Invasive species: the case of the Brussels Psittacidae Impacts, risks assessment and actions range


Invasive species raise many questions regarding their environmental impacts. Literature provides many advices to help field management choices, sometimes contradicting each other. For example, measures should be applied at low level of abundance; but in the same time, it is advised to adapt action to impacts which are often unknown at the settlement moment. The necessity to define present and potential impacts of introduced species is increasingly obvious. In the case of the Psittacidae, there are few examples of feral population management. This could be linked to the fact that these species scarcely induce major economic damages. Furthermore, many people living in town welcome those birds which are to a certain extent a substitute to the contact between man and nature. Brussels avifauna is studied for many years by the birding society Aves, in collaboration with Brussels Capital-Region Environment Institute. Information is partly obtained by common birds monitoring (by point counts) within the framework of the Brussels Environment Survey, but also thanks to a research led in 2002 to assess the impact of the Rind-necked Parakeet Psittacula krameri in Brussels (Weiserbs et al., 2002). Finally, a recent study carried out in 2008 reviewed Psittacidae populations current status in the Brussels Region and analysed their present and potential impacts in order to inform policy-makers about the best management practices able to limit these impacts.
Three Psittacidae breed in Brussels: the Alexandrine Parakeet (Psittacula eupatria), the Ring-necked Parakeet (Psittacula krameri) and the Monk Parakeet (Myiopsitta monachus). The case of the Monk Parakeet will not be further developed here, the Brussels feral population being easier to manage (Weiserbs, in press). If Ring-necked Parakeet is known to be introduced at least in 35 countries, feral populations of Alexandrine Parakeet are much scarcer. In Brussels, Ring-necked Parakeet and Alexandrine Parakeet are strongly increasing, although the second being more recently settled is much less numerous. Both of these populations are mixed in the field, sharing for example roost and feeding sites. Moreover, most of the Alexandrine Parakeet population is located in the North-West of Brussels, where the Ring-necked Parakeet is the most abundant. Feeding by man is supposed to have an important impact on demography, reducing winter mortality and increasing breeding success.

Present and potential impacts of the Ring-necked Parakeet in Brussels could be summarized as followed:
- Competition with indigenous fauna is at present the main threat of the species. In Brussels, a negative impact on Nuthatch has been suggested (Strubbe & Matthysen, 2007) and is observed when competition is experimentally forced (Strubbe & Matthysen, 2009). Point count survey between 1992 and 2008 indicates a favourable status of cavity nesting birds in Brussels (Weiserbs, 2008). Moreover, if no effect is observed for seven cavity nesting species, the Ring-necked Parakeet density has a significant positive effect on the trends of four other hole nesters : Green Woodpecker, Blue Tit, Great Tit (less significant) and Short-toed Treecreeper. This could be explained by the advanced age of most of tree settlements in Brussels parks and the excavating behaviour of Ring-necked Parakeet regularly observed in the field, using any starting wounds on the trees to create new cavity. Besides, research led in 2002 had shown extremely high cavity densities in parks highly populated by the Ring-necked Parakeet (Weiserbs et al., 2002). Nevertheless, a negative impact is feared in the short-term linked to the regeneration of tree settlements and resultant shortage of cavity supplies. Moreover, impact on other groups, like bats, is unknown, but could be real.
- On the fringe of the previous main threats, a local impact is possible on some fruit crops (as observed in Great Britain).
- Finally, very localised impacts are linked to noise disturbance and dirt under the roosts. 

Present impacts of the Alexandrine Parakeet in Brussels are as weak as the population is not very numerous, but are adding to those of Ring-necked Parakeet to which the Alexandrine is associated in the field. Moreover, a strong increase has to be expected in the future, going with growing impacts. The risk assessment is based on two schemes. The “UK non-native organism risk assessment scheme” (Anonymous, 2005), concerning risks for environment and socio-economy, leads for both species to the conclusion of a weak to moderate impact. The “Guidelines for environmental impact assessment and list classification of non-native organisms in Belgium” (Branquart, 2007), assessing risks for Belgian biodiversity, leads to classify both species between categories B (Watch list) and C (low environmental risk)

Since the two species are closely mixed in the field, measures will have to consider both together. The actions range reviews the possible management measures, from the weakest to the strongest:
- Reduce and modify feeding by man to try to slow down demography.
- Act at the cavity supply level to lower potential competition with native cavity-nester (nest boxes setting, old trees preservation,…).
- Sterilization using a chemical substance (as Diazacon) could be possible, requiring to catch birds at roosts, for example by cannon netting.
- Eradication is difficult to plan in an urban context and discouraged, as the current impacts are assessed as low and as the public reaction could prevent future action against more problematic species.


invasive species, Parakeet, Psittacidae


Name Role Start End
Weiserbs, Anne member


Name Role Start End
Département Etudes Aves-Natagora member


Reference Role
Weiserbs (2008). Surveillance de l’état de l’environnement bruxellois. Groupe de travail Aves – Rapport pour Bruxelles Environnement – IBGE 2008. author
Weiserbs, A. (2009). Espèces invasives : le cas des Psittacidés en Belgique. Incidences, évaluation des risques et éventail de mesures. Aves (in press) author
Weiserbs, A., Jacob, J. P. & Rotsaert, G. (2002). Evaluation de l’incidence du développement des populations de perruches sur les habitats et les espèces indigènes en Région bruxelloise. Aves - Rapport pour l’Institut Bruxellois pour la Gestion de l’Environnement. author

created:2011-12-14 14:18:59 UTC, source:web

© 2012 by the Belgian Biodiversity Platform