旗舰级海洋巨型食草动物的增长对海草生态系统多功能性的影响
Seagrass Ecosystem Multifunctionality Under the Rise of a Flagship Marine Megaherbivore
作者: Marjolijn J. A. Christianen, Fee O. H. Smulders, Jan Arie Vonk, Leontine E. Becking, Tjeerd J. Bouma, Sabine M. Engel, Rebecca K. James, Mabel I. Nava, Jaco C. de Smit, Jurjan P. van der Zee, Per J. Palsbøll and Elisabeth S. Bakker
期刊: Global Change Biology
Large grazers (megaherbivores) have a profound impact on ecosystem functioning. However, how ecosystem multifunctionality is affected by changes in megaherbivore populations remains poorly understood. Understanding the total impact on ecosystem multifunctionality requires an integrative ecosystem approach, which is especially challenging to obtain in marine systems. We assessed the effects of experimentally simulated grazing intensity scenarios on ecosystem functions and multifunctionality in a tropical Caribbean seagrass ecosystem. As a model, we selected a key marine megaherbivore, the green turtle, whose ecological role is rapidly unfolding in numerous foraging areas where populations are recovering through conservation after centuries of decline, with an increase in recorded overgrazing episodes. To quantify the effects, we employed a novel integrated index of seagrass ecosystem multifunctionality based upon multiple, well-recognized measures of seagrass ecosystem functions that reflect ecosystem services. Experiments revealed that intermediate turtle grazing resulted in the highest rates of nutrient cycling and carbon storage, while sediment stabilization, decomposition rates, epifauna richness, and fish biomass are highest in the absence of turtle grazing. In contrast, intense grazing resulted in disproportionally large effects on ecosystem functions and a collapse of multifunctionality. These results imply that (i) the return of a megaherbivore can exert strong effects on coastal ecosystem functions and multifunctionality, (ii) conservation efforts that are skewed toward megaherbivores, but ignore their key drivers like predators or habitat, will likely result in overgrazing-induced loss of multifunctionality, and (iii) the multifunctionality index shows great potential as a quantitative tool to assess ecosystem performance. Considerable and rapid alterations in megaherbivore abundance (both through extinction and conservation) cause an imbalance in ecosystem functioning and substantially alter or even compromise ecosystem services that help to negate global change effects. An integrative ecosystem approach in environmental management is urgently required to protect and enhance ecosystem multifunctionality.
Fig. 3 (a) The number of publications on seagrass and green turtle grazing in peer-reviewed journals is accelerating over time (Web of Science, Scopus, Google Scholar 1960–2022, Supplementary information text S1) mirroring the recovery of green turtle populations. Arrow 1: (McRoy & Helfferich, 1977; Thayer et al., 1977); arrow 2: (Jackson, 1997), arrow 3: (Chaloupka et al., 2008; Mazaris et al., 2017; Weber et al., 2014). (b) A selection of sites illustrates that all three different grazing scenarios for green turtles occur in coastal (sub-)tropical seagrass ecosystems around the world, in all three ocean basins where green turtles are found. Green dots: global seagrass distribution (UNEP-WCMC & Short, 2021), blue: distribution of the green turtle, Chelonia mydas, (Kot et al., 2022). (Scenario 1) (Gaubert-Boussarie et al., 2021; Jackson, 1997; Jones et al., 2018; van der Laan & Wolff, 2006; Vonk et al., 2008); (Scenario 2) (Ballorain et al., 2010; Christianen et al., 2019; Gulick et al., 2020; Molina Hernández & van Tussenbroek, 2014; Rodriguez & Heck, 2020; Scott et al., 2020); (Scenario 3) (Christianen et al., 2014; Fourqurean et al., 2019; Gangal et al., 2021).
大型食草动物（巨型食草动物）对生态系统功能有深远的影响。然而，对于大型食草动物种群变化如何影响生态系统多功能性仍然知之甚少。全面理解其对生态系统多功能性的总体影响需要采取一种综合性生态系统方法，尤其是在海洋系统中找到这样一种方法则更具挑战性。我们在一个热带加勒比海海草生态系统中用实验模拟了不同放牧强度并评估了其对于生态系统功能及其多功能性的影响。我们选择了一种重要的海洋巨型食草动物绿海龟作为实验模型，其在生态环境中的角色在众多觅食区中日渐凸显，这些觅食区的种群数量经历了过去几个世纪的大量减少后正通过保育工作逐渐得以恢复，与此同时历史记载的过度放牧事件也有所增加。为了量化这些影响，我们采用了一种新的海草生态系统多功能综合指数，该指数基于反映生态系统服务中的多种公认的海草生态系统功能度量。结果表明，中龟放牧产生的养分循环速率和碳储量最高，而在没有海龟放牧的情况下，沉积物的稳定性、分解率、表层动物的丰富度和鱼类生物量是最高的。相反，密集放牧对生态系统功能产生了不同程度的巨大影响和多功能性的崩溃。这些结果表明，（i）大型食草动物回归会对沿海生态系统功能和多功能性产生巨大影响，（ii）保育工作偏重于巨型食草动物，但忽视捕食者或栖息地等关键驱动因素，可能会导致过度放牧引起的多功能丧失，（iii）多功能指数作为评估生态系统性能的定量工具，显示出了巨大的潜力。巨型食草动物丰度的显著和迅速变化（通过灭绝和保护）导致生态系统功能的不平衡，并大大改变甚至损害生态系统服务，从而有助于抵消全球变化的影响。迫切需要在环境管理中采用综合生态系统方法，以保护和增强生态系统的多功能性。
（实习生吕晴编译）
