Invasion trajectories of introduced alien species usually begin with a long establishment phase of low abundance, often followed by exponential expansion and subsequent adjustment phases. We review the first 26 years of feral Pacific oysters Crassostrea gigas around the island of Sylt in the Wadden Sea (North Sea, NE Atlantic), and reveal causal conditions for the invasion phases. Sea-based oyster farming with repeated introductions made establishment of feral oysters almost inevitable. Beds of mussels Mytilus edulis on mud flats offered firm substrate for attachment and ideal growth conditions around low tide level. C. gigas mapped on to the spatial pattern of mussel beds. During the 1990s, cold summers often hampered recruitment and abundances remained low but oyster longevity secured persistence. Since the 2000s, summers were often warmer and recruitment more regular. Young oysters attached to adult oysters and abundances of >1000 m⁻² were achieved. However, peak abundance was followed by recruitment failure. The population declined and then was also struck by ice winters causing high mortality. Recovery was fast (>2000 m⁻²) but then recruitment failed again. We expect adjustment phase will proceed with mean abundance of about 1000 m⁻² but density-dependent (e.g., diseases) and density-independent (e.g., weather anomalies) events causing strong fluctuations. With continued global warming, feral C. gigas at the current invasion fronts in British estuaries and Scandinavian fjords may show similar adjustment trajectories as observed in the northern Wadden Sea, and also other marine introductions may follow the invasion trajectory of Pacific oysters.

Unintended species introductions may offer valuable insights into the functioning of species assemblages. A spectacular invasion of introduced Pacific oysters Magallana (formerly Crassostrea) gigas in the northern Wadden Sea (eastern North Sea, NE Atlantic) has relegated resident mussels Mytilus edulis on their beds to subtenant status. At the beginning of feral oyster establishment, mussel beds offered suitable sites with ample substrate to settle upon. After larval attachment to mussels, the fast‐growing M. gigas overtopped and smothered their basibionts. With increasing Pacific oyster abundance and size, oyster larvae preferentially settled upon oysters, and the ecological impact of the invaders on the residents changed from competitive displacement to accommodation of mussels underneath a canopy of oysters. Oysters took the best feeding positions while mussels received shelter from predation and detrimental epibionts. The resident's mono‐dominance has turned into co‐dominance with an alien, persisting in novel, multi‐layered mixed reefs of oysters with mussels, which we term “oyssel reefs.” The first 26 yr of the Pacific oyster's conquest of mussel beds in the northern Wadden Sea may question the overcome notions of natural balance, superiority of pristine over novel species combinations, and of introduced alien species threatening biodiversity and ecosystem stability in general.

For about a century, biodiversity in the tidal Wadden Sea (North Sea, European Atlantic) has increased by more than one hundred introduced species from overseas. Most originate from warmer waters and could facilitate the transformation of this coastal ecosystem to comply with climate warming. Some introduced species promote sediment stabilization and mud accretion. This could help tidal flats to keep up with sea level rise. Although some introduced species also entail negative effects, introductions have diversified lower food web levels, and may benefit foraging birds. So far, no resident populations have gone extinct because an introduced species had established. Rather than degrading the ecosystem, the establishment of introduced species seems to have raised the capacity to follow environmental change. We support increasing efforts against introductions to avoid risk. However, once species are integrated, the common condemnation attitude against “non-natives” or “aliens” ought to be reconsidered for tidal ecosystems of low biodiversity.