Patterns of success in passeriform bird introductions on Saint Helena

Abstract

Ecologists have long attempted to predict the success of species that are introduced into foreign environments. Some have emphasized qualities intrinsic to the species themselves, whereas others have argued that extrinsic forces such as competition may be more important. We test some of the predictions made by both the extrinsic and intrinsic hypotheses using passeriform birds introduced onto the island of Saint Helena. We found direct evidence that extrinsic forces are more important predictors of successful invasion. Species introduced when fewer other species were present were more likely to be successful. In a direct test of the alternative hypothesis that intrinsic forces play a more prominent role in success or failure, we found a tendency for species which successfully established on Saint Helena to be also successful when introduced elsewhere. However, the vast majority of species unsuccessful at establishing on Saint Helena had probabilities of success outside Saint Helena of 50% or greater, making this result somewhat equivocal. Finally, we found no evidence to support the hypothesis that species that are successful early are those that are intrinsically superior invaders. These results are consistent with similar analyses of the introduced avian communities on Oahu, Tahiti, and Bermuda.

The Role of Competition and Introduction Effort in the Success of Passeriform Birds Introduced to New Zealand

“…Following Moulton (1993) and Brooke et al (1995), I calculated the number of introduced species already present in a district at the time each species was first introduced by ranking species by introduction date. For each species, the number of other species present at the time of first introduction (NSP) was calculated by summing the number of introductions and subtracting the number of extinctions that occurred before the introduction date.…”

“…Following Moulton andPiInm (1986a, 1987), Lockwood et al (1993), Moulton (1993), Lockwood and Moulton (1994), and Brooke et al (1995), I considered only the introduced passeriform birds. I compiled the list of passeriform species introduced to the North and South Islands of New Zealand, including information on the number of introductions, the location of each introduction, and the number of birds released, from Buller (1888), Andersen (1916), Thomson (1922), Westerskov (1953), Lamb (1964, Wellwood (1968), andWilliams (1969).…”

“…Other studies have excluded introductions of less than some arbitrary "adequate" propagule size, arguing, for example, that introductions of fewer than five birds were unlikely to succeed because of the high chance of stochastic extinction in such small populations (Brooke et al 1995). I included introductions of at least two birds because recorded instances of successful invasion following the release of only two birds (Drummond 1906) show that two can be an adequate propagule size.…”

“…Moulton and colleagues tested this prediction using the records of birds intentionally introduced to the islands of Hawaii and Saint Helena, and their results supported the competition hypothesis: introduced birds were more likely to invade successfully when fewer other introduced species were present (Moulton and Pimm 1983, 1986a, 1986bMoulton 1993;Brooke et al 1995). Moulton and colleagues have used this and other findings consistent with the competition hypothesis (see Results and Discussion) to argue that competition is a major factor influencing invasion success in introduced bird communities (Moulton 1985;Moulton and Pimm 1987~Moulton and Lockwood 1992~Lockwood et al 1993~Lockwood and Moulton 1994.…”

Causes of exotic bird establishment across oceanic islands

“…Competition could still be important in this situation if it was occurring between previously established exotics (e.g. Moulton & Pimm 1983;Lockwood et al 1993;Moulton 1993;Brooke et al 1995;Moulton et al 2001, but see Duncan & Blackburn 2002). However, this would suggest that establishment probability should decline as number of established exotic species increases, while these two variables are unrelated across our islands (estimate Gs.e.ZK0.20G0.31, pZ0.53).…”