Extirpation or even extinction of freshwater invertebrate species is a neglected conservation issue; declines in abundance and spatial distribution for freshwater invertebrates are far less documented than for vertebrate species. One of the faunal freshwater groups more threatened are bivalves. In this study, we used a long data set (from 2004 to 2011) with samples collected annually over a large area (comprising 16 different sites) to document a rapid decline of the bivalve Pisidium amnicum in the Minho River tidal freshwater wetlands (NW of Iberian Peninsula). In addition, from January 2005 to August 2006 monthly samples were collected to study the abundance, biomass, growth and secondary production of P. amnicum.
Results/Conclusions
A rapid decline in density and biomass of the bivalve Pisidium amnicum was recorded without any sign of a potential recovery. Mean density values reached more than 80 ind./m2 in 2004, but decline to less than 1 ind./m2 in 2009. An identical declining trend was observed for biomass. A significant reduction in the spatial distribution also occurred. The abiotic changes resulting from the 2005 heat wave and possibly the negative interactions imposed by the non-indigenous invasive bivalve Corbicula fluminea were the main factors responsible for the declining trends. Given the very low density, P. amnicum is facing a serious risk of extirpation in this ecosystem and conservational measures are urgently needed.