Science

Tearing the “Invisible Fabric” of Nature

Overfishing of cod may have pushed the northwest Atlantic marine ecosystem to "an alternative state." Photo credit: Brian Skerry/New England Ocean Odyssey

Overfishing of cod may have pushed the northwest Atlantic marine ecosystem to "an alternative state." Photo credit: Brian Skerry/New England Ocean Odyssey

A major recent study documents incidents of overfishing that pushed ecosystems beyond tipping points from which they could not rebound, “flipping” them into new states. The meta-study in the Proceedings of the National Academy of Sciences (subscription required) includes examples from around the world—from the Namibian coast and Nile Delta to right here in New England’s waters. In each, over-exploitation of resources triggered a domino effect in the food web, “fundamentally transforming ecosystems to those that are often less productive for fisheries, more prone to cycles of booms and busts, and thus less manageable.”

The authors call for a fishery management system that takes these ecosystem tipping points into account. “You don’t realize how interdependent species are until it all unravels,” said co-author Dr. Felicia Coleman in a press release from Florida State University.

Cod, kelp, and urchins

One case study in the PNAS paper focuses on the decline of Atlantic cod in the northwest Atlantic and the ripple effects this had on the marine ecosystem. The collapse of cod due to overfishing in the 1980s meant that many species that were once the prey for cod have now become “hyper-abundant” and compete with young cod, even eating them. The authors say this is creating “an alternative state of persistently low cod densities from which it will be difficult to escape.”

The loss of cod as a keystone predator continued to reverberate in other ways. Without cod feeding on them, sea urchins also increased in number. Japanese demand for urchin in the late 1980s sparked an urchin fishery in Maine, which overfished urchins into collapse within a decade. Urchins eat seaweed, and with those herbivores gone seaweed proliferated, “creating ideal settlement habitat for crabs.” These crabs have now become the main predators on sea urchins. “In areas of coastal Maine where urchins were extirpated, crab predation has precluded their [urchin] revival for nearly 20 years, even where urchin fishing is prohibited,” the study said. “It is not clear whether these changes could be reversed.”

The forage foundation

Another example comes from the Northern Benguela ecosystem off the southwestern African coast, where energy-rich forage species such as sardine and anchovy were overfished into collapse in the 1970s. This, in turn, had a profound effect on the region’s complex system of deep water mixing of nutrients and oxygen. The result was a persistent low-oxygen state in which other species such as the bearded goby and jellyfish thrived. These species offer far less energy for predators, and soon populations of hake and other fish, and seabirds such as gannet and penguins sharply declined. The authors describe this as “an alternate and potentially irreversible state.”

The “invisible fabric of nature”

The study’s authors say these examples illustrate the importance of the interaction among species, which they call “the invisible fabric of nature.” The influence of these interactions becomes known only after one or more of the species is removed or depleted, triggering “nonlinear dynamics and feedback loops” that produce sudden shifts in an ecosystem. The authors urge fisheries management to take a lesson from ecology and develop ways to recognize and react to these relationships among the species they manage by “developing a sharper focus on … how disrupting these interactions can push ecosystems in which fisheries are embedded past their tipping points.”

As lead author Professor Joseph Travis puts it, “It’s a lot easier to back up to avoid a tipping point than it is to find a way to return once you’ve crossed it.” That should by now be painfully obvious in New England where cod and many other fish have not recovered from depletion, in some instances for decades.


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