The primacy of density-mediated indirect effects in a community of wolves, elk, and aspen

IF 7.1 1区 环境科学与生态学 Q1 ECOLOGY Ecological Monographs Pub Date : 2024-10-23 DOI:10.1002/ecm.1627
Elaine M. Brice, Eric J. Larsen, Daniel R. Stahler, Daniel R. MacNulty
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Abstract

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.
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狼、麋鹿和杨树群落中以密度为中介的间接效应的重要性
生态系统中捕食者的移除或增加会引发营养级联,捕食者通过对其食草动物猎物的直接影响间接影响植物和/或非生物过程。营养级联可以通过密度介导的间接效应(DMIE)和/或性状介导的间接效应(TMIE)发挥作用,前者是指捕食者通过捕食降低食草动物的密度,后者是指捕食者诱发食草动物的性状反应,从而改变食草动物对植物的影响。人工实验表明,TMIE 与 DMIE 相比,是营养级联的同等或更重要的驱动力。这种情况是否普遍适用于自然界还不确定,因为很少有研究直接比较 TMIE 和 DMIE 对自然界未受操纵的田野模式的影响程度。人们经常引用 TMIE 来解释黄石国家公园北部涉及狼(Canis lupus)、麋鹿(Cervus canadensis)和杨树(Populus tremuloides)的教科书式营养级联。这一假说认为,狼主要通过减少麋鹿的啃食来间接增加杨树幼苗进入上层林木的数量,以应对狼捕食风险的空间变化,而不是通过降低麋鹿的种群密度。为了验证这一假设,我们比较了狼捕食风险的时空变化和麋鹿种群密度的时空变化对黄石国家公园北部 113 个杨树林区 21 年间(1999-2019 年)未受人为干预的杨树幼树浏览和吸收模式的影响。在 10 个狼捕食风险指数中,只有 2 个指数对杨树幼树的啃食和吸收有统计学意义的影响,这些影响比麋鹿密度的影响弱 8-28 倍。如果麋鹿密度的时间变化可归因于狼的捕食,那么我们的结果表明,狼-麋鹿-杨树的营养级联主要是密度介导的,而不是性状介导的。这与另一种假设相吻合,即狼和其他积极捕猎的捕食者具有广阔的栖息地领域,当猎物(如麋鹿)也具有广阔的栖息地领域时,DMIEs就会占据主导地位。至少对于这类捕食者-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,同时仍然可以准确地理解营养级联。
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来源期刊
Ecological Monographs
Ecological Monographs 环境科学-生态学
CiteScore
12.20
自引率
0.00%
发文量
61
审稿时长
3 months
期刊介绍: The vision for Ecological Monographs is that it should be the place for publishing integrative, synthetic papers that elaborate new directions for the field of ecology. Original Research Papers published in Ecological Monographs will continue to document complex observational, experimental, or theoretical studies that by their very integrated nature defy dissolution into shorter publications focused on a single topic or message. Reviews will be comprehensive and synthetic papers that establish new benchmarks in the field, define directions for future research, contribute to fundamental understanding of ecological principles, and derive principles for ecological management in its broadest sense (including, but not limited to: conservation, mitigation, restoration, and pro-active protection of the environment). Reviews should reflect the full development of a topic and encompass relevant natural history, observational and experimental data, analyses, models, and theory. Reviews published in Ecological Monographs should further blur the boundaries between “basic” and “applied” ecology. Concepts and Synthesis papers will conceptually advance the field of ecology. These papers are expected to go well beyond works being reviewed and include discussion of new directions, new syntheses, and resolutions of old questions. In this world of rapid scientific advancement and never-ending environmental change, there needs to be room for the thoughtful integration of scientific ideas, data, and concepts that feeds the mind and guides the development of the maturing science of ecology. Ecological Monographs provides that room, with an expansive view to a sustainable future.
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