滨海盐沼湿地是处于海陆交替过渡区的潮滩生态系统，会间歇性或者周期性地受到海洋潮涨潮落的影响，一般生长有许多草本或者低灌木的盐生植被，有着极高的生态系统服务功能，比如提供较高的初级生产力，维持生物多样性，调节营养物质循环，抵御风暴潮保护海岸等等。我国的黄河三角洲，拥有大面积的滨海盐沼湿地，其中一年生盐沼植被盐地碱蓬营造的特色“红地毯”生态系统是最常见的生态景观之一，有着非常重要的生态、经济以及旅游价值。然而在极端气候和围填海活动的影响下，大面积的盐沼湿地出现了退化现象，急需采取科学有效的修复措施来对其进行恢复。
本研究发现在黄河口中位盐沼湿地中，盐地碱蓬-蟹类-微地形三者之间存在一定的生态学互作关系。在这里盐地碱蓬和蟹类洞穴经常同时密集出现，并且在其中不规则地分布有许多盐地碱蓬和蟹类都比较稀疏的平坦小裸斑。在大面积退化盐沼区域更是不具备这种相互作用关系。于是从2016年到2019年，我们采用野外现场观测调查和控制实验相结合的研究方法，对中位盐沼区域盐地碱蓬-蟹类-微地形三者之间的互作机理进行了深入探究，主要包括调查盐地碱蓬-蟹类-微地形的海陆梯度分布特征，进而分析蟹类活动对微地形土壤环境的影响机制及微地形环境对蟹类挖洞的反馈作用机制，探讨蟹类洞穴及植被根部微地形促进盐地碱蓬种子保留与幼苗定植的作用机理，分析蟹类营造微地形促进盐地碱蓬植株生长的作用机理，探讨盐地碱蓬对蟹类的食物供给及遮蔽御敌的促进机制，最后提出了基于盐地碱蓬-蟹类-微地形三者互作关系的生态修复调控模式并应用到大面积退化光板地实际修复工程中。研究成果对开展基于生态系统过程的滨海湿地生态修复具有重要的科学研究价值和实践指导意义。具体研究结果如下：
（1）阐明了盐地碱蓬-蟹类-微地形的海陆梯度分布特征。由海向陆不同区域的水文特征和地形梯度等环境因素的差异，导致了盐地碱蓬、蟹类与微地形的分布存在一定的梯度变化。在相对低位区，潮汐频率很强，蟹类可以在频繁潮汐的保护和提供更多食源的情况下在此大量生存，但凹凸微地形在冬季频繁潮汐作用下被冲刷平坦，加上很强潮汐冲刷作用，植被的种子在这里难以保留，因此在低位区盐地碱蓬少蟹类多，互作现象不明显。在相对中位区，潮汐频率适中，凹凸微地形在冬季适中潮汐作用下得以一定程度的保留，进而能够促进盐地碱蓬的种子保留和幼苗定植，而在夏季茂密的植被也能够促进蟹类的挖洞定居，两者形成了一定的互作关系。在相对高位区，潮汐频率较低，产生了一定的水盐胁迫使得自然状态下植被和蟹类都比较难以生存，互作现象也不明显。
（2）揭示了蟹类与微地形环境的相互促进作用机理。黄河三角洲盐沼湿地中的蟹类通过洞穴挖掘活动，引起了潮滩地貌地形的显著改变，其营造的凹凸起伏的微地形也显著改变了土壤理化指标，相对于未经过蟹类扰动的平坦微地形区域，蟹类洞穴凹凸微地形显著降低了土壤硬度和盐度，并显著提升了土壤含水率和土壤有机质含量。此外，模拟蟹类洞穴挖掘活动，在平坦微地形区域通过人为翻耕营造了人工凹凸微地形样方，也显著改善了潮滩土壤微地形环境指标，其作用与蟹类洞穴凹凸微地形相似。另外人工凹凸微地形中改善了的微地形土壤环境进而还能吸引和促进更多蟹类前来挖洞定居，从而对进一步维持凹凸微地形起到了一定的反馈作用。实验室斜坡模拟实验进一步证明了高含水率、低硬度、低盐度的土壤环境能够吸引更多的蟹类挖洞定居。
（3）阐明了蟹类与微地形促进盐地碱蓬幼苗定植的作用机理。研究通过调查和实验发现在受潮汐影响的中位盐沼生态系统中，蟹类洞穴挖掘活动营造的凹凸微地形结构、潮汐侵蚀对植被根部的冲刷作用形成的凹坑微地形结构，以及植株母体结构保留种子并在潮汐作用下倒伏地面等生物地貌结构或过程都能够促进盐地碱蓬的种子保留和幼苗定植。其中在中位盐沼区域蟹类洞穴挖掘活动营造的凹凸微地形结构能够在冬季得以一定程度上的维持，进而保留盐地碱蓬的种子，并促进春季的幼苗定植。大面积野外调查也发现春季盐地碱蓬幼苗密度与微地形的粗糙度以及前一年的蟹类洞穴密度都成正相关关系。另外模拟自然生物地貌过程而进行的人工微地形改造，也可以促进盐地碱蓬幼苗的定植。同时模拟实验中营造的人工凹凸微地形结构和植被的移栽都能够对蟹类的洞穴挖掘活动起到反馈作用，从而进一步促进和维持了凹凸微地形的形成。
（4）揭示了蟹类与微地形促进盐地碱蓬植株生长的作用机理。研究通过调查和实验发现在黄河三角洲中位盐沼湿地中，蟹类通过洞穴挖掘活动营造和维持的凹凸微地形，还可以为盐地碱蓬的生长提供更为合适的土壤环境（降低土壤硬度和土壤盐度，提高土壤含水率、氧化还原电位、有机质含量等）和更多的聚蔟生长的机会（植被在一定程度内的聚蔟生长所产生的种内促进作用可能会相对高于种内竞争作用，聚蔟植被之间可以共同抵抗环境胁迫，比如潮汐侵蚀、缺氧等），从而能够在一定程度上促进盐地碱蓬植株的生长，对盐地碱蓬的密度、单位面积上的总生物量、以及平均单株生物量等都具有一定的的促进作用。
