RAPID: Testing the ability of the 2015-2017 El Nino Southern Oscillation (ENSO) to drive a community-level regime shift in the Galapagos marine ecosystem

所有类型的生境都发生了生态结构、生物多样性和生态系统功能的变化。有时，这种变化是如此巨大和突然，以至于生态系统切换到另一种状态或制度，这种状态或制度持续了很长一段时间（即几十年到几千年），例如撒哈拉沙漠的植被生态系统和沙漠生态系统之间的切换。由于制度变化可能会极大地改变提供给人类的生态系统产品和服务，因此，从实践和理论上都需要了解导致不同制度之间出现临界点的条件和驱动因素。迄今为止，人们对制度变化的先决条件或驱动因素知之甚少，特别是在潮汐下栖息地，在那里很难获得制度变化之前相互作用的物种群落的生态状态的长期数据。大多数关于海洋栖息地热带政权转移的知识都集中在珊瑚和大型藻类之间的转移上，尽管这些生物只是物种丰富的生态系统的一部分，具有许多可能的轨迹和政权变化的结果。因此，本次研究的首要目标是验证本文开发的一个概念模型，该模型预测了2016年异常强烈的ENSO （El Niño南方涛动）的El Niño和La Niña阶段如何在整个群落水平上驱动加拉帕戈斯群岛岩石潮下的政权转变。由于目标群落包括海绵作为关键组成部分，除了藤壶，甲壳珊瑚藻类（CCA），珊瑚，海胆，海星和掠食性鱼类，该模型测试了海绵珊瑚礁假说（SRH）的预测，这是一种新兴的范式，预测海绵可能越来越多地主导空间，因为珊瑚从未来的气候变化（代表珊瑚到海绵的制度变化）下降。初步数据表明，与SRH相反，在当前ENSO开始时，加拉帕戈斯群岛潮下群落的海绵在异常温暖的温度下减少。然而，海胆对海绵的捕食和CCA似乎同时加速，因此提出了操作性的野外实验，以严格测试和区分ENSO升高的温度对海绵死亡率的影响与海胆对海绵的捕食增强的影响。这些实验结果将在整个ENSO期间通过对加拉帕戈斯中部12个地点的底栖生物群落进行定量重新采样获得的整个群落的实际长期（13-16年）变化的背景下进行评估。重新采样这一基线也将使分析导致假设的制度变化的指标成为可能。从高中学生到研究生和公众，该项目将产生更广泛的教育影响。虽然在许多生态系统中，制度变化被描述为响应自然或人为驱动因素而突然转变为交替的、持续的状态，但对热带海洋底栖群落制度变化的研究主要集中在从珊瑚主导到大型藻类主导的制度转变上。这种狭隘的关注忽视了各种无根无脊椎动物组合的潜在影响，例如海绵，它们与珊瑚共享珊瑚礁硬基质上的空间，如果珊瑚减少，它们可能会作为一种新的制度增殖。SRH是一种新兴的群落生态范式，它假设随着珊瑚因未来气候变化和海洋酸化而减少，海绵可能会日益主导空间，但它尚未经过严格的测试。自1999年以来，在加拉帕戈斯群岛中部的多个地点至少每年对潮下底栖生物群落结构进行量化，因此，在加拉帕戈斯群岛发生的异常强烈的厄尔尼诺Niño现象提供了一个独特的机会，可以评估气候振荡在群落水平上产生制度转变的可能性，并测试SRH。最近的2015年调查显示，与目前的厄尔尼诺Niño变暖相吻合的多个地点的海绵死亡率显著上升，这与SRH相反。然而，海胆对海绵和甲壳珊瑚藻（CCA）的捕食似乎同时加速，因此，我们提出了可操作的野外实验，以严格测试和区分ENSO升高的温度对海绵死亡率的影响与海胆对海绵捕食增强的影响。更具体地说，根据一个新的概念模型，提出了8个主要假设和4个替代假设，预测了CCA、海绵、藤壶、珊瑚、海胆、海星和掠食性鱼类群落的直接和间接政权变化途径。该研究将在加拉帕戈斯群岛中部的12个社区基线地点的潮下岩石中进行，在2016-2017年的四次旅行中，包括El Niño和La Niña阶段。拟议的实验和观测（即基线重新采样）相结合的方法将能够严格评估气候引起的热带底栖生态系统制度变化的直接和间接途径。

Changes in the ecological structure, biodiversity and functioning of ecosystems have occurred in all types of habitats. Sometimes the change is so large and abrupt that the ecosystem switches to an alternate state, or regime, that persists for long periods of time (i.e. decades to millennia) such as the switch between a vegetated and desert ecosystem in the Sahara. Since regime changes may drastically alter the ecosystem goods and services provided to humankind, there is a practical as well as theoretical need to understand the conditions and drivers leading to tipping points between alternate regimes. To date, little is known about either the pre-conditions or drivers of regime change, particularly in subtidal habitats where long-term data on the ecological state of communities of interacting species prior to regime change is difficult to obtain. Most of the knowledge about tropical regime shifts in marine habitats has focused on shifts between corals and macroalgae even though these organisms represent only part of a species-rich ecosystem with many possible trajectories and outcomes of regime change. Consequently, the overarching goal of the proposed investigation is to test a conceptual model developed herein predicting how both El Niño and La Niña phases of the unusually strong 2016 ENSO (El Niño Southern Oscillation) may drive a regime shift in the Galapagos rocky subtidal at the whole community level. As the target community involves sponges as a key component, in addition to barnacles, Crustose Coralline Algae (CCA), corals, sea urchins, sea stars and predatory fish, the model tests predictions from the Sponge Reef Hypothesis (SRH), an emerging paradigm predicting that sponges may increasingly dominate