On the instincts of birds

Abstract

I use three separate data bases to examine recipient community and site factors that might be influencing the establishment, persistence, and distribution of avian exotics. All in all, about half the variance between islands/regions in their numbers of successfully and unsuccessfully introduced species can be accounted for by recipient site-specific variables; the most important correlate of success is the number of native species extinctions over about the last 3000 years, which reflects the degree of human activity and habitat destruction and deterioration through intrusions of exotic predators, herbivores, and parasites. Consequently, the number of exotic species gained is close to the number of species lost through extinction. Even after controlling for avian extinctions, island area correlates positively with introduced species number. Invasion success does not decline significantly with the richness of the native avifauna (after controlling for the effects of extinctions and island area) nor the variety of potential mammalian predators. The relative proportion of extinct native species across islands/regions is negatively correlated with area and positively correlated with introduced species number and the number of endemic species. A strong correlation exists between the number of successes and the number of failures, attesting to the role of persistent acclimatization societies in increasing species numbers despite high failure rates. The relative success to failure rate increases with the number of extinct native species. The correlation between introductions and native extinctions seems to arise because native birds are usually more common, if not restricted, to native habitats while introduced birds are primary occupants of disturbed and open habitats. As more of an island's area is converted to urban, agricultural and disturbed habitats or altered through the introduction of herbivores and exotic predators, most natives lose good living space while most introduced birds, that frequent open and disturbed areas and have evolved in predator-rich areas, gain habitat.I find little support for the notion that rich avifaunas in themselves repel the establishment of avian invaders at the level of whole islands or archipelagoes. However, interactions between established exotics and natives may be influencing habitat distributions of species in both sets within islands. In both man-made habitats and native forest habitats, exotic species number and the relative abundance of exotic birds is negatively related to the number of native species. After accounting for this local variation, exotic species number is positively related to exotic species number for the entire island/region. In local surveys the relative abundance of exotic birds compared to native birds is affected by habitat (non-native habitats have more exotics) and also by the numbers of species of exotics and natives on the island. The relative importance of biotic interactions like competition, apparent competition through differential disease transmission or susceptibility, and predation in shaping the abundance and habitat affinities of exotics and native species can be difficult to unravel when regional affects are so important.