A primary goal of restoration ecology is to understand the factors that generate variability in species diversity and composition among restorations. Plant communities may assemble deterministically toward a common community type, or they may assemble stochastically, ending differently because of weather conditions during establishment, soil legacy effects, or exotic species propagule pressure. To test these alternative hypotheses, we sampled plant communities and soil at 93 randomly selected restored prairies distributed throughout Iowa, USA. Five remnant sites were sampled as a reference. We tested our hypotheses using multiple regressions and investigated the strength of direct and indirect effects on species diversity and richness using structural equation models. The prairie restorations were highly variable in their age, size, diversity, soil characteristics, and how they were managed post‐seeding. The strongest predictor of plant species richness and diversity was the degree of invasion, as measured by the abundance of exotic species. Restorations planted with species‐rich seed mixes had reduced exotic species abundance, which led indirectly to higher species richness of restorations. Sites with higher organic matter and a more linear shape had a direct positive effect on exotic abundance, which in turn decreased diversity. We found little support for deterministic assembly, and diversity did not increase with the age of planting. Our results indicate that restored prairie communities tend to assemble into states of high or low diversity, driven by invasion from exotic plant species. Management of exotic species is essential for maximizing species diversity in temperate grassland restorations.

中文翻译：

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外来物种驱动93个恢复的草丛大草原植物物种多样性的模式

恢复生态学的主要目标是了解引起恢复物种多样性和组成变化的因素。植物群落可能会确定性地向一个共同的群落类型聚集，或者它们可能会随机聚集，由于建立过程中的天气条件，土壤遗留效应或外来物种繁殖压力而结束不同的结局。为了检验这些替代假设，我们在分布于美国爱荷华州的93个随机选择的恢复大草原上对植物群落和土壤进行了采样。采样了五个残余位点作为参考。我们使用多元回归检验了我们的假设，并使用结构方程模型研究了对物种多样性和丰富度的直接和间接影响的强度。草原修复物的年龄，大小，多样性，土壤特性以及播种后如何对其进行管理。植物物种丰富度和多样性的最强预测因子是入侵程度，通过外来物种的丰富度来衡量。用物种丰富的种子混合物种植的恢复物减少了外来物种的丰度，从而间接导致恢复物的物种丰富度更高。具有较高有机质和线性形状的位点对外来丰度具有直接的积极影响，反过来又降低了多样性。我们发现对确定性装配的支持很少，并且多样性没有随着种植年龄的增长而增加。我们的结果表明，在受到外来植物物种入侵的驱使下，恢复的草原群落趋向于聚集成高或低多样性状态。