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

The invasion of ring-necked parakeet (Psittacula krameri) in
Europe and Belgium: mechanisms and consequences for native
biota.
Diederik STRUBBE & Erik MATTHYSEN
Evolutionary Ecology Group, Department of Biology, University of Antwerp,
Groenenborgerlaan 171, 2020 Antwerp
Establishment of ring-necked parakeets in Europe
In Europe, at least 75 nonnative bird species established feral populations (Chiron et al. 2009)
and, due in large part to their popularity as cage birds, parrots (Psittacidae) are well
represented as invaders, accounting for about 18 % of Europe’s established exotic avifauna
(DAISIE 2009). Among parrots, the ring-necked parakeet is undoubtedly the most successful
invader. Although this parakeet originates from mostly subtropical regions in Africa and
Asia, it has formed at least 65 populations in Europe (Lever 2005, Strubbe & Matthysen
2009a), with population sizes varying from several tens to several thousands of birds (Strubbe
& Matthysen 2007). In order to identify the mechanisms that allow this parakeet to invade
Europe, we gathered data on parakeet releases and correlated the outcome of an introduction
event with human population density and climatic factors, thereby testing two of the major
hypotheses on the establishment success of non-native species, i.e. the ‘human activity’
(Taylor & Irwin 2004) and the ‘climate matching’ hypothesis (Williamson 1996). The former
hypothesis states that human activity facilitates the establishment of alien species while the
latter postulates that species have a higher probability to establish if they are introduced to
regions with a climate similar to that in their native area. We found that parakeet
establishment correlated positively with measures of human activity such as human
population density, but negatively with the number of frost days, providing support for both
hypotheses (Strubbe & Matthysen 2009a). Human activity is a root cause of species
introductions (Westphal et al. 2008), but human-dominated habitats are often characterized by
abundant food and we argue that this increased food supply could well explain the link
between the parakeets’ establishment success and human population density, as food
availability is one of the most important factors limiting bird populations and supplementary
feeding may enhance breeding success (Robb et al. 2008), thus increasing population
persistence probability. The negative relationship between parakeet establishment and the
number of frost days indicates that parakeets may suffer from climate mismatch, although
population crashes during harsh winter are rare, and establishment failure could also be
caused by a reduced breeding performance, e.g. due to a lower body condition. Low
temperatures can also impact on avian embryonic development, and further support for the
climate matching hypothesis comes from Shwartz et al. (2009), who found that European
parakeet populations have a much higher rate of egg infertility than parakeets in the native
range (India), or populations introduced to warmer regions such as Israel. Predation pressure,
however, is much lower in the introduced regions compared to the native range, and this
‘predator release’ (Liu & Stiling 2006) partly offsets the climate mismatch, allowing the
parakeet to flourish throughout much of temperate and Mediterranean Europe (Shwartz et al.
2009).
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