space as corals decline from future climate change (representing a coral to sponge regime change). Preliminary data indicate that counter to the SRH, sponges declined during the unusually warm temperatures at the outset of the present ENSO in Galapagos subtidal communities. However, sea urchin predation on sponges and CCA appears to have accelerated at the same time, so manipulative field experiments are proposed to rigorously test and differentiate the effects of ENSO elevated temperature on sponge mortality from the effects of enhanced sea urchin predation on sponges. These experimental results will be evaluated in the context of actual, long-term (13-16 year) changes in the whole community obtained by quantitative re-sampling of the benthic community at 12 sites in the central Galapagos throughout the present ENSO. Re-sampling this baseline will also enable the analysis of indicators leading up to the hypothesized regime change. Broader educational impacts of the project will transpire at all levels from high school students to graduate students and the public.Although regime changes have been described as abrupt shifts to alternate, persistent states in many ecosystems in response to natural or anthropogenic drivers, research on regime change in bottom-dwelling communities of tropical oceans has largely focused on a switch from coral-dominated to macroalgal- dominated regimes. This narrow focus overlooks potential influences of the diverse assemblages of sessile invertebrates such as sponges that share space on the hard substrate of reefs with corals and could proliferate as a new regime if corals are diminished. The SRH is an emerging community ecological paradigm that posits that sponges may increasingly dominate space as corals decline from future climate change and ocean acidification, yet it has not been rigorously tested. The exceptionally strong El Niño occurring in the Galapagos Islands presents a unique opportunity evaluate the potential for climate oscillations to create regime shifts at the community level and to test the SRH because subtidal benthic community structure has been quantified at least annually since 1999 at multiple sites in the central Galapagos Islands. Recent 2015 surveys indicated significant mortality of sponges at multiple sites coincident with the present El Niño warming, counter to the SRH. However, sea urchin predation on sponges and Crustose Coralline Algae (CCA) appears to have accelerated at the same time, so manipulative field experiments are proposed to rigorously test and differentiate the effects of ENSO elevated temperature on sponge mortality from the effects of enhanced sea urchin predation on sponges. More specifically, eight main hypotheses along with four alternate hypotheses are developed and proposed from a new conceptual model predicting direct and indirect pathways of regime change in a community of CCA, sponges, barnacles, corals, sea urchins, sea stars and predatory fish. The research will be performed in the rocky subtidal at the 12 community baseline sites in the central Galapagos archipelago during four trips in 2016-2017 bracketing the El Niño and La Niña phases. The proposed combination of experimental and observational (i.e. baseline re-sampling) methods will enable a rigorous evaluation of climate-induced direct and indirect pathways of regime change in tropical benthic ecosystems.