Climate and competition in abundance trends in native and invasive Tasmanian gulls

Abstract

Climate and competition influence seabird population size yet are rarely considered simultaneously. Here, we consider the influence of climate on nominal abundance trends, and test for evidence of interspecific competition based on 31 yr of count data from 3 co-occurring gull spe- cies in southeastern Tasmania. The silver gull Chroicocephalus novaehollandiae and Pacific gull Larus pacificus are native, while the kelp gull L. dominicanus established in Tasmania from New Zealand in the 1950s. We applied population growth models where either growth rate or carrying capacity was a function of both large-scale climate variables and local conditions. For the kelp gull, a null model without any climate variables was selected, consistent with recent population establishment and increase. For the 2 native species, climate covariates were included, and for both, wind speed was important; for Pacific gulls, the El Nino-Southern Oscillation and regional sea surface temperature were also included in the selected model. These results are consistent with bottom-up forcing in the southeastern Tasmanian marine ecosystem; increased wind forcing leads to increased productivity and higher abundance of an important euphausiid prey species (Nyctiphanes australis). In years with lower wind speeds, warmer waters and higher water column stability, N. australis production is reduced. Models allowing competition effects by the kelp gull on the 2 native species performed poorly relative to models with climate covariates. Thus, compe- tition alone is not a sufficient explanatory factor for observed changes in the 2 native species, and management strategies to maintain populations of the endemic Pacific gull should seek to reduce other stressors, including factors related to climate change.