Complexity-functioning relationships differ across different environmental conditions

ABSTRACT Habitat complexity is widely considered an important determinant of biodiversity, and enhancing complexity can play a key role in restoring degraded habitats. However, the effects of habitat complexity on ecosystem functioning – as opposed to biodiversity and community structure – are relatively poorly understood for artificial habitats, which dominate many coastlines. With Greening of Grey Infrastructure (GGI) approaches, or eco-engineering, increasingly being applied around the globe, it is important to understand the effects that modifying habitat complexity has on both biodiversity and ecological functioning in these highly modified habitats. We assessed how manipulating physical (primary substrate) and/or biogenic habitat (bivalves) complexity on intertidal artificial substrata affected filtration rates, net and gross primary productivity (NPP and GPP, respectively) and community respiration (CR) – as well as abundance of filter feeders and macro-algae and habitat use by cryptobenthic fish across six locations in three continents. We manipulated both physical and biogenic complexity using 1) flat or ridged (2.5 cm or 5 cm) settlement tiles that were either 2) unseeded or seeded with oysters or mussels. Across all locations, increasing physical and biogenic complexity (5 cm seeded tiles) had a significant effect on most ecological functioning variables, increasing overall filtration rates of the assemblages on tiles but decreasing productivity (both GPP and NPP) across all locations. There were no overall effects of increasing either type of habitat complexity on cryptobenthic fish MaxN, total time in frame or macro-algal cover. Within each location, there were marked differences in the effects of habitat complexity. In Hobart, Australia, we found higher GPP on flat tiles than on 5 cm tiles. In Sydney, Australia, we found higher filtration rates and biomass of filter feeders on seeded than unseeded tiles and greater abundance (MaxN) of cryptobenthic fish on 5 cm tiles than on flat tiles, whereas in Dublin, Ireland, there were higher filtration rates on 5 cm complex tiles than on flat tiles. Our findings suggest that GGI solutions via increased habitat complexity are likely to have trade-offs among potentially desired functions, such as productivity and filtration rates, and variable effects on cryptobenthic fish communities. Importantly, our results show that the effects of GGI practices can vary markedly according to the environmental context and therefore should not be blindly and uniformly applied across the globe.摘要 生境复杂性被广泛视为生物多样性的重要决定因素，提升生境复杂性在退化生境修复中可发挥关键作用。然而，相较于其对生物多样性与群落结构的影响，广泛分布于诸多海岸线的人工生境，其复杂性对生态系统功能的影响仍相对缺乏研究。随着灰色基础设施绿色化（Greening of Grey Infrastructure, GGI）或称生态工程技术在全球范围内的应用日益普及，明晰生境复杂性改造对这类高度人工化生境中的生物多样性与生态功能的影响，已成为至关重要的研究课题。本研究在三大洲的六个采样点开展实验，探究潮间带人工基底上的物理（原始基质）与/或生物成因生境（双壳类）复杂性的调控方式，如何影响滤水率、净初级生产力（Net Primary Productivity, NPP）与总初级生产力（Gross Primary Productivity, GPP）、群落呼吸作用（Community Respiration, CR），以及滤食性生物与大型藻类的丰度，还有隐秘底栖鱼类（cryptobenthic fish）的生境利用情况。本研究通过两种方案调控物理与生物成因复杂性：一是使用平面或脊状（高度为2.5 cm或5 cm）的附着基瓷砖；二是将瓷砖设置为未接种或接种牡蛎、贻贝两种状态。在所有采样点中，提升物理与生物成因复杂性（5 cm接种瓷砖）对多数生态功能变量均产生了显著影响：可提升瓷砖上群落的整体滤水率，但会降低所有采样点的初级生产力（包括GPP与NPP）。而提升任一类型的生境复杂性，对隐秘底栖鱼类的最大个体数（MaxN）、帧内总停留时间以及大型藻类盖度均无整体显著影响。各采样点内部的生境复杂性影响存在显著差异：在澳大利亚霍巴特，平面瓷砖上的GPP显著高于5 cm脊状瓷砖；在澳大利亚悉尼，接种瓷砖的滤水率与滤食性生物生物量均高于未接种瓷砖，且5 cm瓷砖上的隐秘底栖鱼类最大个体数（MaxN）高于平面瓷砖；而在爱尔兰都柏林，5 cm复杂结构瓷砖的滤水率显著高于平面瓷砖。本研究结果表明，通过提升生境复杂性实现的灰色基础设施绿色化方案，可能在潜在期望的多项生态功能之间存在权衡，例如初级生产力与滤水率，且对隐秘底栖鱼类群落的影响存在空间异质性。尤为重要的是，本研究结果显示，灰色基础设施绿色化实践的影响会因环境背景的不同而产生显著差异，因此不应在全球范围内盲目统一推行。