（5）阐明了盐地碱蓬植株促进蟹类挖洞定居的作用机理。通过对中位盐沼湿地夏季蟹类洞穴与盐地碱蓬植株密度关系的大范围野外调查、盐地碱蓬移除实验、野外盐地碱蓬模拟实验及实验室捕食风险实验等方法，对盐地碱蓬植株促进蟹类洞穴挖掘活动进而促进蟹洞凹凸微地形营造作用机制进行了深入研究。在盐地碱蓬生长茂盛时期，中位盐沼大面积红地毯区域的盐地碱蓬的密度与蟹类洞穴的密度呈现出一定的线性关系。盐地碱蓬移除之后蟹类洞穴的密度会在一定时间内明显减少。野外盐地碱蓬模拟实验及实验室捕食风险实验进一步充分证明了盐地碱蓬可以通过为蟹类提供食源（盐地碱蓬的生长其自身会为蟹类带来充足的食物，同时还能在潮涨潮落过程中为蟹类拦截藻类等其他潜在食源）、提供抵御鸟类等天敌以及减缓物理胁迫的适宜生长环境，从而对蟹类洞穴挖掘活动产生促进作用，进而能够促进蟹类营造和维持凹凸微地形。
（6）提出了基于盐地碱蓬-蟹类-微地形互作关系的生态修复调控模式并应用到实际修复中。根据盐地碱蓬、蟹类与微地形的互作机理，研究发现蟹类营造的凹凸微地形结构对这种相互作用的稳定性起着非常重要的作用。于是本研究对前人提出的基于植被和地貌环境作用的机会窗口框架模型进行了拓展，提出了基于植被-动物-地貌环境之间相互作用的新的机会窗口模型用来指导生态修复，即模拟动物生态系统工程师改造微地形结构的行为，通过单一人为干预微地形的措施为动植物提供水动力相对平静的微环境条件，从而在一个更短时间的潮汐水动力平静机会窗口中把动植物互作关系重新建立起来。然后进一步提出了基于黄河三角洲盐沼湿地中蟹类-盐地碱蓬-微地形之间互作关系的生态修复调控模式。即模拟蟹类洞穴挖掘活动，在冬季种子散布期通过人工翻耕的方法将平坦的微地形改造为凹凸微地形，从而能够减轻春季土壤环境物理胁迫，并促进盐地碱蓬的种子保留和幼苗定植；盐地碱蓬的生长和改善后的土壤环境则可以吸引更多蟹类前来挖掘洞穴，从而进一步维持凹凸微地形，最终恢复盐地碱蓬、蟹类以及微地形环境之间的相互促进作用状态。然后，本研究在由海向陆潮汐水文梯度上对该生态修复调控模式进行了实验研究，并找到了该生态修复调控模式的修复适用区域：在潮汐水文强度较大的相对低位区不适用，因为频繁的潮汐会很快把人工凹凸微地形冲刷平坦而起不到截留种子的作用；但是能够对潮汐水文作用相对适中或较弱的相对中位区和相对较高位区的光斑裸地起到很好的修复效果。最后，本研究将该生态修复调控模式在中高位区域的大面积退化光板地中进行了大尺度修复的应用与评估，并且取得了非常显著的修复效果。

Coastal salt marshes is a kind of wetland ecosystems, which are covered by some halophytes such as tall herbs or low shrubs, located in the transition region of marine and terrestrial ecosystems, and periodically or intermittently affected by the ocean ebbs and flows. The coastal salt marsh has important ecological, economic, and tourism values, such as providing high primary productivity, sustaining rich biodiversity, regulating nutrient cycling, and protecting storm for coastal cities. There are large areas of the salt marshes in the Yellow River Delta, China. The “red carpet” Suaeda salsa salt marsh is one of the most common ecological landscapes in the Yellow River Delta, which has important ecological, economic and tourist value. However, due to effects of natural climatechange such as extreme drought and of artificial activities such as dams and roads, salt marsh degradation has started to occur and restoration of degraded areas is becoming more important. 
