Herbivory mediates the long‐term shift in the relative importance of microsite and propagule limitation

Abstract

Summary Microsite and propagule limitation are predicted to jointly influence plant community assembly and diversity, with shifts in their relative contributions under different ecological conditions. Mammalian herbivory can also exhibit strong impact on community assembly and diversity. However, to date few studies have considered how herbivory might interact with propagule and microsite limitation and how herbivory might alter their relative importance. Even fewer studies have examined how these processes manifest over time to influence community assembly. In fenced and grazed tundra communities that varied in soil moisture, we manipulated propagule limitation by adding seeds of 14 species and manipulated microsite limitation through a one‐time disturbance treatment which reduced resident community biomass. We then followed these communities for 11 years to assess the long‐term impacts of these processes on community assembly and richness. Herbivory interacted with soil moisture to promote long‐term establishment of seeded species: seed addition increased species richness and this effect persisted over 11 years but only in grazed plots, and in drier conditions. Seed addition and herbivory also interacted to drive community composition. Disturbance initially resulted in greater richness and community divergence, but the effect weakened over time, whereas the effects of herbivory in general strengthened. Synthesis. Our results show that herbivory interacts with environmental conditions to mediate the relative importance of microsite and propagule limitation on community assembly; however, its impacts may only become detectable over longer time‐scales. Moreover, our results suggest that herbivory may be a key biotic modulator of community assembly in low‐productivity ecosystems and that incorporating trophic interactions (such as herbivory) into hypotheses about community assembly may provide a better understanding of the relative importance of different assembly mechanisms.