Positive indirect effects more than balance negative direct effects of ungulate grazers on population growth of a grassland herb

Abstract

Herbivores can affect plant population dynamics both directly because of the damage they inflict, and indirectly by moderating conditions for plant recruitment, competition and other biotic interactions. Still, the relative importance of indirect effects of herbivores on plant population dynamics is poorly known. We quantified direct and indirect effects of ungulate grazers on population growth rate of the short-lived perennial herb Primula farinosa, using integral projection models based on demographic data collected over 7 years in exclosures and open control plots at nine grassland sites in southern Sweden. In addition, we explored the mechanisms behind indirect effects with simulations. Grazers had negative direct effects on P. farinosa population growth rate, but these were more than balanced by positive indirect effects. The positive indirect effects were mainly linked to improved conditions for plant recruitment. Simulations indicated that indirect effects of ungulate grazers on population growth rate via interactions with pollinators, seed predators, and small herbivores were weak in this system. Synthesis. The results demonstrate that a full understanding of the effects of grazing on plant population dynamics requires that both direct and indirect effects are identified and quantified. Plant species vary considerably in their response to shifts in grazing regime. Our study sets an example for how the causes of such variation can be assessed, and thus providing a better understanding of the variable effects of herbivores on plant fitness, abundance and distribution.