Increasing species richness of the macrozoobenthic fauna on tidal flats of the Wadden Sea by local range expansion and invasion of exotic species

Abstract

A 40-y series of consistently collected samples (15 fixed sampling sites, constant sampled area of 15 9 0.95 m 2 , annual sampling only in late-winter/earlyspring seasons, and consistent sieving and sorting procedures; restriction to 50 easily recognizable species) of macrozoobenthos on Balgzand, a tidal flat area in the westernmost part of the Wadden Sea (The Netherlands), revealed significantly increasing trends of species richness. Total numbers of species annually encountered increased from *28 to *38. Mean species density (number of species found per sampling site) increased from *13 to *18 per 0.95 m 2 . During the 40 years of the 1970-2009 period of observation, 4 exotic species invaded the area: (in order of first appearance) Ensis directus, Marenzelleria viridis, Crassostrea gigas, and Hemigrapsus takanoi. Another 5 species recently moved to Balgzand from nearby (subtidal) locations. Together, these 9 new species on the tidal flats explained by far most of the increase in total species numbers, but accounted for only one-third of the observed increase in species density (as a consequence of the restricted distribution of most of them). Species density increased particularly by a substantial number of species that showed increasing trends in the numbers of tidal flat sites they occupied. Most of these wider-spreading species were found to suffer from cold winters. During the 40-y period of observation, winter temperatures rose by about 2°C and cold winters became less frequent. The mean number of cold-sensitive species found per site significantly increased by almost 2 per 0.95 m 2 . Among the other species (not sensitive to low winter temperatures), 6 showed a rising and 2 a declining trend in number of occupied sites, resulting in a net long-term increase in species density amounting to another gain of 1.6 per 0.95 m 2 . Half of the 50 studied species did not show such long-term trend, nor were invaders. Thus, each of 3 groups (local or alien invaders/winter-sensitive species/other increasing species) contributed to a roughly similar extent to the overall increase in species density.

The fate of an immigrant: Ensis directus in the eastern German Bight

“…In contrast, recent studies of Markert et al (2010) and Troost (2010) on mussel beds of Crassotrea gigas, as well as Beukema and Dekker (2011) and this study on sandy habitats with E. directus, found increased species numbers, abundances, biomass and diversity in mussel habitats changed by the invasive species in the North Sea. Nevertheless, E. directus is patchy distributed and variable in occurrence, e.g., mass mortalities and large spat falls (this study, Beukema and Dekker 2011).…”

Transitions in sandflat biota since the 1930s: effects of sea-level rise, eutrophication and biological globalization in the tidal bay Königshafen, northern Wadden Sea

“…Both introduced species may provide habitat structures for further nonnatives. Other studies on long-term change in the benthos of the Wadden Sea have shown that alien species significantly increased species richness in this ecosystem (Beukema and Dekker 2011;Schückel and Kröncke 2013).…”

A supplement to the amphipod (Crustacea) species inventory of Helgoland (German Bight, North Sea): indication of rapid recent change

“…As to the hard-bottom communities at Helgoland, we probably will see them growing closer to those in south-west Britain and southern Norway with respect to species composition and diversity. Increases in local biodiversity in the context of warming have also been shown and predicted, respectively, for intertidal communities of the British Isles (Hawkins et al, 2009) and tidal flats of the Dutch Wadden Sea (Beukema & Dekker, 2010).…”