Biological Conservation 78 (1996) 69-96 Copyright © 1996 Elsevier Science Limited Printed in Great Britain. All rights reserved PIi: S0006-3207(96)00019-5 0006-3207/96/$15.00 +.00 ELSEVIER GLOBAL PATTERNS IN THE ESTABLISHMENT AND DISTRIBUTION OF EXOTIC BIRDS Ted J. Case Department of Biology, 0116 University of California at San Diego, La Jolla, CA 92093, USA Abstract be influencing habitat distributions of species in both sets I use three separate data bases to examine recipient com- within islands. In both man-made habitats and native munity and site factors that might be influencing the forest habitats, exotic species number and the relative establishment, persistence, and distribution of avian abundance of exotic birds is negatively related to the exotics. All in all, about half the variance between number of native species. After accounting for this local islands~regions in their numbers of successfully and variation, exotic species number is positively related to unsuccessfully introduced species can be accounted for exotic species number for the entire island~region. In by recipient site-specific variables; the most important local surveys the relative abundance of exotic birds com- correlate of success is the number of native species pared to native birds is affected by habitat (non-native extinctions over about the last 3000 years, which reflects habitats have more exotics) and also by the numbers of the degree of human activity and habitat destruction and species of exotics and natives on the island. The relative deterioration through intrusions of exotic predators, her- importance of biotic interactions like competition, appar- bivores, and parasites. Consequently, the number of ent competition through differential disease transmission exotic species gained is close to the number of species or susceptibility, and predation in shaping the abundance lost through extinction. Even after controlling for avian and habitat affinities of exotics and native species can be extinctions, island area correlates positively with intro- difficult to unravel when regional affects are so impor- duced species number. Invasion success does not decline tant. Copyright © 1996 Elsevier Science Limited significantly with the richness of the native avifauna (after controlling for the effects of extinctions and island Keywords: birds, extinction, exotic species, invasibility, area) nor the variety of potential mammalian predators. habitat destruction. The relative proportion of extinct native species across islands~regions is negatively correlated with area and positively correlated with introduced species number and INTRODUCTION the number of endemic species. A strong correlation exists between the number of successes and the number For assorted reasons, some practical and some purely of failures, attesting to the role of persistent acclimatiza- aesthetic, people have purposely introduced various tion societies in increasing species numbers despite high plants and animals from one place to another across .failure rates. The relative success to failure rate increases the globe. What factors determine whether a species with the number of extinct native species. The correlation will become established or not? What site properties between introductions and native extinctions seems to determine whether an ecological system will accept arise because native birds are usually more common, if or repel invaders? Given answers to these questions, not restricted, to native habitats while introduced birds how can we best manage ecological systems to avoid are primary occupants of disturbed and open habitats. As problems? more of an island's area is converted to urban, agricul- One ecological issue affecting answers to these ques- tural and disturbed habitats or altered through the intro- tions is whether natural communities vary in their resis- duction of herbivores and exotic predators, most natives tance to invasions because of intrinsic properties of the lose good living space while most introduced birds, that natural system. Elton (1958) suggested that species-poor frequent open and disturbed areas and have evolved in communities, characteristic of islands or very disturbed predator-rich areas, gain habitat. habitats, were more susceptible to invasion than species- l find little support for the notion that rich avifaunas in rich communities. More recent theoretical explorations themselves repel the establishment of avian invaders at of model communities lend some credence to this the level of whole islands or archipelagoes. However, hypothesis (Case, 1991; Post & Pimm, 1983; Drake, interactions between established exotics and natives may 1983, 1988). The role of habitat disturbance in promoting biologi- Correspondence to: T. J. Case cal invasions is well supported (Rejm~nek, 1989). But e-mail: [email protected] how would we test Elton's hypothesis regarding 69 70 T.J. Case species-richness experimentally? Ideally we would con- failed as well as successful introduction attempts. For struct islands identical in all respects but differing in these locations, mostly places that had acclimatization the richness of the resident fauna. We then would societies in the 1800s, I ask what site and community- introduce the same set of species to each island, con- level factors contribute to establishment success com- trolling for individual numbers released, sex ratio, pared to failure. physiological condition, and other variables that might Finally, since competitive interactions between influence reproductive and survival success. Since this is natives and exotics may potentially be important in generally impossible except at the microcosm scale shaping the proclivity of native species to enter human- (Drake, 1991; Drake et al., 1993; Robinson & modified habitats and exotic species to penetrate native Dickerson, 1984), for real islands we must rely on habitats, I examine the habitat distributions of native interpreting the results of past historical introductions and introduced birds in various habitats and locations where many confounding factors, beyond the nature of in the Pacific region. the recipient community, can and do influence invasion The present study is entirely devoted to identifying success. With introductions there are often no controls factors in the recipient location that potentially infl- and, like all historical work, we must often rely on the uence the establishment, persistence, and spread of sometimes conflicting accounts described by different introduced bird species. It is also important to make actors and observers, no longer living; but the vast comparisons between species: are the same species that number of introductions performed on literally thou- are successful in one place successful in others? What sands of different islands and mainlands worldwide biological attributes of species are associated with estab- offers an opportunity to attempt answers to the ques- lishment success? How similar must climates be in the tions raised in the opening paragraph. old and new homes of species? I reserve these analyses The method of the introduction may influence its for a later paper. likelihood of success, e.g. how many individuals are released, and how they are transported and acclimated METHODS to the new location. Different species have been intro- duced to different places with different climates so Native and introduced birds on islands species-specific autecological factors will also be important. Table 1 contains a list of locations with tallies for their Some species may be better adapted to physical and numbers of native and introduced birds along with climatic features of the new environment and others other geographic and biotic features of the location. may have behavioral and life-history attributes that References are given in Appendix 1. The general refer- enhance population growth when they are rare. We ences at the bottom of Appendix 1 were used for nearly might get spurious associations between invasion resis- all locations. Islands were selected for inclusion in the tance and native biodiversity because of confounding analysis based on two criteria: a complete avifaunal interactions with other variables. For example, the list, and an attempt to balance representation across number of native species on an island generally different regions of the globe (excluding polar regions). increases with its area (Preston, 1962; MacArthur & The analysis is restricted to land and freshwater Wilson, 1967). Yet island area directly can affect intro- species (excluding shorebirds, waders, and strictly sea- duction success by potentially affecting mate-finding going ducks) that maintain breeding populations in the ability. A few individuals of a new exotic species locations (i.e. migrants are excluded). Table 1 is based released into a large area may have difficulty locating on archipelagoes with some minor exceptions (see mates compared to the same number released into a below). Using individual islands as separate data points smaller area. Without experimentally or statistically (rather than lumping islands into archipelagoes) com- partialling out the possible affects of area per se on mits pseudoreplication since the native faunas and introduction success, we might incorrectly conclude introduction histories of nearby islands are typically that species-richness (which covaries with area) was closely linked and birds introduced to one island often explaining the results when, in this scenario, it was colonize nearby islands