This study found that there is an ecological relationship among salt marsh plants (S. salsa), crabs (mainly Helice tientsinensis) and microtopography in the middle-elevation salt marshes of the Yellow River delta. It is found that the high density of S. salsa and high density of crab burrows often appear together in these areas, and there are also some small bare patches with low density of S. salsa, low density of crab burrows, and flat microtopography. Moreover, there is no such stable interaction among S. salsa, crabs and microtopography in the large areas of degraded bare areas. Thus, in this study, field investigations and manipulative experiments were therefore conducted to examine the mechanisms of the interaction among S. salsa, crabs and microtopography in the intertidal salt marsh of the Yellow River Delta from 2016 to 2019. Specifically, we investigated: the regional distribution and ecological responses of the interaction among S. salsa, crabs and microtopography; the influence mechanism of crab activities on microtopographic environment and the feedback mechanism of microtopographic environment on crabs; the facilitation mechanism of the seed retention and seedling establishment of S. salsa by the microtopography created by crab burrowing and the interaction of plants and tides; the facilitation mechanism of the plant growth of S. salsa by the microtopography created by crab burrowing; the facilitation mechanism of the food providing and shelter againsting the enemy of crabs by S. salsa plants. Finally, according to the mechanisms of the interaction, we proposed a biogeomorphic restoration and regulation model based on the interaction among S. salsa, crabs and microtopography, and applied this model into the field restoration engineerings to restore the large degraded bare areas. This study has important scientific value and practical significance for the ecological restoration of coastal salt marshes based on ecosystem processes. The specific results are as follows:
(1) The different regional distribution of the S. salsa, crabs and microtopography along the coastal topography gradient have been illustrated, which was resulted mainly from the hydrological characteristics and topographic gradient of different regions. In the relatively low zones, the high tidal frequency makes the seed retention of S. salsa difficult, but can protect the crabs, thus there are really fewer S. salsa plants but many crabs. In the relatively middle zones, the moderate tidal frequency are suitable for both S. salsa plants and crabs, the concave-convex microtopography could trap plant seeds, and the plants could provide food and protective shelter for crabs, thus there are significant facilitative interactions among crabs, S. salsa and microtopography which result in the patchy landscape. In the relatively high zones, the low tidal frequency makes higher physical stresses (water and salinity), which are unsuitable for crabs to live, thus the flat microtopography makes the seed retention of S. salsa difficult although the tide can dispersal seeds here. 
(2) The influence mechanisms of crab activities on microtopographic environment and the feedback mechanism of microtopographic environment on crabs have been revealed. Crabs living in the intertidal salt marshes build and maintain their burrows through constant digging. At the same time, the burrowing behavior of crabs generates obvious concave-convex microtopography on the intertidal surface. Compared with the relatively flat microtopography without crabs, the concave-convex microtopography generated by crabs significantly changed the physical and chemical characteristics of the microenvironment, such as increased the soil moisture content and soil organic matter content, and decreased the soil salinity and soil hardness. Artificially modifying the microtopography can also effectively change the soil texture, and play a similar role to the concave-convex microtopography generated by crabs, also increased the soil moisture content and soil organic matter content, and decreased the soil salinity and soil hardness. Artificial concave-convex microtopography can also play a feedback effect on crabs and attract more crabs to burrow and settle down, then generate and maintain the concave-convex microtopography. This may be due to the fact that the soil environment indices of the concave-convex microtopography are more suitable for the burrowing behavior of crabs. The laboratory slope simulation experiment furtherly showed that the soil with high moisture content, low hardness and low salinity attracted more crabs to burrow and settle down. 
