Assessing the potential impact of invasive ring-necked parakeets Psittacula krameri on native nuthatches Sitta europeae in Belgium.
Abstract
Biological invasions are a major threat to biodiversity. Given the large number of invasive species, we need to be able to identify invaders with large effects in order to prioritize management efforts. Here, we present a framework using species distribution modelling to predict how abundance of native species will change as a result of competition with an invasive species using ring-necked parakeets Psittacula krameri and nuthatches Sitta europaea as a case study. Ring-necked parakeets are widely introduced throughout Europe and compete with native cavity-nesters. Using the relationship between parakeet abundance and environmental variables in Brussels (Belgium), we predict parakeet abundance across Flanders. A competition coefficient, quantifying the parakeets' impact on nuthatches, was obtained by a regression on a data set of parakeet and nuthatch abundance. An estimate of the number of nuthatches that will be lost when parakeets have occupied all suitable sites was calculated by superimposing the abundance maps of the two species and applying the competition coefficient. Our results predict a potential population of about 22 000 parakeet pairs, indicating that they could become one of the most numerous cavity-nesters in the region. Parakeet abundance is the highest in older, more fragmented forest in urban areas whereas nuthatches prefer larger, old and oak-dominated forests. Our models indicate that throughout much of their range, and in a variety of habitats, parakeets and nuthatches will compete for nesting cavities, but as the competition strength is only moderate, the total impact of parakeets on nuthatch populations will be limited, with at most one-third of the population at risk. Synthesis and applications. Species distribution models combined with empirical estimates of competition strength can be used as a general tool to make an assessment of the potential impact of established invasive species. Such information is required to make effective decisions on how to prioritize management effort and resources across the multitude of invasive species that currently threaten native ecosystems. For the ring-necked parakeet, our results indicate that there is no compelling evidence indicating that parakeets pose a threat large enough to justify an eradication campaign where they are currently present.