A large net containing multiple European green crabs is examined at a shellfish farm in Willapa Bay, Washington.
Credit: Abby Keller
As ecosystems around the planet undergo rapid transformations, policymakers and land managers are grappling with a pressing question: When is it time to stop resisting ecological changes and accept a new normal?
A new study published in the Journal of Applied Ecology may offer decision-makers guidance and support in navigating the increasingly difficult—if not impossible—task of conserving historical ecological baseline conditions.
"The decision of when to resist and when to accept involves a lot of uncertainty," stressed lead author Abby Keller, a Ph.D. candidate in the Department of Environmental Science, Policy, and Management at UC Berkeley. "Luckily, we can use mathematical concepts from decision theory to support rational decision-making under uncertainty."
The study authors—who include Associate Professor Carl Boettiger—examined efforts to manage the European green crab, an invasive marine species that has spread to every continent except Antarctica.
Green crabs are an extremely difficult species to control, Keller said, as their larvae develop in open waters for several months and are capable of traveling hundreds of kilometers across the ocean. And even if adult crabs are physically removed from a bay, a deluge of new larvae can rapidly replenish their numbers.
Keller likened this to "trying to remove water from a lake that is replenished by a rain-fed mountain stream, except the amount of water flowing into the lake is only increasing."
Once introduced, green crabs aggressively prey on juvenile shellfish and fish species that are endemic to the region; destroy eelgrass beds that provide critical habitat for salmon; and threaten commercial fishery operations. There has been no documented successful example of green crab eradication, and Keller notes that the species has begun spreading through the northeast Pacific Ocean as climate change drives an increase in temperatures.
Prioritizing management decisions is difficult, given the species' potentially wide footprint and the need to strategically allocate resources to locations where resistance is most feasible. "But the outcomes of removal actions are difficult to predict as the biological dynamics are random and uncertain," Keller said.
Credit: Laura Kraft
To determine how to minimize the ecological impact and economic cost of a green crab invasion on affected ecosystems, the authors used stochastic dynamic programming—an optimization technique used to solve decision-making under uncertainty over time—across four distinct scenarios where the crab population responds differently to removal.
"Our study should provide some clarity about whether the green crab population is at a point where you could continue to act and see positive results, or if the abundance across the broader region has grown so much that it's best to focus on adapting to make the ecosystem more resilient," Keller said. "You don't want to be doing something that's wasteful or futile."
In ecosystems where green crab populations are isolated within individual bays, the study found that decision-makers could continue to see positive outcomes by removing crabs from marine areas even as the population grows more dense.
As the populations of crabs across ecosystems grow increasingly more connected, the authors show, it becomes more difficult for decision-makers to rationalize the cost of intervention. "Even if your area has a relatively low density of green crabs, the breeding and dispersal of crabs and larvae from connected areas might be an insurmountable challenge to overcome in the future," Keller said.
As the costs of resistance continue to scale, Keller and her co-authors hope their findings could help ecosystem managers prioritize the type of targeted interventions that offer the most bang for their buck.
"We're entering a new era of ecosystem management where we have to explore decision frameworks that move beyond conserving a historical baseline," said Boettiger. "We hope our study can enable the type of informed, strategic decisions needed to navigate a changing world."
To determine how to minimize the ecological impact and economic cost of a green crab invasion on affected ecosystems, the authors used stochastic dynamic programming—an optimization technique used to solve decision-making under uncertainty over time—across four distinct scenarios where the crab population responds differently to removal.
"Our study should provide some clarity about whether the green crab population is at a point where you could continue to act and see positive results, or if the abundance across the broader region has grown so much that it's best to focus on adapting to make the ecosystem more resilient," Keller said. "You don't want to be doing something that's wasteful or futile."
In ecosystems where green crab populations are isolated within individual bays, the study found that decision-makers could continue to see positive outcomes by removing crabs from marine areas even as the population grows more dense.
As the populations of crabs across ecosystems grow increasingly more connected, the authors show, it becomes more difficult for decision-makers to rationalize the cost of intervention. "Even if your area has a relatively low density of green crabs, the breeding and dispersal of crabs and larvae from connected areas might be an insurmountable challenge to overcome in the future," Keller said.
As the costs of resistance continue to scale, Keller and her co-authors hope their findings could help ecosystem managers prioritize the type of targeted interventions that offer the most bang for their buck.
"We're entering a new era of ecosystem management where we have to explore decision frameworks that move beyond conserving a historical baseline," said Boettiger. "We hope our study can enable the type of informed, strategic decisions needed to navigate a changing world."
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