Section of Integrative Biology, University of Texas at Austin, C0930, Austin, Texas 78712, USA.
Ecology. 2013 Mar;94(3):751-60. doi: 10.1890/12-0732.1.
Although negative relationships between diversity (frequently measured as species richness) and invasibility at neighborhood or community scales have often been reported, realistic natural diversity gradients have rarely been studied at this scale. We recreated a naturally occurring gradient in species richness to test the effects of species richness on community invasibility. In central Texas savannas, as the proportion of woody plants increases (a process known as woody plant encroachment), herbaceous habitat is both lost and fragmented, and native herbaceous species richness declines. We examined the effects of these species losses on invasibility in situ by removing species that occur less frequently in herbaceous patches as woody plant encroachment advances. This realistic species removal was accompanied by a parallel and equivalent removal of biomass with no changes in species richness. Over two springs, the nonnative bunchgrass Bothriochloa ischaemum germinated significantly more often in the biomass-removal treatment than in unmanipulated control plots, suggesting an effect of native plant density independent of diversity. Additionally, significantly more germination occurred in the species-removal treatment than in the biomass-removal treatment. Changes in species richness had a stronger effect on B. ischaemum germination than changes in plant density, demonstrating that niche-related processes contributed more to biotic resistance in this system than did species-neutral competitive interactions. Similar treatment effects were found on transplant growth. Thus we show that woody plant encroachment indirectly facilitates the establishment of an invasive grass by reducing native diversity. Although we found a negative relationship between species richness and invasibility at the scale of plots with similar composition and environmental conditions, we found a positive relationship between species richness and invasibility at larger scales. This apparent paradox is consistent with reports from other systems and may be the result of variation in environmental factors at larger scales similarly influencing both invasibility and richness. The habitat loss and fragmentation associated with woody plant encroachment are two of many processes that commonly threaten biodiversity, including climate change. Many of these processes are similarly likely to increase invasibility via their negative effects on native diversity.
尽管在邻里或社区尺度上，多样性（通常以物种丰富度衡量）与可入侵性之间的负相关关系经常被报道，但在该尺度上很少研究现实自然多样性梯度。我们重新创建了一个自然发生的物种丰富度梯度，以测试物种丰富度对群落可入侵性的影响。在德克萨斯州中部的草原上，随着木本植物比例的增加（这一过程称为木本植物入侵），草本栖息地既会丧失又会破碎化，本地草本物种丰富度下降。我们通过去除木本植物入侵过程中草本斑块中较少出现的物种，来原位检查这些物种损失对可入侵性的影响。这种现实的物种去除伴随着生物量的平行和等效去除，而物种丰富度没有变化。在两个春天，外来丛生禾草 Bothriochloa ischaemum 在生物量去除处理中的萌发频率明显高于未处理的对照样地，这表明与多样性无关的本地植物密度有影响。此外，在物种去除处理中的萌发频率明显高于在生物量去除处理中的萌发频率。物种丰富度的变化对 B. ischaemum 的萌发有更强的影响，而植物密度的变化则没有，这表明在这个系统中，与生态位相关的过程比物种中性的竞争相互作用对生物抗性的贡献更大。在移植生长中也发现了类似的处理效果。因此，我们表明，木本植物入侵通过降低本地多样性间接促进了入侵草的建立。尽管我们在具有相似组成和环境条件的斑块尺度上发现了物种丰富度与可入侵性之间的负相关关系，但在更大尺度上我们发现了物种丰富度与可入侵性之间的正相关关系。这种明显的悖论与其他系统的报告一致，可能是由于较大尺度上环境因素的变化同样影响可入侵性和丰富度。与木本植物入侵相关的生境丧失和破碎化是许多共同威胁生物多样性的过程之一，包括气候变化。这些过程中的许多同样可能通过对本地多样性的负面影响而增加可入侵性。
