If this conclusion, held alongside the decades-old consensus that top- down control is unimportant in the oceans, looks like nothing short of a paradigm shift in marine science, what seems a bit mystifying is the creepingly slow conceptual migration that preceded it—the half-century it took for theories of trophic cascade to move from small ponds out to the open ocean. It's especially puzzling because, in truth, there were revealing data floating around all along—albeit in rather obscure places—and the relevant theory, too, was already in circulation. As early as the late seventies, for instance, a South African marine biologist had suggested that shark-exclusion nets around Durban's beaches had killed so many large sharks that their prey were proliferating. And in the mid-eighties, a group of mathematical ecologists, meeting to talk about fisheries, asserted that managers ought to pay more attention to threats posed by trophic cascade.
So why did it take so long for the idea to take hold? It's possible that the greatest intellectual success of late-twentieth century oceanography incidentally confounded thinking about top-down control. Beginning in the late-eighties, oceanographers realized that every few decades or so, entire ocean basins—the North Pacific or the North Atlantic, for instance—experience sudden shifts in their patterns of circulation, drastically changing physical conditions for the entire region. Oceanographers called these events "decadal switches" or "regime shifts." Unfortunately, ecologists had recently been using the term "regime shift" to describe what happens when you extirpate a top-predator and the effects cascade through the entire system. But obviously, the common term belies contrasting explanations of change. And dangerously, the two explanations confounded each other.
In the North Sea, for instance, a decadal switch—of the physical, oceanographic variety—occurred in the late eighties. But this was also a time and place of intensive fishing. So was the upsurge of herring due to shifts in the physical environment, or to the cascading effects of cod fishing? Here's an analogy that gets at the difficulty: imagine how hard it would have been to prove that global warming is caused by humans if, just as climatologists were struggling to get their message across, astrophysicists had made the fantastic new discovery that the sun sometimes shifts to hotter temperatures.
The coastal towns of eastern Canada, snug clusters of peaked roofs and weathered wood, tucked in rocky coves so as to be near the boats but out of the weather, are communities that cannot survive without their haul from the sea. So when the groundfish collapsed in the early nineties, and the government, intent on bringing them back, declared a moratorium on fishing cod, haddock, and pollack, the towns unraveled. In the first year of the ban, forty thousand jobs were lost, most by people with little to fall back on. Thousands of families up and left. Schools closed. Rates of mental illness rose.
All that by way of sacrifice, and still, the groundfish did not recover. Many took this as clear evidence that new physical conditions, ushered in by a decadal switch, had to be to blame; otherwise, why wouldn't the fish have rebounded? Moreover, a similar case of strangely stubborn collapse was unfolding, at the very same time, in the cod fishery of the Baltic Sea, so it seemed plausible that the parallel tragedies had their common origin in a vast, North Atlantic decadal switch. But the groundfish's failure to recover, it now appears, does not necessarily implicate a decadal switch, because it can also result from the inner-workings of a food web.
Kenneth Frank, at the Bedford Institute of Oceanography, was able to reconstruct the cascade that transpired off the coast of Nova Scotia. As groundfish numbers fell, their prey—herring, capelin, and sand lance—surged, causing a decline in zooplankton and an expansion of phytoplankton: a cascade through four full stories of the food web. Still, nothing in this picture suggests why the process did not simply reverse itself once the fishing relented. But researchers from Germany and Sweden, studying the Baltic catastrophe, recognized that the extended trophic cascade may have triggered another kind of change, a bit like a Rube Goldberg machine that, as its final step, opens a trapdoor beneath itself.