Climate matching drives spread rate but not establishment success in recent unintentional bird introductions

Abstract

Understanding factors driving successful invasions is one of the cornerstones of invasion biology. Bird invasions have been frequently used as study models, and the foundation of current knowledge largely relies on species purposefully introduced during the 19th and early 20th centuries in countries colonized by Europeans. However, the profile of exotic bird species has changed radically in the last decades, as birds are now mostly introduced into the invasion process through unplanned releases from the worldwide pet and avicultural trade. Here we assessed the role of the three main drivers of invasion success (i.e., event-, species-, and location-level factors) on the establishment and spatial spread of exotic birds using an unprecedented dataset recorded throughout the last 100 y in the Iberian Peninsula. Our multimodel inference phylogenetic approach showed that the barriers that need to be overcome by a species to successfully establish or spread are not the same. Whereas establishment is largely related to event-level factors, apparently stochastic features of the introduction (time since first introduction and propagule pressure) and to the origin of introduced species (wild-caught species show higher invasiveness than captive-bred ones), the spread across the invaded region seems to be determined by the extent to which climatic conditions in the new region resemble those of the species' native range. Overall, these results contrast with what we learned from successful deliberate introductions and highlight that different management interventions should apply at different invasion stages, the most efficient strategies being related to event-level factors.

The European trade ban on wild birds reduced invasion risks

“…Second, captive‐bred birds have less ability to survive in the wild than wild‐caught, due to changes in behavioral and physiological traits (Cabezas, Carrete, Tella, Marchant, & Bortolotti, ; Carrete & Tella ), and would have lower probabilities of being successfully introduced (e.g., they have lower escaping abilities Carrete & Tella ) and subsequently established (e.g., almost all escaped individuals are recaptured or predated, Carrete & Tella, , ). As such, breeding origin is one of the main factors influencing invasion success of current non‐intentional bird introductions (Abellán et al., ). Third, our observation of market prices suggested that captive‐bred birds were more expensive than their wild‐caught conspecifics at the beginning of the ban.…”

“…It was initially a temporal measure aimed at preventing the spread of the avian flu, but it was made permanent in 2007 and its focus was broadened to include the conservation of traded species and animal welfare. Although not directly related to invasion management, this regulation is likely to have affected invasion risks, given the invasive potential of wild‐caught traded birds (Abellán, Tella, Carrete, Cardador, & Anadón, ; Carrete & Tella, , ). Two recent modeling approaches have predicted that the EU ban may have reduced invasion risks in target regions, while legal trade fluxes were redirected to other regions along with predicted risks (Cardador et al., ; Reino et al.…”

Alien species richness is currently unbounded in all but the most urbanized bird communities

“…When introductions occur repeatedly, propagule pressure accumulates over time and establishment success increases, therefore the current alien species richness-alien species pool size relationship might be more linear than relationships from the past. Indeed, Abellán et al (2017) found that the time since the first introduction event increased establishment success of introduced birds, possibly explained by higher cumulative numbers of propagules. By this logic, sites with a longer introduction history may have accumulated more propagules, and hence exhibit a more linear alien species richness-alien species pool size relationship.…”

“…The form of the alien species richness-alien species pool size relationship is based on historical introductions, and future introductions may enhance its linearity. First, as economic activities continue, propagule pressure will continue to accumulate and some species may eventually establish (Abellán et al 2017), thus promoting a more linear alien species richness-alien species pool size relationship. Additionally, the sources of introduced birds have gradually changed from European colonies to the Indian subcontinent, Indochina and sub-Saharan Africa (Dyer et al 2017a), which expand alien species pools and might prevent alien species richness from reaching an asymptote (Liebhold et al 2017).…”

“…Understanding what determines alien species richness in urban communities is crucial, since urban areas can act as propagule sources for alien species to invade adjacent natural habitats (McLean et al 2017). Once established, alien species that are active dispersers can spread to nearby natural habitats if they find the environment preferable (Abellán et al 2017), while the spread of passive dispersers can be facilitated by human transportation (Von der Lippe and Kowarik 2008) or dispersal corridors such as rivers (Säumel and Kowarik 2010). Urban areas are expected to increase by 1.2 million km 2 by 2030 (tripling the urban area present in 2000), with most increases projected to take place in biodiversity hotspots (Seto et al 2012).…”