Within a few years of their escape in the early 1990s from farms off the coast of Germany, Pacific oysters established feral populations along the North Sea’s eastern shores. The oysters were invasive, spreading without restriction, and smothered native mussels, which are an important bottom-of-the-food-chain food source for the region’s seabirds. Ecological catastrophe appeared imminent.
Yet that’s not what happened. Twenty-six years after their arrival, Pacific oysters and mussels now seem to be coexisting. The resulting communities, dubbed “oyssel reefs” by researchers who describe the invasion’s dynamics in the journal Ecosphere, may even be healthier than the mussels alone. “The introduction and spread of Pacific oysters has entailed more resistant, more resilient and more diverse communities at sites of the former mussel beds,” said Karsten Reise, the study’s lead author and an ecologist at the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research. “Oyssel reefs are likely to better cope with the challenges of the Anthropocene.”
During the invasion’s early stages, mussel beds made inviting anchor points for larval oysters. It didn’t take long for those beds to be lost beneath a carpet of big, fast-growing invaders. Hence the alarm. Yet as Reise’s team observed, subsequent generations of oysters grew preferentially on earlier oysters, and new generations of mussels settled undisturbed in the spaces between them. There aren’t as many mussels as before, but they have persisted — and the resulting communities have some potentially useful new properties.
Oysters anchor deeper in the mud than mussels did alone, says Reise, so the reefs are less likely to be dislocated by strong waves or ice floes. They’re also preadapted to warmer waters, which should come in handy as Earth continues to warm. The two species together should also possess a wider range of responses to disease and stress than either species would alone. It’s not an entirely happy picture — fewer mussels means less food for birds — but the oyssel reefs do appear to have a greater total species richness.
The invasion’s trajectory challenges “notions of natural balance, superiority of pristine over novel species combinations, and of introduced alien species threatening biodiversity and ecosystem stability in general,” wrote Reise’s team, referring directly to an ongoing debate in conservation circles over whether people are sometimes too hasty bemoan alien species. Which isn’t to say “that oyssel reefs are better than mussel beds,” they write; that would be “too simplistic.” The point is that invasion itself isn’t simple.
“This conclusion is specific to Pacific oysters at European shores,” says Reise. “With other introduced species the outcome might be similar or very different. The lesson is to put not all aliens in the same conceptual box.”
Source: Reise et al. “Invading oysters and native mussels: from hostile takeover to compatible bedfellows.” Ecosphere, 2017.