Predation, global change, and natural resource conflict
Drought Increases Consumer Pressure on Oyster Reefs
While predation can help maintain the structure of ecological communities, sudden environmental shifts and/or the harvesting of higher order predators can release other species that consume and deplete important habitat-forming or foundation species (e.g., trees, coral, oysters). My graduate student (Hanna Tillotson) and I examined whether outbreaks of predators explained a recent loss of commercially important oyster reefs on the Atlantic coast of Florida, USA. Using the comparative-experimental approach, we found that an outbreak of predatory marine snails (crown conch) was the proximal cause of oyster losses. After uncovering that snails were the proximal cause of oyster loss, we tested whether changing water salinity was the ultimate cause of the oyster declines by controlling the growth and survival of the snails and/or by controlling the distribution of higher-order predators that consume the snails. When we conducted these field experiments across a salinity gradient, we failed to detect spatial variation in predation on snails or in snail growth and survival. However, in laboratory experiments, we determined the role of salinity by showing that snail larvae failed to survive at low salinities. Because this estuary’s salinity increased in 2006 in response to reduced inputs of freshwater, we concluded that the ultimate cause of oyster decline was a drought-induced increase in salinity.
Crown conch (Melongena corona) eating an oyster.
Map of study sites in the estuary.
Garland, H. G., D. L. Kimbro. 2015. Drought Increases Consumer Pressure on Oyster Reefs in Florida, USA. PLoS ONE 10(8): e0125095.
The Dynamics of Open Populations
I previously used relatively short-term field experiments to show that the loss of native oysters from an estuary in California and failure of oyster restoration efforts may be due to the biological invasion of predatory crabs and snails. But many other processes that affect the growth, survival and recruitment of these oysters may have also been in play or even more important. The continual challenge for ecologists has been the difficulty of distinguishing the relative importance of effects that are merely statistically significant from those that are of greatest biological importance. My colleague (Dr. Will White, Oregon State University) and graduate mentor (Dr. Ted Grosholz, UC Davis) took a novel approach to understanding the relative contributions of top-down, bottom-up (i.e., food delivery) and supply-side (i.e., recruitment) forces in a coastal ecosystem that hybridizes experimental field studies, long-term surveys and Integral Projection Models (IPMs). Specifically, we used data from the well-studied system involving Olympia oysters in a California estuary, where experimental and observational studies have documented strong influences of predation (top-down), primary production (bottom-up) and larval recruitment (supply-side) on population dynamics. We used these experimental results together with a ten year time-series of size-abundance and recruitment data in a state space model with a model selection approach that quantified the temporal and spatial scales of variation for each factor. Our results produced a synthetic understanding of the relative importance of these factors beyond that which could be obtained by field observation and experimentation alone.
Figure 1. Schematic of study sites in Tomales Bay, CA and the multiple gradients of top-down (TD), bottom-up (BU), and supply-side (SS) factors that may cause variation in oyster population dynamics over space and time.
Kimbro, D. L., J. W. White, E. D. Grosholz. 2018. The dynamics of open populations: integration of top-down, bottom-up and supply-side influences on intertidal oysters. Oikos.
The application of our predation research approach to resolve natural resource conflict
Building upon this research, a multi-university team and I evaluated whether predator outbreaks can help explain the collapse of Florida’s largest oyster fishery in Apalachicola Bay, which was declared a Federal Fisheries Disaster in 2013. From 2013–2017, we used the comparative-experimental approach and a mathematical moding approach desribed above to disentangle the relative roles of harvesting pressure, upstream freshwater removal by the State of Georgia that increases the salinity of Apalachicola Bay, and the increasing frequency of natural drought. We found that oyster harvesting has remained constant over the last 30 years and that natural drought has recently allowed predators to deplete oysters. However, this runaway consumption of oysters by predators was greater in Apalachicola Bay than in nearby estuaries with the same local meteorological conditions, because of the upstream removal of freshwater and increasing salinity beyond that due to natural environmental variation alone. This research was part of interstate litigation before the Supreme Court of the United States, and our findings were supported by the Court. Our lab’s study of predation demonstrates how basic ecological research on predation can be used to help resolve conflict over natural resources in a period of global change.
Five drills attacking an oyster.
Figure 1. (A) Map of study sites along the Florida panhandle. Apalachicola Bay and Ochlockonee Bay are highlighted in red. Shading distinguishes the watersheds of the Apalachicola River (Dark gray) and the Ochlockonee River (Darker gray). Map of Apalachicola Bay (B) and Ochlockonee Bay (C). In Apalachicola, dark shading illustrates distribution of oyster reefs. In both estuaries, concentric circles illustrate proportional distances (close, mid, far) of oyster reefs from river discharge. In Apalachicola, proportional distances extend West (W) and East (E) of the river. (D) Time series of weekly mean salinity and weekly salinity anomaly at Cat Point oyster bar in Apalachicola Bay from 1992–2016. Anomaly was calculated based on the climatological mean from 1992-2002; data obtained from Apalachicola National Estuarine Research Reserve ([http://cdmo.baruch.sc.edu)/]http://cdmo.baruch.sc.edu).