Threatened pond endemicity on an oceanic island: the presence of an exotic fish

The EU Water Framework Directive (WFD) prioritises the importance of freshwater conservation areas in the Macaronesian Islands (Azores, Madeira, Canarias and Cape Verde). However, few studies have investigated these freshwater areas. Therefore, research must focus on establishing reference sites for aquatic conservation. We analyse the potential effects of water characteristics and the presence of an exotic predator fish, Gambusia holbrooki (Girard, 1859), on the abundance of an endemic aquatic beetle, Hydroporus guernei (Régimbart, 1891), on an oceanic island. During ten successive months, we sampled the abundance of H. guernei using a protocol that targeted macroinvertebrates in three temporary and two permanent ponds on Terceira Island, Azores archipelago. We also recorded the abundance of G. holbrooki, which was observed for the first time on the island, as well as the environmental characteristics of these ponds. H. guernei contributes a high degree of endemicity to the temporary ponds, while the exotic species only occurred in one permanent pond. The results of multiple regressions suggest that the abundance of H. guernei can be mainly explained by the conductivity (19.1%) and the abundance of G. holbrooki (14.5%). These results suggest that H. guernei is a species with a narrow tolerance to environmental changes. In addition, its abundance may be affected by the presence of G. holbrooki. We suggest that urgent monitoring actions be taken to control the abundance of this exotic fish in these ponds, thereby guaranteeing the conservation of endemic H. guernei populations.