Data from: Shrub encroachment is linked to extirpation of an apex predator

The abundance of shrubs has increased throughout Earth's arid lands. This ‘shrub encroachment’ has been linked to livestock grazing, fire-suppression and elevated atmospheric CO2 concentrations facilitating shrub recruitment. Apex predators initiate trophic cascades which can influence the abundance of many species across multiple trophic levels within ecosystems. Extirpation of apex predators is linked inextricably to pastoralism, but has not been considered as a factor contributing to shrub encroachment. Here, we ask if trophic cascades triggered by the extirpation of Australia's largest terrestrial predator, the dingo (Canis dingo), could be a driver of shrub encroachment in the Strzelecki Desert, Australia. We use aerial photographs spanning a 51-year period to compare shrub cover between areas where dingoes are historically rare and common. We then quantify contemporary patterns of shrub, shrub seedling and mammal abundances, and use structural equation modelling to compare competing trophic cascade hypotheses to explain how dingoes could influence shrub recruitment. Finally, we track the fate of seedlings of an encroaching shrub, hopbush (Dodonaea viscosa angustissima), during a period optimal for seedling recruitment, and quantify removal rates of hopbush seeds by rodents from enriched seed patches. Shrub cover was 26–48% greater in areas where dingoes were rare than common. Our structural equation modelling supported the hypothesis that dingo removal facilitates shrub encroachment by triggering a four level trophic cascade. According to this model, increased mesopredator abundance in the absence of dingoes results in suppressed abundance of consumers of shrub seeds and seedlings, rodents and rabbits respectively. In turn, suppressed abundances of rodents and rabbits in the absence of dingoes relaxed a recruitment bottleneck for shrubs. The results of our SEM were supported by results showing that rates of hopbush seedling survival and seed removal were 1·7 times greater and 2·1 times lower in areas where dingoes were rare than common. Our study provides evidence linking the suppression of an apex predator to the historic encroachment of shrubs. We contend that trophic cascades induced by apex predator extirpation may be an overlooked driver of shrub encroachment.全球干旱地区的灌木丰度持续攀升。这种“灌木扩张（shrub encroachment）”现象与家畜放牧、火灾抑制以及促进灌木定植的大气CO₂浓度升高密切相关。顶级捕食者可引发营养级联效应，进而影响生态系统内多个营养级中众多物种的种群丰度。顶级捕食者的灭绝与游牧放牧有着密不可分的联系，但此前并未被视作导致灌木扩张的潜在因素。 本研究旨在探究：澳大利亚最大陆生捕食者——澳洲野犬（Canis dingo）的灭绝所引发的营养级联效应，是否会成为澳大利亚斯特泽尔基沙漠（Strzelecki Desert）灌木扩张的驱动因素。 本研究采用跨越51年的航空影像，对比历史上澳洲野犬稀有与常见区域的灌木盖度。随后，我们量化当前灌木、灌木幼苗及哺乳动物的丰度模式，并通过结构方程模型（structural equation modelling）对比竞争性营养级联假说，以阐释澳洲野犬如何影响灌木定植。最后，我们在幼苗定植的最佳窗口期内，追踪入侵性灌木车桑子（Dodonaea viscosa angustissima，hopbush）幼苗的存活动态，并量化啮齿类动物对富集种子斑块中车桑子种子的移除速率。 澳洲野犬稀有区域的灌木盖度较常见区域高出26%~48%。本研究的结构方程模型验证了如下假说：澳洲野犬的消失通过引发四级营养级联效应，促进了灌木扩张。根据该模型，在澳洲野犬缺失的情况下，中型捕食者的丰度上升，进而分别抑制了灌木种子与幼苗的消费者——啮齿类和兔类的种群丰度。反之，澳洲野犬缺失区域内啮齿类与兔类的丰度下降，缓解了灌木定植的瓶颈效应。本研究的结构方程模型结果得到了野外观测数据的支持：车桑子幼苗的存活率在澳洲野犬稀有区域是常见区域的1.7倍，而种子移除速率则仅为后者的1/2.1（即低2.1倍）。 本研究为顶级捕食者的抑制与历史上的灌木扩张之间的关联提供了实证依据。我们认为，顶级捕食者灭绝所引发的营养级联效应，或许是此前被忽视的灌木扩张驱动因素。