Localized control of opportunistic, overabundant species in protected areas: A retrospective modelling approach encompassing future scenarios

The yellow-legged gull (Larus michahellis) is an opportunistic seabird species, whose worldwide demographic increase in the second half of the 20th century was associated to anthropogenic activities, such as the proliferation of open-air dumps in urban coastal areas. In the small Berlengas archipelago (Peniche, Portugal), the species breeding population reached about 44,000 individuals in 1994, representing a severe ecological pressure in this Biosphere Reserve. In an attempt to control this overpopulation, management actions were implemented in the area, namely, the culling of breeding adults and later egg destruction campaigns. Although the population has been decreasing in recent years, it is unknown how much of this reduction is due to management actions since the closure of open-air dumps in Portugal might also have influenced the yellow-legged gull population trends. We developed a dynamic model to test pertinent explanatory hypotheses for this problem, taking into account retrospective population trends under alternative contexts of food availability and management actions. The model was also used to predict population growth patterns under future management scenarios. Our results show that, despite contributing to reduce birds local abundance, egg destruction alone would not be able to trigger the population decrease observed in the last two decades. Instead, the permanent closure of open-air dumps in 2001 was likely the major driver for the species local decline. In the current context of restricted anthropogenic food sources, our study also suggests that the tendency of the population is to naturally decline. Thus, continuing fertility control campaigns can compromise the future viability of the yellow-legged gull population on the Berlengas Natural Reserve. Our study highlights the interplay between model-based research and ecological monitoring to test the effectiveness of ongoing management programmes and to anticipate the ecological consequences of future control of native species.