Data from: Regime shifts shorten food chains for mesopredators with potential sublethal effects

1. Predator populations are in decline globally. Exploitation, as well as habitat degradation and associated changes in prey availability are key drivers of this process of trophic downgrading. In the short term, longevity and dietary adaptability of large-bodied consumers can mask potential sub-lethal effects of a changing prey base, producing a delayed effect that may be difficult to detect. 2. In coral reef ecosystems, regime shifts from coral- to algae-dominated states caused by coral bleaching significantly alter the assemblage of small-bodied reef fish associated with a reef. The effects of this changing prey community on reef-associated mesopredators remains poorly understood. 3. This study found that the total diversity, abundance and biomass of piscivorous mesopredators was lower on regime-shifted reefs than recovering reefs, 16 years after the 1998 mass coral bleaching event. 4. We used stable isotope analyses to test for habitat-driven changes in the trophic niche occupied by a key piscivorous fishery target species on reefs that had regime-shifted or recovered following climatic disturbance. Using morphometric indices, histology, and lipid analyses, we also investigated whether there were sub-lethal costs for fish on regime-shifted reefs. 5. Stable isotopes demonstrated that fish from regime-shifted reefs fed further down the food chain, compared to recovering reefs. Lower densities of hepatocyte vacuoles in fish from regime-shifted reefs, and reduced lipid concentrations in spawning females from these reefs, indicated a reduction in energy stores, constituting a sub-lethal and potential delayed effect on populations. 6. Reduced energy reserves in mesopredators could lead to energy allocation trade-offs, and decreased growth rates, fecundity, and survivorship, resulting in potential population declines in the longer term.

1. 全球范围内捕食者种群均呈衰退趋势。过度开发、生境退化以及随之而来的猎物可获得性变化，是引发这一营养降级（trophic downgrading）过程的关键驱动因素。从短期来看，大型消费者的寿命与饮食适应性，能够掩盖猎物基础变化所带来的潜在亚致死效应，进而产生难以被及时察觉的延迟影响。
2. 在珊瑚礁生态系统中，由珊瑚白化引发的珊瑚向藻类主导状态的稳态转换（regime shifts），会显著改变与珊瑚礁相关联的小型礁鱼群落组成。目前学界对这类猎物群落的变化会对礁栖中型捕食者产生何种影响仍知之甚少。
3. 本研究发现，在1998年大规模珊瑚白化事件发生16年后，经历稳态转换的珊瑚礁上，食鱼中型捕食者的总多样性、丰度与生物量均低于正在恢复的珊瑚礁。
4. 本研究借助稳定同位素分析（stable isotope analyses），针对经历气候扰动后发生稳态转换或已恢复的珊瑚礁上一种关键的食鱼性渔业目标物种所占据的营养生态位，检验其是否存在生境驱动的变化。此外，我们还通过形态计量学指标、组织学分析以及脂质分析（lipid analyses），探究了稳态转换珊瑚礁上的鱼类是否存在亚致死代价。
5. 稳定同位素分析结果显示，相较于恢复中的珊瑚礁，来自稳态转换礁区的鱼类在食物链中的取食层级更低。稳态转换礁区鱼类的肝细胞空泡（hepatocyte vacuoles）密度更低，且该礁区产卵雌鱼的脂质浓度也有所下降，这表明其能量储备出现减少，印证了种群所面临的亚致死且可能存在延迟的影响。
6. 中型捕食者的能量储备减少，可能会引发能量分配权衡，并导致生长速率、繁殖力与存活率下降，进而在长期内造成种群衰退的潜在风险。