Dramatic impact of alien carp Cyprinus carpio on globally threatened diving ducks and other waterbirds in Mediterranean shallow lakes

Mediterranean shallow lakes support high biodiversity but suffer many anthropogenic threats, including introductions of alien fish. We studied the impact of introduction of common carp (Cyprinus carpio) to Medina and Zoñar lakes in SW Spain. Both lakes were protected as Ramsar sites because of their importance for waterbirds, particularly the globally threatened white-headed duck (Oxyura leucocephala IUCN Endangered) and common pochard (Aythya ferina IUCN Vulnerable). Two carp introduction events in Medina lake, with total eradication of carp in between, provided a unique opportunity to study the impacts of carp on the waterbird community (counted monthly from 2001 to 2013, with up to 69 species) and submerged macrophyte cover (quantified with satellite images). A comparison of waterbird abundance before and after carp eradication in the smaller Zoñar lake supported the results from Medina lake. Carp consistently led to the destruction of macrophyte beds and a radical change in the waterbird community. After controlling for the influence of depth fluctuations, the numbers and species richness of diving ducks were significantly reduced by carp, whilst the opposite effect was observed for piscivores such as herons. Negative impacts on O. leucocephala, A. ferina, red-crested pochard (Netta rufina) and herbivorous coots (Fulica spp.) were particularly pronounced. A significant negative impact of carp was also recorded on greater flamingos (Phoenicopterus ruber), black-necked grebes (Podiceps nigricollis), little grebes (Tachybaptus ruficollis) and gadwall (Anas strepera). In contrast, carp presence had a positive impact on grey herons (Ardea cinerea). The ongoing expansion of alien cyprinids in the Mediterranean region constitutes a major threat for waterbirds and particularly for sedentary, threatened taxa such as the white-headed duck and red-knobbed coot (F. cristata). Of 22 key sites for the isolated Iberian population of white-headed duck identified in a European action plan in 1996, at least 14 have since suffered carp invasions. Further development of successful control methods for carp populations is urgently required to support the conservation of waterbirds in the Mediterranean region.