Changing Numbers of Three Gull Species in the British Isles

Between-population variation of changes in numbers can provide insights into factors influencing variation in demography and how population size or density is regulated. Here, spatio-temporal patterns of population change of Herring Gull (Larus argentatus), Lesser Black-backed Gull (L. fuscus) and Great Black-backed Gull (L. marinus) in the British Isles are described. The aim of this study was to test for density-dependence and spatial variation in population trends as two possible, but not mutually exclusive, explanations of population changes with important implications for the understanding of these changes. Between 1969 and 2013, the three species showed different population trends with Herring Gulls showing a strong decline, Lesser Black-backed Gulls an increase until 2000 but then a decline since, and Great Black-backed Gulls showing no clear pattern. Population changes in Herring Gulls varied between different regions of the British Isles with decreases in the northern and western parts of the British Isles and no clear trends elsewhere. Population changes were density-dependent in the Herring Gull, and Lesser Black-backed Gulls showed faster population increases at lower Herring Gull densities. Herring Gulls seemed to seek refuge in urban environments, whereas Lesser Black-backed Gulls expanded their range into the urban environment. The large declines in hitherto abundant species create a dilemma for conservation bodies in prioritizing conservation policies. The spatial variation in population changes and the differences between species suggest that there is no single cause for the observed changes, thus requiring region and species-specific conservation management strategies.