Abstract The fluctuations in densities or biomass of a number of invading and native polychaete and mollusc species in the S outhwest N etherlands were compared over a period of 20 years. For recent invaders a lag phase of 7–10 years occurred after their first appearance, followed by an exponential increase in abundance or biomass for 2–3 years. High numbers and biomass then continued for about 5 years, followed by a strong decline. The total sequence from introduction to decline lasted about 15 years. The densities or biomass of invaders appearing decades or even centuries ago in the D elta area have fluctuated in a similar manner to those of native species, indicating that the densities or biomass of invading species after a 15‐year period of strong changes become governed largely by the same environmental factors as native species. The conclusion may be that after some decades, invading species can become part of a balanced co‐existence with the native species, and that this may yield a net gain in the overall diversity.