Exotic invaders of the meso-oligohaline zone of estuaries in the Netherlands: why are there so many?

The numbers of exotic species introduced into brackish waters (5-20 psu) an d high-salinity waters (> 20 psu) in the Netherlands are hypothesized to re flect species richness in such waters elsewhere in the world. Notwithstandi ng the fact that species numbers in brackish waters all over the world are lower than in high-salinity waters, the numbers of introduced species in th ese waters in the Netherlands are about equal. Alternative hypotheses to ex plain this phenomenon are: (1) because most ports are situated in brackish regions, brackish-water species stand a better chance of being transported; (2) because brackish-water species are more tolerant of conditions in ball ast water tanks, these species have a better chance of being transported al ive than high-salinity species; and (3) because brackish waters have few sp ecies, it is easier for an introduced species to establish itself in bracki sh waters. None of the latter three hypotheses can be rejected and probably they all play a part in explaining the phenomenon. The third hypothesis, h owever, seems most likely.