes over and is assumed to be extreme at high densities. The characteristics of r- and α-strategists are found to be similar to those of the docile and aggressive forms presumed in Chitty's theory for fluctuating populations. Literature supports the attributes predicted by the theory. I argue that sensitivity to density-independent factors is higher in the α-strategy. On the basis of a graphic model, I show that the α-strategists' high sensitivity to extrinsic factors will account for the crash in microtine cycles. On the basis of these theoretical considerations, Chitty's theory for fluctuating populations is interpreted to imply that interactions between intrinsic and extrinsic (random) factors will result in cycles. A graphic model for this interpretation of Chitty's theory is discussed. The heterogeneity of the habitat is an important aspect. According to theory, dispersal of pregnant females is explained as an adaptation leading to increased current reproductive output. This behaviour is presumed to dominate numerically during the increase phase of a cycle, a prediction supported in literature.