Fish and invertebrate communities show greater day/night partitioning on tropical than temperate reefs

Diel partitioning of animals within ecological communities is widely acknowledged, yet rarely quantified. Investigation of most ecological patterns and processes involves convenient daylight sampling, with little consideration of the contributions of nocturnal taxa, particularly in marine environments. Here we assess diel partitioning of reef faunal assemblages at a continental scale utilizing paired day and night visual census across 54 shallow tropical and temperate reefs around Australia. Day/night differences were most pronounced in the tropics, with fishes and invertebrates displaying distinct and opposing diel occupancy on coral reefs. Tropical reefs in daytime were occupied primarily by fishes not observed at night (64% of all species sighted across day and night, and 71% of all individuals). By night, substantial emergence of invertebrates not otherwise detected during sunlit hours occurred (56% of all species, and 45% of individuals). Nocturnal emergence of tropical invertebrates corresponded with significant declines in the richness and biomass of predatory and herbivorous diurnal fishes. In contrast, relatively small diel changes in fishes active on temperate reefs corresponded to limited nocturnal emergence of temperate invertebrates. This reduced partitioning may, at least in part, be a result of strong top-down pressures from fishes on invertebrate communities, either by predation or competitive interference. For shallow reefs, the diel cycle triggers distinct emergence and retreat of faunal assemblages and associated trophic patterns and processes, which otherwise go unnoticed during hours of regular scientific monitoring. Improved understanding of reef ecology, and management of reef ecosystems, requires greater consideration of nocturnal interactions. Without explicit sampling of nocturnal patterns and processes, we may be missing up to half of the story when assessing ecological interactions.

生态群落内动物的昼夜生态位分隔（diel partitioning）已得到广泛认可，但相关量化研究却极为匮乏。当前多数生态格局与过程的研究多采用便捷的日间采样方案，极少关注夜行性类群（nocturnal taxa）的贡献，在海洋环境中这一问题尤为突出。本研究针对澳大利亚周边54处浅海热带与温带礁体开展配对昼夜目视普查（visual census），以此在大陆尺度上评估珊瑚礁动物群落的昼夜生态位分隔情况。热带海域的昼夜差异最为显著，珊瑚礁上的鱼类与无脊椎动物呈现出截然不同且相互对立的昼夜栖息模式。日间热带礁体的栖息者以夜间未被观测到的鱼类为主——此类鱼类占昼夜总观测物种数的64%，占总个体数的71%。至夜间，日间光照时段未被检出的无脊椎动物大量出现，其物种数占昼夜总观测物种数的56%，个体数占总个体数的45%。热带无脊椎动物的夜间活动，与昼行性捕食鱼类和植食鱼类的物种丰富度及生物量显著下降存在关联。与之形成对比的是，温带礁体上活动的鱼类昼夜变化幅度相对较小，对应的温带无脊椎动物夜间出现量也较为有限。这种减弱的昼夜分隔现象，至少部分可归因于鱼类对无脊椎动物群落施加的强自上而下调控压力，包括捕食作用与竞争干扰两种途径。对于浅海礁体而言，昼夜循环会驱动动物群落发生显著的活动与栖息转移，同时改变相关的营养格局与生态过程——这些现象在常规日间科学监测中往往被忽略。要深化对珊瑚礁生态学的认知并完善礁体生态系统管理，需更加重视夜行性生物间的相互作用。若未对夜间生态格局与过程开展针对性采样，我们在评估生态相互作用时可能会错失多达一半的关键信息。