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，这种状态或制度将持续很长一段时间(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转换。由于政权更迭可能会极大地改变提供给人类的生态系统产品和服务，因此有必要在实践和理论上了解导致不同政权之间临界点的条件和驱动因素。迄今为止，人们对制度变化的前提条件或驱动因素知之甚少，特别是在潮下生境，在那里很难获得关于制度变化前相互作用物种群落生态状况的长期数据。关于海洋生境中热带制度变化的大部分知识集中在珊瑚和大型藻类之间的变化上，尽管这些生物只代表物种丰富的生态系统的一部分，具有许多可能的制度变化轨迹和结果。因此，拟议调查的总体目标是测试在此建立的概念模型，该模型预测2016年异常强烈的ENSO(厄尔尼诺南方涛动)的厄尔尼诺和拉尼娜阶段如何在整个社区一级推动加拉帕戈斯岩石次潮汐的制度转变。由于目标社区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、海星和捕食性鱼类外，该模型测试了海绵礁假说(SRH)的预测，该假说是一种新兴范式，预测随着未来气候变化(代表珊瑚从海绵制度变化)的珊瑚减少，海绵可能越来越多地主导空间。初步数据表明，与SRH相反，在加拉帕戈斯潮下群落目前的ENSO开始时，海绵在异常温暖的温度期间下降。然而，海胆对海绵和CCA的捕食似乎同时加速，因此建议进行现场操纵性实验，以严格测试和区分ENSO升温对海绵死亡率的影响和海胆对海绵的增强捕食作用。这些实验结果将根据整个群落的实际、长期(13-16年)变化进行评估，这些变化是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而获得的，整个ENSO。对这一基线进行重新抽样也将有助于分析导致假设的政权更迭的指标。该项目的更广泛的教育影响将体现在从高中生到研究生和公众的各个层面。尽管许多生态系统的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、持久的状态，但对热带海洋海底群落制度变化的研究主要集中在从以珊瑚为主向以大型藻类为主的制度转变上。这种狭隘的关注忽略了固着无脊椎动物的各种组合的潜在影响，例如海绵，它们与珊瑚共享珊瑚礁坚硬底物上的空间，如果珊瑚减少，它们可能会作为一种新的制度繁殖。SRH是一种新兴的群落生态范式，它假设随着珊瑚在未来气候变化和海洋酸化的影响下衰落，海绵可能会越来越多地主导空间，但它尚未经过严格的测试。加拉帕戈斯群岛发生的异常强烈的厄尔尼诺现象提供了一个独特的机会，评估气候振荡在社区一级造成制度变化的可能性，并测试可持续发展指数，因为自1999年以来，至少每年对加拉帕戈斯群岛中部多个地点的潮下底栖群落结构进行了量化。最近的2015年调查表明，与目前的厄尔尼诺变暖相吻合的是，多个地点的海绵死亡率很高，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，因此建议进行现场操纵性实验，严格测试和区分ENSO升温对海绵死亡率的影响和海胆对海绵的增强捕食作用。更具体地说，从一个新的概念模型中发展和提出了8个主要假说和4个备选假说，预测了CCA、海绵、藤壶、珊瑚、海胆、海星和捕食性鱼类群落中制度变化的直接和间接路径。这项研究将在2016-2017年四次旅行期间在加拉帕戈斯群岛中部12个社区基准点的岩石潮下进行，期间包括厄尔尼诺和拉尼娜阶段。拟议的实验方法和观测方法(即基线重采样)相结合，将能够对气候引起的热带底栖生态系统状况变化的直接和间接途径进行严格评估。

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.