The removal or addition of a predator in an ecosystem can trigger a trophic cascade, whereby the predator indirectly influences plants and/or abiotic processes via direct effects on its herbivore prey. A trophic cascade can operate through a density‐mediated indirect effect (DMIE), where the predator reduces herbivore density via predation, and/or through a trait‐mediated indirect effect (TMIE), where the predator induces an herbivore trait response that modifies the herbivore's effect on plants. Manipulative experiments suggest that TMIEs are an equivalent or more important driver of trophic cascades than are DMIEs. Whether this applies generally in nature is uncertain because few studies have directly compared the magnitudes of TMIEs and DMIEs on natural unmanipulated field patterns. A TMIE is often invoked to explain the textbook trophic cascade involving wolves (Canis lupus), elk (Cervus canadensis), and aspen (Populus tremuloides) in northern Yellowstone National Park. This hypothesis posits that wolves indirectly increase recruitment of young aspen into the overstory primarily through reduced elk browsing in response to spatial variation in wolf predation risk rather than through reduced elk population density. To test this hypothesis, we compared the effects of spatiotemporal variation in wolf predation risk and temporal variation in elk population density on unmanipulated patterns of browsing and recruitment of young aspen across 113 aspen stands over a 21‐year period (1999–2019) in northern Yellowstone National Park. Only 2 of 10 indices of wolf predation risk had statistically meaningful effects on browsing and recruitment of young aspen, and these effects were 8–28 times weaker than the effect of elk density. To the extent that temporal variation in elk density was attributable to wolf predation, our results suggest that the wolf–elk–aspen trophic cascade was primarily density‐mediated rather than trait‐mediated. This aligns with the alternative hypothesis that wolves and other actively hunting predators with broad habitat domains cause DMIEs to dominate whenever prey, such as elk, also have a broad habitat domain. For at least this type of predator–prey community, our study suggests that risk‐induced trait responses can be abstracted or ignored while still achieving an accurate understanding of trophic cascades.

中文翻译：

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密度介导的间接效应在狼、麋鹿和白杨群落中的首要地位


在生态系统中移除或添加捕食者可以触发营养级联反应，即捕食者通过直接影响其食草动物猎物来间接影响植物和/或非生物过程。营养级联可以通过密度介导的间接效应 （DMIE） 运作，其中捕食者通过捕食降低食草动物密度，和/或通过性状介导的间接效应 （TMIE），其中捕食者诱导食草动物性状反应，从而改变食草动物对植物的影响。操作实验表明，TMIEs 是比 DMIE 等效或更重要的营养级联驱动力。这是否普遍适用于自然界尚不确定，因为很少有研究直接比较 TMIE 和 DMIE 在自然未操纵的场模式上的大小。TMIE 经常被用来解释教科书中涉及黄石国家公园北部狼 （Canis lupus）、麋鹿 （Cervus canadensis） 和白杨 （Populus tremuloides） 的营养级联。该假设假设，狼间接增加了年轻白杨到上层的招募，主要是通过减少麋鹿浏览以应对狼捕食风险的空间变化，而不是通过降低麋鹿种群密度。为了检验这一假设，我们比较了狼捕食风险的时空变化和麋鹿种群密度的时间变化对黄石国家公园北部 21 年（1999-2019 年）113 个白杨林中未操纵的年轻白杨浏览和招募模式的影响。狼捕食风险的 10 个指标中只有 2 个对年轻白杨的浏览和招募具有统计学意义的影响，并且这些影响比麋鹿密度的影响弱 8-28 倍。 在某种程度上，麋鹿密度的时间变化归因于狼的捕食，我们的结果表明，狼-麋鹿-白杨营养级联主要是密度介导的，而不是性状介导的。这与另一种假设一致，即每当猎物（如麋鹿）也具有广泛的栖息地时，狼和其他具有广泛栖息地域的积极捕食者会导致 DMIE 占据主导地位。至少对于这种类型的捕食者-猎物群落，我们的研究表明，风险诱导的性状反应可以被抽象或忽略，同时仍然可以准确理解营养级联反应。