Northern range shift may be due to increased competition induced by protection of species rather than to climate change alone

Abstract

Few long-term, large-scale studies have been conducted about the factors likely toexplain changes in species abundance and distribution in winter. Range shifts are generally attributed to the climate change or land use. This study shows that other factors such as species protection and the ensuing increasing numbers of individuals andcompetition could be involved. It details the progressive conquest of France, the mostimportant European wintering area for great cormorant, in three decades as its legalprotection by the EU Birds Directive. It is based on 13 exhaustive national counts.Cormorants first occupied the farthest areas (Atlantic and Mediterranean lagoons,then larger rivers) from the main-core European breeding area, with only progressiveoccupancy of the northeastern part later. This strategy mainly resulted from competition for optimal available feeding areas. Suboptimal areas (smaller wetlands harboringsmaller night roosts, colder northeastern French areas) and progressive fragmentation of large night roosts into smaller, better located ones minimized flight costs. Thecoldest areas were occupied last, once other areas were saturated. Their occupancywas favored locally by the global climate change, but it played a minor role in thesestrategies. Both factors induced only a small NNE shift of the weighted centroid rangeof the wintering population (2.6 km/year) which mainly resulted from competition(buffer effect). Only the 2009 cold wave decreased the total number of winteringcormorants at the national scale, once the population had probably reached the carrying capacity of the country, while the previous cold waves had a minor effect.Comparatively, there was a greater SSE range shift of the weighted centroid of thebreeding population (4.66 km/year). Range shifts of other recently protected specieshave been attributed to the sole climate change in the literature, but competition dueto the saturation of usual wintering or breeding areas should be considered too.