Numerical response of small mustelids to vole abundance: delayed or not?

Abstract

One of the most studied problems in population ecology has been to understand the relative roles of top–down and bottom–up forces in regulating animal populations. This has also been a key issue in studies of vole population dyna mics. Vole populations exhibit a wide variation of dynamics, from seasonal fluctuations to multiannual variations or cyclicity. One of the hypotheses to explain cyclic population dynamics is predation by the specialist predators. A common counterargument against the predation hypothesis has been the lack of conclusive observations of the time delay in the predators’ numerical response. We studied the interaction between voles and their specialist small mustelid predators, the stoat Mustela erminea and the least weasel Mustela n. nivalis, by modelling their interaction to data sets that cover large areas of Finland. Vole abundance was monitored with biannual trappings and their predators with snow‐tracking. Results show a high dependence of the predators on the voles, and this connection is generally tighter in weasels than in stoats. Weasel abundance is affected most strongly by the vole abundance in previous spring, 8.5– 10 months earlier, while in stoats the effect of autumn abundance of voles, 2.5–6 months earlier, was the strongest. These results, together with the observation that the weasels’ effects on voles are stronger after a time lag of 6–9.5 than 2–4.5 months, indicate the existence of a time lag in weasels’ numerical response. A time lag in the predators’ numerical response is a necessary condition for the predators to drive population cycles in its prey, and therefore our results support the specialist predation hypothesis.

Predator–vole interactions in northern Europe: the role of small mustelids revised

“…R. Soc. B 281: 20142119 supporting earlier results [9,15,24]. Earlier mechanistic modelling studies on vole-predator interactions have suffered from the lack of predator time series to confirm the results.…”

“…The changes in the time lags of the predators' numerical responses may also be owing to temporal variations in the relative abundances of the different species. Different raptor species respond to vole densities with different time lags [40], and the stoat responds to vole densities with a shorter time lag than the least weasel [15]. Furthermore, the ability of the predators to track vole abundances and to adjust their reproductive effort accordingly may depend on weather conditions, which could lead to the observed temporal variation in the responses.…”

“…The reliability of the snow track indices is a matter of debate, especially with respect to the small mustelids (discussed recently in [15,41]). Our results indicate a marked amount of measurement error in the small mustelid index, as only ca 50% of variation in the index was attributable to the vole abundances, although these predators are known to rely heavily on voles [9].…”

“…They are small and highly adapted to preying on voles even in their nests and under the snow cover, and can be the main cause of mortality in vole populations [14]. They depend almost exclusively on voles [9], but have a much lower intrinsic growth rate than their prey, and hence a strong but delayed numerical response to vole densities, resulting in delayed density-dependent predation pressure on vole populations [9,15]. Of the two species, the stoat is considered to be more of a generalist than the least weasel [9].…”

Why Do the Boreal Forest Ecosystems of Northwestern Europe Differ from Those of Western North America?

“…(b) An example of the dynamics of voles and stoats from the central Finnish boreal forest (Sundell et al 2013). …”