Identifying spatial drivers of long-term population growth in three large gull species: the importance of mink farms and urban areas

Abstract

Population growth generally shows extensive spatial variation within species, but the proximate and ultimate drivers of this variation are often poorly understood. For highly mobile colonial breeders, population growth is expected to be linked to resource availability within a considerable radius of the colony. We analyzed the relationship between population growth over the period 2000–2020 and resource availability for three large gull species in several hundred colonies in Denmark. Colony growth rates showed strong spatial autocorrelation for Herring Gull <em>Larus argentatus</em>, whereas no such relationship was apparent for the other two species (Great Black-backed Gull <em>L. marinus</em> and Lesser Black-backed Gull <em>L. fuscus</em>). Colony growth rates of Herring Gulls were correlated with relevant proxies of food availability within species-specific foraging ranges, including the extent of urban and subtidal foraging habitats, and the number of mink farms. No such correlations were found for the other two species. The positive relationships of Herring Gull colony growth with the number of mink farms and the extent of built-up area were particularly interesting, as they highlighted the strong dependency of this species on human-associated food sources. Furthermore, Denmark closed all mink farms in late 2020 because of concerns about the spread of SARS-CoV-2 virus between farms and between minks and humans, culling approximately 17 million minks. This dramatic change in food availability is expected to have a negative impact on the Danish Herring Gull population, which in recent years has fared better than in the neighboring countries.