Rosenstiel School of Marine and Atmospheric Science, Department of Marine Ecosystems and Society, University of Miami, Miami, FL, USA.
Leonard and Jayne Abess Center for Ecosystem Science and Policy, University of Miami, Coral Gables, Coral Gables, FL, USA.
Sci Rep. 2019 Feb 13;9(1):1908. doi: 10.1038/s41598-018-37576-6.
Despite global declines of apex predatory sharks, evidence for ecosystem consequences remains limited and debated. This is likely a result of both the logistical difficulties of measuring such processes in marine systems and also due to shifting baselines, whereby the ecosystem changes have occurred prior to monitoring. Between 2000-2018, we conducted standardized monitoring of white shark (Carcharodon carcharias) abundance patterns (N = 6,333 shark sightings) and predatory activity (N = 8,076 attacks on seals) at Seal Island, a Cape fur seal (Arctocephalus pusillus pusillus) colony in False Bay, South Africa. Over the 18-year study, declines in white shark abundance and attack rates were documented between 2015-2018, with anomalous lows occurring in 2017 and 2018. This included prolonged periods of complete white shark absence from Seal Island. The disappearance of white sharks from Seal Island coincided with the unprecedented appearance of sevengill sharks (Notorynchus cepedianus; N = 120 sightings), an otherwise allopatric kelp-associated apex predator in False Bay. We also recorded a sevengill shark attacking a live seal in the absence of white sharks. These data provide empirical evidence for behavioral shifts in an allopatric marine predator following the decline and disappearance of white sharks from a foraging site. This study demonstrates the importance of historical data and long-term monitoring for disentangling ecological consequences of apex predator declines.
尽管全球顶级掠食性鲨鱼的数量有所下降，但仍缺乏有关生态系统后果的证据，并且这一问题仍存在争议。这可能是由于在海洋系统中测量这些过程具有后勤方面的困难，也可能是由于基线变化，即生态系统的变化发生在监测之前。在 2000 年至 2018 年期间，我们在南非 False 湾的 Cape fur 海豹（Arctocephalus pusillus pusillus）聚居地 Seal Island 对大白鲨（Carcharodon carcharias）数量模式（N=6333 次鲨鱼目击事件）和捕食活动（N=8076 次海豹攻击事件）进行了标准化监测。在 18 年的研究中，记录到 2015 年至 2018 年期间大白鲨数量和攻击率下降，2017 年和 2018 年出现异常低值。这包括大白鲨长时间完全从 Seal Island 消失。大白鲨从 Seal Island 消失恰逢七鳃鲨（Notorynchus cepedianus；N=120 次目击事件）的空前出现，七鳃鲨是 False 湾中另一种同域的、与巨藻有关的顶级掠食者。我们还记录到在没有大白鲨的情况下，一条七鳃鲨攻击了一只活海豹。这些数据提供了有关在远离觅食地的地方，一种同域海洋捕食者的行为发生转变的经验证据。本研究表明，历史数据和长期监测对于理清顶级掠食者减少对生态系统的影响具有重要意义。
