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In the late 1970s the American jackknife Ensis americanus (syn. E. directus) was introduced into the North Sea. The history of its dispersal along the coast is used to derive an estimate of the species' mobility. On average, the population spread by 125 km per year to the north and, at the same time, by 75 km per year to the west, against the residual currents. There- fore, recruitment in a given area may depend on larvae pro- duced some 125 km upstream while the offspring produced in the study area may recruit another 125 km downstream. As a consequence, the population dynamics cannot be explained from single-site sampling and even studies on local effects need knowledge of the population dynamics on a large spatial scale to distinguish between general trends and local peculiarities. For E. americanus in the North Sea a minimum longshore extension of the sampling area of some 250 km is suggested. Provided other benthic species with pelagic larvae have similar dispersal capabilities, community studies need a similar spatial scale. This implies that the number of distinguishable major communities is very limited in the coastal North Sea. To over- come the logistic difficulties in studying a sufficiently large area, a permanent network of institutes engaged in benthic popula- tion and community dynamics is recommended.

What an introduced species can tell us about the spatial extension of benthic populations