Exotic invasive plants threaten ecosystem integrity, and their success depends on a combination of abiotic factors, disturbances, and interactions with existing communities. In dryland ecosystems, soil biocrusts (communities of lichens, bryophytes, and microorganisms) can limit favorable microsites needed for invasive species establishment, but the relative importance of biocrusts for landscape-scale invasion patterns remains poorly understood. We examine effects of livestock grazing in habitats at high risk for invasion to test the hypothesis that disturbance indirectly favors exotic annual grasses by reducing biocrust cover. We present some of the first evidence that biocrusts increase site resistance to invasion at a landscape scale and mediate the effects of disturbance. Biocrust species richness, which is reduced by livestock grazing, also appears to promote native perennial grasses. Short mosses, as a functional group, appear to be particularly valuable for preventing invasion by exotic annual grasses. Our study suggests that maintaining biocrust communities with high cover, species richness, and cover of short mosses can increase resistance to invasion. These results highlight the potential of soil surface communities to mediate invasion dynamics and suggest promising avenues for restoration in dryland ecosystems.

中文翻译：

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放牧干扰通过降解土壤生物地壳群落促进外来一年生草的生长。

外来入侵植物威胁着生态系统的完整性，它们的成功取决于非生物因素、干扰以及与现有群落的相互作用。在旱地生态系统中，土壤生物结皮（地衣、苔藓植物和微生物群落）可以限制入侵物种建立所需的有利微型场所，但生物结皮对景观尺度入侵模式的相对重要性仍知之甚少。我们研究了在入侵风险高的栖息地放牧牲畜的影响，以检验干扰通过减少生物地壳覆盖而间接有利于外来一年生牧草的假设。我们提出了一些初步证据，表明生物结壳在景观尺度上增加了场地对入侵的抵抗力，并调节了干扰的影响。生物结皮物种丰富度，因牲畜放牧而减少，似乎也促进本地多年生草。短苔藓作为一个功能群，似乎对防止外来一年生草的入侵特别有价值。我们的研究表明，保持生物结皮群落具有高覆盖度、物种丰富度和短苔藓覆盖度可以增加对入侵的抵抗力。这些结果突出了土壤表面群落调节入侵动态的潜力，并为旱地生态系统的恢复提出了有希望的途径。