Strong seasonality may attenuate trophic cascades: vertebrate predator exclusion in boreal grassland

We studied the indirect effects of vertebrate predator exclusion on plant communities in boreal grassland in western Finland to find out whether the removal of the top trophic level would result in a trophic cascade. Predators were excluded from 1996 to 2000 by eight predator-proof fences (each 0.5 ha) constructed on old fields. Despite a major increase in vole densities, the expected trophic cascade attenuated rapidly so that the indirect effects of predator exclusion were restricted to a few plant species. The cause for the rapid attenuation of the trophic cascade appeared to be strong seasonality, as peak densities of voles were attained at the end of the growing season of vegetation, and vole populations declined before the next growing season so that the herbivory pressure during the growing season remained low or moderate. Accordingly, most plants escaped the heaviest grazing pressure either in time (plants completed their reproduction and withered before winter) or in space (living parts hidden under frozen ground and ice). However, heavy winter herbivory reduced the biomass of available vegetation and killed woody species (willows) at vole peaks, which implies that predator exclusion may have a strong effect on secondary succession. During summer, voles reduced the coverage of only a few preferred food plants ( Elymus repens , Phleum pratense , Vicia cracca ). Voles also maintained annual and biennial species in the community by creating gaps in the closed vegetation. We conclude that abiotic factors (harsh winter conditions) limited peak numbers of herbivores below a threshold density where herbivores could have caused a community-level decline in the biomass of herbaceous plants during summer.