Simulated owl predation risk to voles modifies browsing effects on tree seedling growth
​A vole shaping the vegetation (picture by Mari Lyly)
​Large herbivores, like ungulates, are known to be able to shape vegetation, including preventing areas becoming forested by seedling browsing. Smaller herbivores, such as voles, can be numerous and thus also cause substantial gnawing damage to especially smaller seedlings in commercial seedling nurseries. Protecting seedlings mechanically from browsing is both laborious and expensive. Thus we planned an experiment to see if vole damages could be reduced by simulated predation risk.
Seedlings of three locally common and commercially important tree species (silver birch, Scots pine, Norway spruce) were planted and Microtus voles were released in large seminatural outdoor enclosures. In those enclosures either owl calls (owl treatment, predation risk) or calls of song birds harmless to voles (control, no predation risk) were played, and the vole numbers as well as the growth of the seedlings were regularly followed.

DSC_9680 Mari Lyly.JPG

We found that simulated owl predation risk was associated with higher growth rates in seedlings, but only in deciduous silver birch and only in late season. Deciduous seedlings are more palatable to voles and they also show much faster growth compared to coniferous trees, which most likely explains the difference in response between the tree species. The seasonal effect, on the other hand, is probably caused by the other food items of voles: earlier in the season and in the absence of predation risk voles were able to move freely and search for their preferred food, grasses. Later in the season grasses were less palatable and voles were thus more likely to attack the tree seedlings.
Our results suggest that owl calls could potentially help to protect at least birch seedlings, but the timing of the treatment is important.

9.1.2018 Elina Koivisto

Lyly, M. S., Koivisto, E., Huitu, O. & Korpimäki, E. 2018: Simulated owl predation risk to voles modifies browsing effects on tree seedling growth. — Ann. Zool. Fennici 55: 93–101.

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