Commentary: Geographic patterns of establishment success among exotic bird populations

Abstract

ISSN 1948-6596 the capacity to deal with numerous statistical models, ranges of initial conditions, different cli- mate projections, and many more effects. Person- ally, I never liked the idea of automatically proc- essing hundreds of species without careful consid- eration of predictor sets with regard to a specific species. However, I will now use BIOMOD even for single species projections, simply because the general framework makes ensemble forecasting so easy for us. And if one chooses to use only a few carefully chosen predictors, then BIOMOD can successfully process many species simultane- ously. news and update Thuiller, W., Lafourcade, B., Engler, R., & Araujo, M.B. (2009) BIOMOD – a platform for ensemble fore- casting of species distributions. Ecography, 32, Niklaus E. Zimmermann Swiss Federal Research Institute WSL e-mail: niklaus.zimmermann@wsl.ch http://www.wsl.ch/staff/ niklaus.zimmermann/ Edited by Thiago F. Rangel commentary Geographic patterns of establishment success among exotic bird populations Human-mediated species invasions are a major component of global environmental change and there remains a pressing need to understand the mechanisms by which a species becomes invasive. With this in mind, the key stage in the invasion pathway (sensu Williamson and Brown 1986) is species establishment. It is at this stage, when a species has been transported outside its native geographic range and released in a novel recipient environment, that a species either fails or suc- ceeds to establish an exotic population. However, while much attention has been focussed on what species-level traits determine establishment suc- cess in exotic bird populations (Blackburn et al. 2009), relatively less research has concerned the role of location-level variables. The most obvious reason for this relative ‘lack of research’ is that in most cases, compara- tive analyses have consisted of large datasets of good quality records of different species to single specific locations (e.g. Hawaii, Moulton et al. 2001; Australia, Duncan et al. 2001; New Zealand, Duncan et al. 2006) rather than repeated intro- ductions of the same species to multiple locations. Bird species are conspicuous elements of the envi- ronment and their distribution and abundance have attracted considerable research attention in the discipline of biogeography (e.g. Gaston and Blackburn 2000). It is therefore somewhat surpris- ing that autecological studies of exotic birds have received disproportionately limited research at- tention within invasion biology (Pysek et al. 2008). A recent article in the Journal of Biogeography (Strubbe and Matthysen 2009), provides a timely example of a study that addresses this bias. Diederik Strubbe and Erik Matthysen com- piled data on the introductions and establishment success of two parakeet species (ring-necked Psit- tacula krameri and monk Myiopsitta monachus parakeets) across their exotic European distribu- tion. In doing so, the authors provide one of the first comparative biogeographic studies to test environment-level features of establishment suc- cess among exotic birds. In total, 181 introduction events were used for their analysis in the exotic range. In their native ranges, parakeet occurrence was estimated using the presence-only method MAXENT (Phillips et al. 2006). For both parakeet species, individually, their establishment success in Europe was 53%. Data from environmental and climatological variables were used to test the rela- tive influence of alternative 'climate-matching' and 'human-activity' hypotheses. The authors found that parakeet establishment success was greater in areas of more dense human population settlement and, both in the native and exotic ranges, their distribution was associated with a smaller number of annual frost days. Further ex- amination revealed that both species were equally sensitive to frost in their exotic range, the major- ity of failed introductions occurring in regions with more than 50 frost days per year. Human activity can influence establishment success both indi- rectly, through habitat modification and provision frontiers of biogeography 1.2, 2009 — © 2009 the authors; journal compilation © 2009 The International Biogeography Society