Science Facing Aliens - Invasive Alien Species in Belgium - Belgian ...

Nuthatches defend their cavities by adjusting the entrance size of cavities to their own size by
plastering up the entrance with mud. However, this does not protect the nuthatches from ringnecked
parakeets, as parakeets start laying eggs already at the end of February (pers. obs.)
while in western Europe, nuthatches start breeding only in the second half of April
(Matthysen 1998). When breeding cavities were blocked, parakeets were forced to look for
new breeding sites and the difference in timing of breeding enabled the parakeets to take the
best nesting sites first.
Parakeet range expansion in Belgium and consequences for native avifauna.
Although the bulk of the parakeet population still occurs in the Brussels metropolitan area,
parakeets can now be found breeding in a range of about 40 km around their release site and
they are steadily increasing their range (Vermeersch et al. 2004, Vermeersch et al. 2006). To
assess the potential distribution of parakeets in Belgium, we used a species distribution model
(SDM) to identify areas suitable for parakeets. SDM use information on species occurrence
data and environmental variables to generate statistical functions characterizing a species’
ecological requirements, and these functions are then projected onto the geographical area of
interest to obtain the species (potential) distribution (Guisan & Zimmermann 2000). Using a
dataset of ± 400 point locations of breeding parakeets and a number of habitat variables
derived from region-wide land-use and forestry maps, we modeled parakeet distribution using
an Ecological Niche Factor Analysis (ENFA, Hirzel et al. 2002). ENFA is a presence-only
method and searches for an environmental ‘envelope’ characterizing the areas in which the
species is present. Results show that parakeet distribution is mainly governed by variables
representing cavity availability (i.e. the presence of parks and old forests) and parakeets are
again strongly associated with urban habitats (Strubbe & Matthysen 2009c). The resulting
distribution map (not shown here) shows that parakeets have ample room to spread into. Most
highly suitable habitats are found along the urbanized north-south axis from Brussels to
Antwerp, indicating that known nuthatch strongholds such as the regions south and east of
Antwerp are highly likely to be invaded (Strubbe & Matthysen 2009c).
However, as the real threat this parakeet poses depends on a combination of its
potential distribution and abundance and its per capita impact on native birds, we not only
need an assessment of its potential geographic distribution but also an estimate of its
abundance (Parker et al. 1999). In order to obtain an estimate of the parakeets' expected
abundance, we took advantage of recent improvements in the SDM field, and reanalyzed our
data of parakeet abundance (see above, but point counts now converted to breeding densities,
pairs/ha) using Boosted Regression Trees (Elith et al. 2008), a new technique capable of
modeling abundance. BRT, coupled with the availability of exceptionally detailed geographic
data layers containing information on land-use and vegetation types (the Biological Valuation
Map and the Forest Reference Layer), allowed us to model parakeet abundance across the
country at the level of individual forest patches. Region-wide predictions of nuthatch
abundance are obtained using patch-level data obtained from a breeding bird atlas project. In
order to quantify the parakeets' impact on nuthatches, we applied a regression model on our
Brussels dataset of parakeet and nuthatch abundances, and we use the correlation coefficient
between parakeet and nuthatch abundance as an indicator of competitive strength. The
competition between parakeets and nuthatches was then quantified by superimposing their
abundance maps and applying the competition coefficient, resulting in an estimate of the
number of nuthatches that will be lost when parakeets have occupied all suitable sites.
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