Fennoscandian Voles

As mentioned above, Fennoscandian voles show dramatic population cycles over a wide geographic range with a pronounced decrease in cycle strength with decreasing latitude. Time-series analyses point to a significant second-order component in the system, again suggesting that delayed density-dependent time lags are in operation. In contrast to the previous systems, this example includes a suite of small mammals that cycle in partial synchrony: the field vole, Microtus agrestis, the sibling vole, M. rossiae-meridionalis, and the bank vole, Clethrionomys glareolus. The abrupt crashes in population densities that characterize the decline portion of the cycles occur despite seemingly adequate environmental conditions for survival and reproduction.

Time-series analysis again suggested that there is a significant lag dynamics, with an approximate order of 2. Thus, we are dealing with density-dependent lags that are of medium length (3-5 years). Model-fitting using specialist predators as the causal mechanism fit the dynamics of the actual populations quite well, and can mimic the declines in periodicity with increasing latitude given the correct model parameters.

In contrast to the LBM example, there are experimental data that support the time-series data and the mechanistic model results. Experimental removal of both avian predators and weasel predators eliminated the decline in population densities during two separate cycles. Elimination of weasels alone, however, did not preclude the population crashes, perhaps because weasels do not make up a majority of vole-eating predators in the study areas. Although these results are highly significant, they do not fully explain the cycles. They only are able to mechanistically account for the population crashes, but not the subsequent increases in population sizes as the next cycle begins. This may not be a large obstacle, as small herbivores have a high intrinsic rate of population growth, and the increases may be a case of standard exponential growth dynamics as described in Lotka-Volterra models.

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