Rocky Mountain Research Station, USDA Forest Service, 800 East Beckwith Avenue, Missoula, Montana 59801, USA.
Ecology. 2012 Apr;93(4):783-92. doi: 10.1890/11-0750.1.
The intentional introduction of specialist insect herbivores for biological control of exotic weeds provides ideal but understudied systems for evaluating important ecological concepts related to top-down control, plant compensatory responses, indirect effects, and the influence of environmental context on these processes. Centaurea stoebe (spotted knapweed) is a notorious rangeland weed that exhibited regional declines in the early 2000s, attributed to drought by some and to successful biocontrol by others. We initiated an experiment to quantify the effects of the biocontrol agent, Cyphocleonus achates, on Ce. stoebe and its interaction with a dominant native grass competitor, Pseudoroegneria spicata, under contrasting precipitation conditions. Plots containing monocultures of each plant species or equal mixtures of the two received factorial combinations of Cy. achates herbivory (exclusion or addition) and precipitation (May-June drought or "normal," defined by the 50-year average) for three years. Cy. achates herbivory reduced survival of adult Ce. stoebe plants by 9% overall, but this effect was stronger under normal precipitation compared to drought conditions, and stronger in mixed-species plots compared to monocultures. Herbivory had no effect on Ce. stoebe per capita seed production or on recruitment of seedlings or juveniles. In normal-precipitation plots of mixed composition, greater adult mortality due to Cy. achates herbivory resulted in increased recruitment of new adult Ce. stoebe. Due to this compensatory response to adult mortality, final Ce. stoebe densities did not differ between herbivory treatments regardless of context. Experimental drought reduced adult Ce. stoebe survival in mixed-species plots but did not impede recruitment of new adults or reduce final Ce. stoebe densities, perhaps due to the limited duration of the treatment. Ce. stoebe strongly depressed P. spicata reproduction and recruitment, but these impacts were not substantively alleviated by herbivory on Ce. stoebe. Population-level compensation by dominant plants may be an important factor inhibiting top-down effects in herbivore-driven and predator-driven cascades.
有意引入专门的昆虫食草动物来进行外来杂草的生物防治，为评估与自上而下的控制、植物补偿性反应、间接效应以及环境背景对这些过程的影响相关的重要生态概念提供了理想但研究不足的系统。矢车菊（斑点矢车菊）是一种臭名昭著的牧场杂草，它在 21 世纪初表现出区域性减少，一些人归因于干旱，另一些人归因于生物防治的成功。我们开始进行一项实验，以量化生物防治剂 Cyphocleonus achates 对 Ce. stoebe 的影响及其与占主导地位的本地草种竞争种 Pseudoroegneria spicata 的相互作用，这是在不同降水条件下进行的。在三年的时间里，包含每个植物物种的单一种植或两者相等混合物的地块接受了 Cy. achates 食草动物（排除或添加）和降水（5 月至 6 月干旱或“正常”，由 50 年平均值定义）的因子组合的实验。Cy. achates 食草动物总体上降低了成年 Ce. stoebe 植物的存活率 9%，但在正常降水条件下比干旱条件下的效果更强，在混合物种的斑块中比在单一种植的斑块中更强。食草动物对 Ce. stoebe 的个体种子产量或幼苗或幼体的繁殖没有影响。在混合组成的正常降水斑块中，由于 Cy. achates 食草动物的成虫死亡率增加，导致新的成年 Ce. stoebe 的繁殖增加。由于对成虫死亡率的这种补偿反应，无论上下文如何，食草动物处理之间的最终 Ce. stoebe 密度没有差异。实验性干旱降低了混合物种斑块中成年 Ce. stoebe 的存活率，但并没有阻碍新成虫的繁殖或降低最终 Ce. stoebe 的密度，这可能是由于处理时间有限。Ce. stoebe 强烈抑制了 P. spicata 的繁殖和繁殖，但 Ce. stoebe 上的食草动物对这些影响没有实质性缓解。优势植物的种群水平补偿可能是抑制食草动物驱动和捕食者驱动级联中的自上而下效应的一个重要因素。
