Data from: Livestock grazing reinforces the competitive exclusion of small-bodied birds by large aggressive birds

1.Grazing by domestic livestock is sometimes promoted as a management tool to benefit biodiversity. In many situations, however, it can produce negative outcomes. 2.Here we examine the impacts of recent and historic livestock grazing on bird communities in the semi-arid woodlands in eastern Australia, testing the notion that grazing removes the suppressive effect of structurally complex vegetation on miners, thereby reducing the richness and abundance of small birds. 3.We used time- and area-limited searches of 108 sites varying in livestock grazing history and intensity, to explore the direct and indirect effects of grazing, habitat complexity and the abundance of aggressive, large-bodied birds on smaller-bodied birds using two-way analysis of variance and structural equation modelling. 4.Small birds were less abundant and had lower richness in the presence of miners. Our structural equation models indicated that recent grazing had direct suppressive effects on the abundance of miners, and both richness and abundance of all but the largest-bodied bird groups. However, higher levels of historic livestock grazing reinforced the competitive exclusion of the six small-bodied bird groups (insectivores, nectarivores, declining woodland birds, small ground foraging birds, all small birds, all non-miners) by aggressive miners via reductions in habitat complexity. Moreover, the strength of any suppressive effects on small birds or positive effects on large birds by miners increased with increasing miner abundance. 5.Synthesis and applications. Our results highlight the importance of vegetation structural complexity, not only for providing habitat for woodland birds, but as barriers to the invasion and competitive dominance of miners. Our findings suggest that management actions aimed at reducing tree and shrub density to promote open woodlands are likely to have significant negative consequences for the conservation of small woodland birds.1. 家畜放牧有时被推广为有益于生物多样性的管理手段。然而在诸多情境下，其反而可能产生负面效应。 2. 本研究针对澳大利亚东部半干旱林地，探究近期与历史家畜放牧对当地鸟类群落的影响，旨在检验如下假说：放牧会消除结构复杂植被对矿吸蜜鸟（miners）的抑制作用，进而降低小型鸟类的物种丰富度与种群丰度。 3. 本研究选取108个家畜放牧历史与强度各不相同的样地，采用限时限定面积的调查方法，通过双因素方差分析与结构方程模型，探究放牧、栖息地复杂度以及攻击性大型鸟类丰度对小型鸟类的直接与间接影响。 4. 研究表明，当矿吸蜜鸟存在时，小型鸟类的种群丰度与物种丰富度均更低。本研究的结构方程模型结果显示，近期放牧对矿吸蜜鸟的种群丰度具有直接抑制作用，同时对除体型最大鸟类类群之外的所有类群的物种丰富度与种群丰度均产生抑制效果。然而，更高强度的历史家畜放牧会通过降低栖息地复杂度，强化攻击性矿吸蜜鸟对六大小型鸟类类群（食虫鸟类、食蜜鸟类、衰退型林地鸟类、小型地面觅食鸟类、所有小型鸟类以及所有非矿吸蜜鸟类）的竞争排斥作用。此外，矿吸蜜鸟对小型鸟类的抑制作用以及对大型鸟类的正向促进作用，会随矿吸蜜鸟种群丰度的提升而增强。 5. 综合与应用。本研究结果凸显了植被结构复杂度的重要性：其不仅可为林地鸟类提供栖息环境，同时可作为屏障抵御矿吸蜜鸟的入侵与竞争优势。本研究发现表明，旨在降低乔木与灌木密度以打造开阔林地的管理措施，可能会对小型林地鸟类的保护产生显著的负面效应。