Abstract
Invasive species can take advantage of resources unexploited by natives (opportunism hypothesis) or they can exploit the same resources but more aggressively or efficiently (competition hypothesis), thus impacting native species. However, invasive species tend to exploit anthropogenic habitats that are inefficiently used by natives such as urban environments. Focusing on the ring-necked parakeet (Psittacula krameri), one of the most invasive birds worldwide, we combined observations of interspecific aggressions, species-specific cavity-nest preferences and the spatial distribution of the native cavity-nesting vertebrate community to determine the invasion process as well as its potential impacts on native species in a Mediterranean city. Our results support the competition hypothesis, suggesting that ring-necked parakeets are outcompeting native species sharing nest-site preferences. Parakeets initiated and won most interspecific aggressions, which were directed towards competitors but also towards predators. This behaviour could explain the spatial arrangement of natives, with most bird species breeding close to parakeets possibly to take advantage of their effective antipredatory behaviour. However, temporal and spatial patterns of segregation suggest that a threatened bat species is negatively affected by parakeets. This demonstrates that common species gain benefits and threatened ones (in this study, a bat and possibly a falcon) lose nest sites due to invaders. Therefore, the conservation status of the native species that pay the costs of competition with invaders should be considered. This scenario of winners and losers may, however, shift towards more losers if the ring-necked parakeet population continues to grow, thus requiring close monitoring and control/eradication programs to avoid further impacts.
Highlights
Biological invasions are considered a major threat to global biodiversity, since invasive species may cause negative impacts on natives through increased predation risk, competition, hybridization or the spread of disease [1]
Sol et al [15] examined the invasion paradox by studying the use of food resources by invasive and native bird species in an Australian city, concluding that the success of invaders is explained by their capacity to exploit ecological opportunities that most native species rarely use
We investigated two key aspects behind the establishment success of ring-necked parakeets on a relatively newly invaded urban area, namely: the way they shared nest-site resources with the recipient community and the aggressive interactions they experienced with other species
Summary
Biological invasions are considered a major threat to global biodiversity, since invasive species may cause negative impacts on natives through increased predation risk, competition, hybridization or the spread of disease [1]. The relationship between invasive species and biodiversity measures is less clear [2], as introduced species can contribute to species gain by its establishment [3], can reduce species richness through extinction processes [4] or can have no detectable effects on native biota [5]. These different patterns may be explained by the nature of the invader [6] and by the characteristics of the recipient community [7]. If exotic species are able to out-compete natives by exploiting resources more efficiently or through aggressive behaviours, they can successfully invade the new area causing the displacement of the native competitor [10]
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