I wish I had lived hundreds of years ago, when the skies were black with birds and the waters bubbled with fish so thick you couldn’t swim. A lot has changed since then. Species aren’t what they used to be, they’ve changed size and shape, and we need to recognize and incorporate those changes if we hope to restore nature to the abundant days of old.
In the 1700’s, Cod shoals off the shores of eastern Canada were reportedly “so thick by the shore that we hardly have been able to row a boat through them.” And huge too. Fish seven-feet long and 200 pounds were reported. The dense masses of fish provided a plentiful bounty for centuries and built a provincial economy and culture. But in the early 1990’s, the fishery came crashing down, shattered like the fabled Humpty Dumpty, and regulators and fishers alike are struggling to put it back together again.
Today, three decades after enacting fishing moratoria, there is still little sign of recovery. Why? Why, after removing the initial disturbance - overfishing - haven’t the fish returned? It’s a common conservation problem. We disturb an ecosystem, realize our error, and then try to rebuild what we once had. But we fail. Why? We need to understand our failures if we want to stop repeating our mistakes.
In our recent paper, we provide a possible explanation. We suggest that restoring a shattered ecosystem is like piecing together a complex puzzle. Each puzzle piece is a species, a piece’s size is the number of individuals of the species, and the piece’s shape is the set of species’ traits, like length or coloration. But, unlike a conventional puzzle, the size and shape of the pieces are always changing, such that the puzzle might not go together tomorrow like it did today. So, simply removing an initial disturbance, like overfishing, often fails to restore an ecosystem because the ecosystem isn’t what it used to be. Our concept modernizes what biologists previously referred to as the Humpty-Dumpty effect, and it might explain the Cod conundrum in a few key ways.
First, a once small piece of the puzzle – the grey seal population – might now be too big to allow a large cod population to fit back into the ecosystem. A single adult seal can eat up to 4000 pounds of Cod per year. That’s nearly five times more Cod than Dwayne ‘The Rock’ Johnson eats in a year, and about 650 times more Cod than my family eats. Despite this voracious appetite, seals had little effect on Cod populations throughout the 20th century because they were hunted to near extinction in the early 1900’s. Since 1980, however, the size of the grey seal population, and therefore their Cod consumption, has been increasing exponentially. By 2010, grey seals were likely consuming seven times more fish than commercial fishers were reeling in before the fishing moratorium.
So, why after removing the initial disturbance - overfishing – aren’t the Cod recovering more quickly? In part because overfishing hasn’t stopped at all. The culprit just changed from humans to grey seals.
But seals have been here for hundreds of years, even back when the Cod populations were thick enough to walk across. So, maybe seal abundance is not the only piece of the puzzle that changed. Rapid evolution might also be changing the shape of the Cod population, further slowing their recovery.
Rapid evolution is a genetic change in a species’ traits that occurs over just a few generations, and it is very common in nature. For example, a single hurricane caused the evolution of grip strength in lizards on a Caribbean island, and rampant ivory poaching during a 15-year civil war in Mozambique caused Elephants to evolve tusklessness. It’s a phenomenon that’s revolutionizing ecology, and it might be critical to understanding the story with Cod.
Fishing is one of the strongest human-induced pressures causing rapid evolution in wild populations. For example, targeting big fish during fishing, and therefore removing the biggest fish from the population, can cause the rapid evolution of smaller body size. But, we likely cause evolution even without preferring specific fish. When fishing pressure is high, it is advantageous for a fish to mature quickly to increase the likelihood of reproducing before getting caught. Fish that mature quickly devote less energy to growth, which makes them smaller at any given age. So, fishing often results in the evolution of smaller, quickly maturing fish, even if we let the big ones get away.
But, maturing quickly and at a small size is a major disadvantage for a population recovering from a disturbance like overfishing. Small fish are more susceptible to predators and have less offspring with lower survival. More predation and less babies mean slower increases in population size over time. So, a fish population that evolves in response to overfishing will likely recover much more slowly than if it hadn’t evolved at all.
Cod off the coast of Canada show potential signs of fisheries-induced evolution: they now mature earlier and are 20% smaller than they were before the impacts of overfishing. Reduction in the age and size at maturity of Cod is likely slowing their recovery, but experts debate whether the changes are due to evolution or whether Cod will quickly re-evolve their old ways. Nonetheless, this high-profile example has highlighted how rapid evolution might complicate conservation puzzles, and explain why simply removing an initial disturbance might not recover a shattered ecosystem.
The Cod puzzle is another example of why we need detailed natural history research to effectively rebuild shattered ecosystems, which was the lesson learned from the story of the Island Fox in my last post of this series. But, we also need targeted, long-term monitoring programs to better understand how the pieces of conservation puzzles are changing. Without such monitoring off the shores of eastern Canada, we’d be baffled by the lack of Cod recovery, not knowing that the puzzle pieces had changed size and shape. Worse, we would likely be wasting limited conservation resources on ineffective management, repeating our past mistakes, and perpetuating distrust among opposing sides of important conservation debates.
If we want to get back to the abundant days of old, we need to recognize the ever-shifting nature of ecosystems. We need to work with the puzzle pieces we’ve created, or reshape the pieces to rebuild what we once had. It’s a framework that can increase conservation success, especially under the complexities of climate change, which will be the subject of my final post in this series.
Learn more about the Humpty-Dumpty effect, interesting case studies, and its importance in conservation in our paper “An Eco-evolutionary Perspective on the Humpty-Dumpty Effect and Community Restoration”. Get notified about new posts in this series and others by subscribing here.
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