Predictable anthropogenic food subsides and regime shift in a generalist seabird

Abstract

[eng] Perturbations are a common feature of any biological system. When changes in the environmental conditions are drastic and of great magnitude, they may induce a regime shift. Regime shifts in population size, for example, are typically determined by a drastic change in the rate of population growth or in the attained carrying capacity. Detecting a regime shift is important to understand the mechanisms underlying the observed population fluctuations. I considered the temporal variation of population size in the colony of Yellow legged gull, Larus michahellis, at Sa Dragonera Island, Balearic archipelago, Spain. The population of yellow-legged gulls at Sa Dragonera has undergone a variation in its population size during the last 11 years. I investigated whether the closure of the main open-air landfill of the Balearic archipelago, an open-air landfill has provoked a regime shift. I applied the Ricker’s model of population growth to several time windows of the series and assessed when the model parameters changed significantly, i.e. the presence of a breaking point. By means of this method, a regime change was identified in 2011, one year after the closure of the landfill in 2010. This perturbation caused a drastic variation in the carrying capacity of the system. The short temporal series did not allow a numerical comparison of the parameters before and after the regime shift, but nevertheless the method detected a breaking point. The results suggest that managing the Predictable Anthropogenic Food Subsides might induce a regime shift, pushing the population of opportunistic species to a new (lower) equilibrium