Extremely high abundances or outbreaks of predators can decimate economically and ecologically important prey populations. Such outbreaks will become more common as global climate change and food-web modifications increasingly facilitate predators and stress prey. Natural systems are organized hierarchically, so outbreaks may be influenced by both large-scale stressors (e.g., drought) and local-scale stressors (e.g., overfishing). Because drivers acting at different scales require different management responses, predator outbreaks must be investigated in a manner that can disentangle the effects of regional- and local-scale forcing. Here, we repeatedly conducted paired experiments and monitoring for four years in two different bays and evaluated long-term records of precipitation as well as river flow to investigate whether the collapse of a nationally important oyster fishery in the Gulf of Mexico (Apalachicola Bay, Florida) was (1) caused proximally by a predator outbreak and (2) whether this outbreak was mediated by regional- and/or local-scale forces.
During the oyster fishery collapse, we found elevated water salinity, high abundance of predatory snails (drills), and massive oyster mortality due to predation. By repeating these experiments for 4 years, we found that periods of reduced water salinity inhibited predation on oysters. To parse the influence of regional versus local factors on drill outbreaks, we simultaneously replicated the paired experiments and monitoring in a nearby bay (Ochlockonee Bay) that has shared the same regional-scale rainfall conditions as Apalachicola Bay over the past 30 years. However, 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. Thus, Apalachicola Bay experienced an additional, localized anthropogenic stress. In Ochlockonee Bay, our experiments demonstrated that the river maintained sufficiently low water salinity to provide ~ 40% of oyster reefs with a refuge from predation. However in Apalachicola Bay, salinity-dependent predation extended in breadth over the estuary, up to the river mouth. Given that massive upstream water withdrawal has occurred in the Apalachicola watershed (but not Ochlockonee), it is reasonable to surmise that these withdrawals exacerbated the stress of the regional drought, created the difference in predation between the two bays, and thus may have precipitated the oyster fishery collapse.