The “extinction vortex” is all too familiar in the world of endangered species: Extremely small populations often become vulnerable to new threats, precipitating a downward spiral toward extinction. But in a world of limited resources, I wonder about society’s obligation to intervene where there is little chance for reversing downward trends. A case in point is the critically endangered monk seal, which is declining toward extinction in one of the most pristine marine wilderness areas on earth.
The Papahānaumokuākea Marine National Monument in the northwest Hawaiian Islands is the largest marine protected area in the US. The monument, established in 2006 by President George W. Bush, has been heralded as a conservation success and the reserve teems with a diversity of marine organisms.
With this intact ecosystem has come the recovery of top predators—sharks. But in an unintended consequence, shark predation has been implicated as a primary cause of the decline of the monk seal.
Most of the remaining 1,000 Hawaiian monk seals live in the monument, and their numbers are in precipitous decline. Their populations became so small that sex ratios skewed towards males, creating social dysfunction and male aggression. That aggression seemed to peak in 1997, which was also the first time researchers observed sharks eating an intriguing new source of food that they continue to rely on today—drowned seal pups in the water.
Should humans intervene to reverse this extinction vortex and to help monk seals recover within their “restored” habitat? This raises a clear question about “conservation triage”: Do we want an ecosystem with a viable population of monk seals, or do we want to maintain the virtues of having a marine ecosystem with a full complement of apex predators?
In its answer to that question, the National Marine Fisheries Service is proposing to cull Galapagos sharks in areas of the monument with a particular problem, such as the French Frigate Shoals. The rationale is that removing a few “nuisance” sharks who have learned to hunt for monk seals will reduce early mortality of monk seals, which is very high in their first two years of life. The hope is that this would stabilize monk seal populations—and still maintain a viable shark population and a diverse community structure within the monument.
Is it scientifically and ethically defensible to kill a few sharks to save monk seals from extinction?
Without intervention, monk seals will likely become extinct before my children grow up to see them. On the other hand, there is no guarantee that culling sharks will lead to the seals’ recovery. A sustained program to remove sharks likely will reduce their take in localized areas, perhaps enough for juvenile monk seals to survive. But, there also is a chance that culling will hurt the viability of the shark population, which is already suffering globally through bycatch.
While scientists hypothesize that only a few sharks—those “nuisance animals”—are eating monk seals, supporting data are sparse, and other species such as tiger sharks are known to prey upon monk seals. This build-up of predators in the Northwestern Hawaiian Islands and the complexity of the ecosystem may make controlling shark predation impractical.
A careful evaluation of the costs and benefits of different management options (e.g., shark predation mitigation, translocation or captive breeding of monk seals) is needed to identify the greatest likelihood of success given societal preferences, logistical, and financial constraints.
As a society, are we prepared to accept that our proverbial Noah’s ark is perhaps not large enough to bring all of the world’s species into the future of our children? More importantly, do we have the proper ethical code in place to justly triage the planet’s declining biodiversity?
