Data from: Herbivory mediates the long-term shifts in the relative importance of microsite and propagule limitation

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.

微生境限制（microsite limitation）与繁殖体限制（propagule limitation）被预测会共同影响植物群落组装（plant community assembly）与物种多样性，且二者的相对贡献会随不同生态条件发生变化。哺乳动物植食作用（mammalian herbivory）同样会对群落组装与多样性产生显著影响。然而迄今为止，鲜有研究探讨食草作用如何与繁殖体限制、微生境限制产生交互作用，以及食草作用如何改变二者的相对重要性；更少研究关注这些过程如何随时间推移影响群落组装。
本研究在土壤湿度（soil moisture）存在差异的围栏封育与放牧苔原群落（tundra communities）中，通过添加14个物种的种子来操控繁殖体限制，并通过一次性干扰处理降低本地群落生物量（resident community biomass），以此操控微生境限制。随后我们对这些群落开展了为期11年的跟踪监测，以评估这些过程对群落组装与物种丰富度（species richness）的长期影响。
研究发现，食草作用与土壤湿度交互作用，促进了播种物种的长期定居：种子添加提升了物种丰富度，且该效应在放牧样地与较干燥生境中持续了11年之久。种子添加与食草作用同样存在交互效应，共同驱动群落组成（community composition）变化。干扰处理最初提升了物种丰富度与群落分化（community divergence），但该效应随时间推移逐渐减弱；而食草作用的整体效应则随时间增强。
综合来看，本研究结果表明，食草作用可与环境条件交互，介导微生境限制与繁殖体限制对群落组装的相对重要性；但其影响可能仅在更长时间尺度下才可被检测到。此外，本研究结果提示，食草作用可能是低生产力生态系统中群落组装的关键生物调控因子，而将食草作用等营养级相互作用（trophic interactions）纳入群落组装假说，将有助于更深入理解不同组装机制的相对重要性。