Invasive ring-necked parakeets Psittacula krameri in Europe: Invasion success, habitat selection and impact on native bird species

Biological invasions, i.e. the human-assisted introduction and subsequent spread of organisms beyond their natural dispersal barriers, are a serious global threat to biodiversity, but also offer a unique opportunity to test several ecological theories. While an enormous number of species is being transported and released into new areas nowadays, only a fraction of these species are able to establish self-sustaining populations and only a small part of these established species are able to spread beyond their point of introduction. Some invasives, however, can have devastating impact on biodiversity, cause huge economic damage and even threaten human health. Thus, while identifying the mechanisms allowing invasive species to establish and spread certainly is an interesting ecological question, it also yields crucial information for management of the problems posed by invasive alien species. Using the invasion of Europe by ring-necked parakeets Psittacula krameri (Scopoli) 1769, this thesis tests some of the most prominent hypotheses on the invasion success of nonnative species, assesses whether parakeets impact on native avifauna and explores how recent improvements in statistical species distribution modeling techniques can be used to predict the expected distribution, abundance and impact of an established invader.;Analyses of data gathered in the Brussels Capital Region and parts of neighbouring Flanders show that also in Belgium, parakeets are associated with human activity, as they reach their highest densities in forests and parks surrounded by built-up areas. Radio-tracking of a small number of breeding parakeets confirmed that parakeets prefer to forage in city parks, gardens and orchards rather than forests or agricultural areas. Parakeet numbers are also strongly associated with tree cavity density, suggesting that cavity availability is a limiting factor. Based on preferred nesting cavity characteristics, parakeets might come into conflict with native hole-nesting birds such as nuthatches (Sitta europaea) and starlings (Sturnus vulgaris). Results show that parakeet abundance was a significant predictor of nuthatch abundance but there was no evidence for a relation with the number of starlings. Starlings may be able to cope with the parakeets, as they themselves are notorious invaders, e.g. in North America, where they are known to evict several larger birds from their breeding cavities. Nuthatches normally defend their cavities by adjusting the entrance size of cavities to their own size by plastering up the entrance with mud, but this does not protect them from parakeets, as parakeets start breeding much earlier than the nuthatches do. These findings were verified by an experimental manipulation of cavity availability in two Brussels city parks, as after blocking all parakeet breeding cavities, a significant decline in nuthatch numbers was observed, largely due to nest take-overs by parakeets.;In order to assess the potential geographic distribution of the ring-necked parakeet in Flanders, the species distribution modeling technique ENFA (Ecological Niche Factor Analysis) was used. ENFA uses information on known occurrence records to identify areas suitable for colonization by parakeets, and the ENFA models predict ample suitable habitat for the parakeets to spread into, especially via a north-south axis from Brussels to Antwerp. Eastwards, suitable areas are scarcer and the best areas are located in the regions surrounding Leuven and Turnhout while in the west, suitable areas are only found around Ghent and Bruges. Parakeet distribution is primarily governed by the amount of older forests, parks and built-up area in the landscape, reflecting the parakeets' need for suitable nesting cavities and its reliance upon urban areas to forage. ENFA results also learn that parakeets are highly likely to invade some known nuthatch strongholds. To predict how the distribution of nuthatches will change as a result of competition with parakeets, existing data on parakeet and nuthatch distribution and abundance were modeled using Boosted Regression Trees (BRT), a novel distribution modeling technique capable of modeling abundance. BRT parakeet distribution maps generally corroborate with the ENFA results and predict a mean number of about 22 000 parakeet breeding pairs (90 % confidence limits 9 000 to 39 800), indicating that these parakeets could become one of the most numerous cavity-nesters in the region. However, despite the high predicted parakeet abundance, total impact on nuthatches will probably only be small, as maximum 'one third of the nuthatch population would be at risk. Thus, the establishment of ring-necked parakeets should be prevented, but in areas where they are currently present, there is no imminent ecological threat that calls for an eradication campaign. (Abstract shortened by UMI.)...