Insights into bird invasion trends from long-term data: Invasion pathways, phylogenetic bias, and the tens rule in Spain and Portugal
Exotic species are now recognized as one of the major drivers of global biodiversity loss, but also provide opportunities to understand better basic ecological processes. Given the historical high amount of data collected by scientists, environmental managers and amateur ornithologists, avian invasions have received much attention in the scientific literature. Most of this research has been performed using past introduced species which were rather well documented due to their intentionality. Because introductions have changed over time, such catalogues of historical invasions omitting current avian invasions can produce biased analyses, losing generality and applicability to management. Here we compile a comprehensive database for exotic birds in Spain and Portugal representing one of the most complete dataset on exotic birds at a regional level, to assess the magnitude of bird invasion in this region, and to characterize the transitions between main stages in the invasion process (transport, introduction and establishment) and its temporal evolution.
Our results show that the magnitude of bird invasions in the study area is much greater than usually described at a regional level. At least 1,010 non-native species have been transported to Spain and Portugal (accounting for ca. 11% of the bird species of the world), among which 375 (ca. 35%) have been introduced from 1912 to 2012. Although most of them were occasionally seen, up to 32 (9%) species have established breeding populations or at least reproduce regularly in the wild. Thus, our findings are in line with the idea that most species fails to transit the different steps of the invasion process (the so-called ‘tens rule’). Our results show also that the set of introduced birds in the study area is phylogenetically clustered in several distantly related clades, and that those taxa that have succeed in establishing are also a non-random subset of all introduced birds in terms of phylogeny. Our results revealed also important temporal changes in the introduction patterns. Introductions were rather scarce during the first years, but rapidly increased since the 50’s, showing an exponential trend until 2006 and a slow down during more recent years. Furthermore, while intentional introductions (e.g. ornamental taxa) were more common in the first decades, most of recent introductions resulted from the accidental escape of traded cage-birds.