Interference competition in a threatened seabird community: A paradox for a successful conservation

It is often assumed that conservation actions targeting a threatened community (e.g. habitat protection) will result in similar benefits for all species. However, complex interactions between species, such as interference competition, may result in displacement of subordinate, vulnerable species. We analysed here the spatio-temporal population dynamics of a threatened seabird community since the protection in the 1980s of several breeding sites at the Ebro Delta, western Mediterranean, Spain during 1980–2007. Competition for the most suitable patches was governed by body size, with smaller species avoiding associations with larger, dominant species. We tested whether the density increase of the larger species (yellow-legged gull Larus michahellis and Audouin’s gull Larus audouinii) at La Banya (the highest quality patch within the Delta) affected species diversity at the local level. As expected, such diversity decreased, resulting also from the colonisation of other sites by smaller species, some abandoning the former area. The conservation paradox appeared because the interference competition was dominated not only by the largest species of the community, the yellow-legged gull, which is sometimes considered a pest species, but also by the vulnerable Audouin’s gull, a flagship species which has ca. 65% of the total world population at La Banya. Nevertheless, the availability of alternative patches within the Ebro Delta ensured a high biodiversity at regional scale. Results highlight the role of turnover of competing species within a community in ecosystem function and stability, and the importance of alternative sites and dispersal abilities for the conservation of vulnerable communities.