The role of a dominant predator in shaping biodiversity over space and time in a marine ecosystem

1. Exploitation of living marine resources has resulted in major changes to populations of targeted species and functional groups of large-bodied species in the ocean. However, the effects of overfishing and collapse of large top predators on the broad-scale biodiversity of oceanic ecosystems remain largely unexplored. 2. Populations of the Atlantic cod (Gadus morhua) were overfished and several collapsed in the early 1990s across Atlantic Canada, providing a unique opportunity to study potential ecosystem-level effects of the reduction of a dominant predator on fish biodiversity, and to identify how such effects might interact with other environmental factors, such as changes in climate, over time. 3. We combined causal modelling with model selection and multimodel inference to analyse 41 years of fishery-independent survey data (1970–2010) and quantify ecosystem-level effects of overfishing and climate variation on the biodiversity of fishes across a broad area (172 000 km2) of the Scotian Shelf. 4. We found that alpha and beta diversity increased with decreases in cod occurrence; fish communities were less homogeneous and more variable in systems where cod no longer dominated. These effects were most pronounced in the colder north-eastern parts of the Scotian Shelf. 5. Our results provide strong evidence that intensive harvesting (and collapse) of marine apex predators can have large impacts on biodiversity, with far-reaching consequences for ecological stability across an entire ecosystem.

1. 海洋生物资源开发已对海洋中目标物种及大型物种功能群的种群数量造成显著改变。然而，过度捕捞与大型顶级捕食者种群崩溃对海洋生态系统大范围生物多样性的影响，目前仍未得到充分探究。2. 20世纪90年代初，加拿大大西洋海域的大西洋鳕鱼（Gadus morhua）种群遭过度捕捞，多个种群发生崩溃，这为研究优势捕食者种群缩减对鱼类生物多样性的潜在生态系统级效应，以及探究此类效应如何随时间与气候变化等其他环境因素产生交互作用，提供了独特的研究契机。3. 本研究将因果建模（causal modelling）与模型选择、多模型推断相结合，对1970年至2010年间共41年的渔业独立调查数据进行分析，量化了斯科舍陆架（Scotian Shelf）17.2万平方公里广阔海域内，过度捕捞与气候变化对鱼类生物多样性的生态系统级效应。4. 研究发现，随着鳕鱼出现率下降，α多样性（alpha diversity）与β多样性（beta diversity）均有所提升；在鳕鱼不再占据优势的生态系统中，鱼类群落的同质性更低、变异性更强。此类效应在斯科舍陆架东北部的寒冷海域最为显著。5. 本研究结果提供了有力证据，表明海洋顶级捕食者（marine apex predators）的高强度捕捞（及其种群崩溃）会对生物多样性造成显著影响，并对整个生态系统的生态稳定性产生深远影响。