(3) The facilitation mechanisms of the seedling establishment of S. salsa by the crabs and microtopography have been illustrated. Field investigations and manipulative experiments were conducted to identify associations between seedling establishment of S. salsa and biogeomorphological processes dominated by crabs and S. salsa plants in the middle salt marshes. Successful seedling establishment was strongly influenced by some natural biogeomorphological processes or structures, mainly including concave-convex microtopography generated by crab burrowing, hollows around root segment caused by the integrative effects of tidal flushing and plant stem, and maternal plants with seeds onto the ground. The concave-convex microtopography generated by burrowing crabs could be maintained to some extend in winter, which facilitated the established seedlings in the middle salt marshes. The field investigations of large areas also found that the seedling density of S. salsa in spring was positively correlated with the roughness of microtopography and the density of crab burrows in the previous autumn. The artificial microtopography modification which imitated natural biogeomorphological processes could really facilitate the seedling establishment and the restoration of degraded salt marshes. The salt marsh plants and modified microtopography had feedbacks to crab burrowing. 
(4) The facilitation mechanisms of the plant growth of S. salsa by the crabs and microtopography have been revealed. Field investigations and manipulative experiments in the middle salt marshes showed that crabs H. tientsinensis can promote density, total biomass per unit area, and average biomass through burrowing-generated concave-convex microtopography, which can improve the edaphic environment (decreased soil hardness and salinity, and increased soil moisture content, oxidation-reduction potential, and soil organic matter content), and provide plants more clustered growth opportunities that could facilitate positive intraspecific plant interactions. 
(5) The facilitation mechanisms of the crab burrowing activities by S. salsa plants have been illustrated. Field investigations of the relationship between the density of crab burrows and S. salsa plants, S. salsa removal experiment, field manipulation experiment of S. salsa and predation risk lab experiment in the middle salt marshes were done to study the facilitation mechanism of crabs by S. salsa plants. The investigation showed that crab densities generally increased with plant densities in summer. The experimental removal of salt marsh plants decreased crab burrow density, while transplanting plants and simulating plants in bare patches promoted crab burrowing. Our study showed that S. salsa plant is a kind of food for crabs, which could also intercept some other potential food for crabs during tides. Additionally, S. salsa plants could also facilitate crabs by resisting predators (mainly sea birds) and reducing physical stresses (increased soil moisture content and soil organic matter, and decreased soil penetration resistance, soil salinity, photosynthetically active radiation, and soil temperature).
(6) A model of ecological restoration and regulation based on the ecological interaction among S. salsa, crabs and microtopography has been put forward and applied in the restoration practice. According to the mechanisms of the interaction among crabs, S. salsa and microtopography, we found the concave-convex microtopograpy generated by crabs plays an essential role on the stability of this interaction. Thus, based on the previous ‘Windows of Opportunity (WoO)’ framework about the interaction between vegetation and geomorphorlogy, we proposed extension of this conceptual framework that includes reciprocal facilitation between plants and bioturbating animals. In this case, a single intervention mimicking the structures created by animals could provide calmer hydrodynamic conditions and restore the reciprocal facilitation between plants and animals within a short ‘WoO’. According to our new ‘WoO’ framework, we furtherly proposed a restoration and regulation model of the degraded bare areas based on the interaction among crabs, S. salsa and microtopography. We could simulate crabs burrowing activity, modify the flat microtopography transform into concave-convex microtopography by artificial ploughing during the seed dispersal period in winter, consequently reducing the physical stresses and promoting the seed retention and seedling establishment of the S. salsa in spring. Then the saltmarsh plants and the improved edaphic environment could both attract increasing crabs to burrow and thus maintain the concave-convex microtopography. Then we regulate the model through small-scale microtopography restoration experiment along coastal topographic gradients, and found the applicable restoration areas of the restoration and regulation model. The restoration effects is not significant in relatively low zones with high tidal hydrology, because the artificial microtopography could be difficult to maintain under frequent tidal effects, which couldn’t intercept much seeds. However, the restoration effects is significant in relatively middle and high zones with moderate and low tidal hydrology. Finally, we applied the restoration and regulation model to the application of the large-scale restoration of degraded bare areas in the middle and high zones of salt marshes and achieved significant restoration success.