Data from: Local and regional stressors interact to drive a salinization-induced outbreak of predators on oyster reefs

Predator outbreaks are predicted to increasingly decimate economically and ecologically important prey populations because global climate change and food-web modifications frequently facilitate predators and stress prey. Natural systems are organized hierarchically, with processes operating at multiple scales giving rise to patterns of biodiversity, so predicting and managing outbreaks requires a framework that accounts for the effects of both local and regional stressors. Here, we used the comparative experimental approach to investigate whether the collapse of a nationally important oyster fishery in the Gulf of Mexico (Apalachicola Bay, Florida) could have been (1) caused proximally by a predator outbreak and (2) whether this outbreak was mediated by local- and/or regional-scale forces. During the fishery collapse, we paired experiments with monitoring in Apalachicola Bay and found elevated water salinity, high abundance of predatory snails, and intense oyster mortality due to predation. By repeating these experiments over 4 yr, we found that periods of reduced water salinity inhibited predation on oysters. To partition the influence of local-versus-regional factors on this predator outbreak, we simultaneously replicated the paired experiments and monitoring in a nearby bay (Ochlockonee Bay) that shares the same regional-scale rainfall conditions. Increasing freshwater withdrawals from the watershed that drains into Apalachicola Bay have increased salinities in that bay, but there have not been similar withdrawals in the Ochlockonee Bay watershed. Therefore, Apalachicola Bay experienced a localized anthropogenic stress, while both bays experienced regional stress from drought. In Ochlockonee Bay, our experiments demonstrated that the river maintained sufficiently low salinity to provide ~50% of oyster reefs with a refuge from predation. In contrast, salinity-dependent predation in Apalachicola Bay extended up to the river mouth. Given the stark differences in upstream water withdrawals between these watersheds, it is reasonable to surmise that these withdrawals exacerbated the stress of regional drought, created the difference in predation between the two bays, and thus may have precipitated the oyster fishery collapse. Our study provides empirical support for recent theory about the hierarchical organization of ecosystems, which predicts that stressors will interact across scales to cause localized predator outbreaks.

据预测，捕食者暴发（predator outbreaks）事件将愈发频繁地大量毁灭兼具经济与生态重要性的猎物种群。究其原因，全球气候变化与食物网改造常可助力捕食者种群、胁迫猎物种群。自然生态系统呈层级化组织架构，多尺度运作的生态过程共同塑造了生物多样性（biodiversity）格局，因此对捕食者暴发进行预测与管理，需构建能够同时考量局地与区域胁迫因子影响的研究框架。本研究采用对照实验法（comparative experimental approach），旨在探究墨西哥湾佛罗里达州阿巴拉契科拉湾（Apalachicola Bay）的国家级重要牡蛎渔业（oyster fishery）崩溃是否（1）由捕食者暴发直接引发，以及（2）该暴发是否由局地或区域尺度的生态作用力所介导。在该渔业崩溃阶段，我们于阿巴拉契科拉湾同步开展实验与监测，发现该海域水体盐度（salinity）升高、捕食性螺类丰度显著偏高，牡蛎因捕食出现了极高的死亡率。通过在4年时间内重复开展上述实验，我们发现水体盐度降低的时段可有效抑制牡蛎遭受的捕食压力。为厘清局地与区域因子对此次捕食者暴发的影响，我们在邻近的奥克洛科尼湾（Ochlockonee Bay）同步重复了上述实验与监测工作——该海湾与阿巴拉契科拉湾具有相同的区域尺度降雨条件。汇入阿巴拉契科拉湾的流域内，持续增加的淡水取水量推高了该海湾的水体盐度；而奥克洛科尼湾流域并未出现类似的淡水取水行为。因此，阿巴拉契科拉湾遭受了局地人为胁迫，而两个海湾均面临由干旱引发的区域尺度胁迫。在奥克洛科尼湾，实验结果表明，河流输入维持了足够低的盐度，为约50%的牡蛎礁（oyster reefs）提供了躲避捕食的庇护所。与之相反，阿巴拉契科拉湾的盐度依赖型捕食压力甚至延伸至河口（river mouth）区域。鉴于两个流域的上游取水行为存在显著差异，我们可以合理推测：此类取水行为加剧了区域干旱带来的胁迫，造成了两个海湾间捕食压力的差异，进而可能促成了该牡蛎渔业的崩溃。本研究为近期提出的生态系统层级组织理论提供了实证支撑——该理论预测，胁迫因子可通过跨尺度交互作用引发局地捕食者暴发事件。