The quick and the slow: Competitive ability of two silica-rich grasses influenced by large and small herbivores

Abstract

Silicate-rich grasses often dominate in heavily grazed areas, presumably because high amounts of silica deter herbivores. Activity of large herbivores and small rodent herbivores increase competitive ability of silicate-rich grasses, possibly through apparent competition. Both types of herbivores often co-exist in grassland ecosystems. Their potential additive effect on competitive ability of silica-rich grasses has seldom been studied. The impact of large and small rodent herbivores on silica-rich grasses, however, can differ. The reason for this can be that small rodent herbivores extensively cut these grasses, for example for nest construction. This suggests that small rodent herbivores actually might have a negative impact on the biomass of the grasses, especially during population peaks. How the grasses respond to rodent activity (i. e. grazing and cutting) may differ depending on growth rate. The ability of a silica-rich grass to dominate the plant community is therefore possibly a result of tolerance through fast growth rate rather than resistance via silicates. I compared the competitive ability (as measured by total and relative biomass) of two common and widespread grasses (Deschampsia cespitosa and Nardus stricta) with similar silica content and different growth rates to that of the surrounding vegetation. I used a three-year exclosure experiment in two sub-arctic riparian valleys, separating the effect of large (reindeer) and small rodent herbivores. None of the grasses showed an exclusively positive biomass response to herbivory, as shown by decline of Deschampsia (-30.8±20.1 g/m2) and Nardus (-57.3±12.3 g/m2) biomass. Total biomass of Deschampsia decreased in response to the impact of only rodents, but recovered when rodent populations decreased. Relative biomass of Deschampsia was almost three times higher in both treatments allowing herbivores as compared to no herbivory, while Nardus gained no biomass advantage from any type of herbivores. This suggests that growth rate is an important deterimant of competitive ability under herbivory. Synthesis: Competitive ability of one unpalatable grass with high growth rate, Deschampsia, increased with large and small herbivore activity, while that of another with low growth rate, Nardus, remained unaltered. Advantages gained by apparent competition is thus likely a result of high tolerance through high growth rates.