Table 1. Vertebrate introductions between Europe and North America.

	No. of species Europe → North America				No. of species North America → Europe				World parrots (ref. 12) (n = 350)
Invasion step	Fish (n = 220)	Mammals (n = 207)	Birds (n = 361)	All (n = 788)	Fish (n = 713)	Mammals (n = 342)	Birds (n = 519)	All (n = 1,574)
Introduction	14	13	40	67	35	9	28	72	54
	6.4 ± 3.2%	6.3 ± 3.3%	11.1 ± 3.2%	7.9 ± 1.9%	4.9 ± 1.7%	2.6 ± 1.7%	5.4 ± 1.9%	4.3 ± 1.0%	15.4 ± 3.8%
Establishment	9	11	12	32	19	7	7	33	38
	64.3 ± 25.1%	84.6 ± 19.6%	30.0 ± 14.2%	59.6 ± 11.6%	54.3 ± 16.5%	77.8 ± 27.2%	25.0 ± 16.0%	52.4 ± 11.9%	70.4 ± 12.2%
Spread	5	8	8	21	12	5	2	19	N/A
	55.6 ± 32.5%	72.7 ± 26.3%	66.7 ± 26.7%	65.0 ± 16.5%	63.2 ± 21.7%	71.4 ± 33.5%	28.6 ± 33.5%	54.4 ± 17.4%	N/A

The given percentages [±95% confidence interval (C.I.), binomial distribution] are based on the species that made the previous step of the invasion process. For example, from the pool of 220 fish species native to Europe but not to North America, 14 were introduced to North America, i.e. 6.4%. The calculations for All species are based on the proportions for fish, mammals, and birds. For example, the given proportion of all species that were introduced from Europe to North America was calculated as the mean for fish (6.4%), mammals (6.3%), and birds (11.1%) rather than as the ratio of 67 to 788; here, the 95% C.I. was calculated by assuming independence of fish, mammal, and bird introductions. The study by Cassey et al. (12) on worldwide parrot introductions is given for comparison.