Table_2_Local Human Impacts Disrupt Relationships Between Benthic Reef Assemblages and Environmental Predictors.DOCX

Human activities are changing ecosystems at an unprecedented rate, yet large-scale studies into how local human impacts alter natural systems and interact with other aspects of global change are still lacking. Here we provide empirical evidence that local human impacts fundamentally alter relationships between ecological communities and environmental drivers. Using tropical coral reefs as a study system, we investigated the influence of contrasting levels of local human impact using a spatially extensive dataset spanning 62 outer reefs around inhabited Pacific islands. We tested how local human impacts (low versus high determined using a threshold of 25 people km−2 reef) affected benthic community (i) structure, and (ii) relationships with environmental predictors using pre-defined models and model selection tools. Data on reef depth, benthic assemblages, and herbivorous fish communities were collected from field surveys. Additional data on thermal stress, storm exposure, and market gravity (a function of human population size and reef accessibility) were extracted from public repositories. Findings revealed that reefs subject to high local human impact were characterised by relatively more turf algae (>10% higher mean absolute coverage) and lower live coral cover (9% less mean absolute coverage) than reefs subject to low local human impact, but had similar macroalgal cover and coral morphological composition. Models based on spatio-physical predictors were significantly more accurate in explaining the variation of benthic assemblages at sites with low (mean adjusted-R2 = 0.35) rather than high local human impact, where relationships became much weaker (mean adjusted-R2 = 0.10). Model selection procedures also identified a distinct shift in the relative importance of different herbivorous fish functional groups in explaining benthic communities depending on the local human impact level. These results demonstrate that local human impacts alter natural systems and indicate that projecting climate change impacts may be particularly challenging at reefs close to higher human populations, where dependency and pressure on ecosystem services are highest.

人类活动正以史无前例的速率改变着生态系统，但目前仍缺乏针对局地人类影响如何改变自然系统、并与全球变化其他维度相互作用的大规模研究。本研究提供实证证据，表明局地人类影响从根本上改变了生态群落（ecological communities）与环境驱动因子（environmental drivers）之间的关联关系。本研究以热带珊瑚礁（tropical coral reefs）为研究对象，依托覆盖太平洋有人居住岛屿周边62个外礁的大范围空间数据集，探究了不同强度局地人类影响的效应。我们采用预设模型与模型选择工具，检验了局地人类影响（以每平方公里礁域25人为阈值划分为低、高影响两组）如何影响（i）底栖生物群落（benthic community）结构，以及（ii）底栖生物群落与环境预测因子之间的关联关系。研究通过野外调查获取了礁体水深、底栖生物群落组成以及植食性鱼类（herbivorous fish）群落的相关数据。此外，我们从公共数据库中提取了热应力（thermal stress）、风暴暴露度（storm exposure）以及市场引力（market gravity，由人口规模与珊瑚礁可达性共同决定）的相关数据。研究结果显示，相较于低影响组礁体，受强局地人类影响的礁体以更高占比的藻席藻类（turf algae，平均绝对盖度高出10%以上）与更低的活珊瑚盖度（live coral cover，平均绝对盖度低9%）为特征，但二者的大型藻类盖度（macroalgal cover）与珊瑚形态组成并无显著差异。基于空间-物理预测因子（spatio-physical predictors）构建的模型，在低局地人类影响站点中对底栖生物群落变异的解释精度显著更高（平均校正决定系数adjusted-R²=0.35）；而在高影响站点中，底栖生物群落与环境因子的关联大幅减弱，模型解释精度仅为平均0.10的校正决定系数。模型选择流程还发现，不同植食性鱼类功能群在解释底栖生物群落时的相对重要性，会随局地人类影响强度发生显著变化。上述结果证实了局地人类影响会改变自然系统，并表明在人类依赖与生态系统服务（ecosystem services）压力均处于高位的人口密集区域周边礁体，预测气候变化影响的难度将尤其突出。