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

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. Therefore, recruitment in a given area may depend on larvae produced some 125 km upstr eam while the offspring produced in the study area may recruit another 125 km downstream. As a consequence, the population dynamics cannot be explaine d from single-site sampling and even studies on local effects need knowledg e of the population dynamics on a large spatial scale to distinguish betwee n general trends and local peculiarities. For E. americanus in the North Se a a minimum longshore extension of the sampling area of some 250 km is sugg ested. Provided other benthic species with pelagic larvae have similar disp ersal capabilities, community studies need a similar spatial scale. This im plies that the number of distinguishable major communities is very limited in the coastal North Sea. To overcome the logistic difficulties in studying a sufficiently large area, a permanent network of institutes engaged in be nthic population and community dynamics is recommended.