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Deconstructing myths on large gulls and their impact on threatened sympatric waterbirds


Daniel Oro, IMEDEA (CSIC-UIB), Miquel Marqués 21, 07190 Esporles, Mallorca, Spain. Tel: +34971611731; Fax: +3497161761


Owing to increasing population trends and facultative predatory habits, large gulls have been identified as significant agents of change in the alteration of many ecological communities. Often, they are perceived as negatively impacting the population trends of most sympatric waterbirds. Consequently, culling programs have been implemented to remove adults, chicks and eggs intensively. Here, we review the interactions recorded in the literature between the yellow-legged gull Larus michahellis and 10 sympatric waterbirds in the Mediterranean region, all threatened and classified as species of conservation concern. We also used 177 long-term population trends derived from previous studies to study the population dynamics of these species and the culling effort performed. We show that gulls negatively affected survival, fecundity, foraging ecology and nesting habitat availability for many species. However, the annual population growth rates of most sympatric waterbirds showed positive values, even at sites where culling has yet to be initiated and local yellow-legged gull populations are large and increasing. Our results suggest clearly that population increase has not been exclusive of yellow-legged gulls, especially at the regional level. Yet, growth rates of both yellow-legged gulls and sympatric waterbirds were positively associated. Strikingly, the population extinction rate was similar between colonies of yellow-legged gulls and those of sympatric species. Thus, evidence exists to state that the success of gull control programs is relatively low in the long term. We recommend that conservation agencies heed several basic principles of population and community ecology before initiating control, for instance that (1) yellow-legged gulls have bred historically with other bird species and have likely developed defensive mechanisms against this predator and (2) populations of large gulls are regulated by density-dependent mechanisms in both space and time. Incoming European environmental policies on fishing discards and rubbish management should control more naturally and efficiently the density of large gulls and the composition of seabird communities in the long term.