Empty habitat in coastal sediments for populations of macrozoobenthos

Species with wide dispersal are expected to have little empty habitat. This was tested by analysing habitat use in the Wadden Sea near the island of Sylt (North Sea). Sampling covered 222 intertidal sites spread across the Konigshafen tidal flats (4.5 km(2), mapping approach conducted once) and 270 subtidal sites along 12 km of a tidal channel system (stratified random sampling in spring and autumn). For any single species 'suitable habitat' was extrapolated from the ranges of water depth and sediment composition present at the ten sites with the highest frequency or abundance of the species. On average, macrobenthic species actually used less than half of the suitable sites within the scale of local populations; this was far less than expected from a lognormal distribution. In the subtidal, abundance of most species changed between the two sampling seasons. A 50% increase in overall abundance was accompanied by a decrease in empty habitat of only about 25%. Thus, a doubling of abundances would not fill the empty space but just of half of it. The polychaete Scoloplos armiger was an exception in occupying almost all of its suitable habitat in the intertidal and subtidal sediments. A distinct patch of high species richness occurred where flood waters persistently form a large gyre which may enhance larval settlement. We suggest that limitations to larval settlement and/or juvenile survival primarily cause the extent of the observed habitat emptiness.