Table 4. Optimal radial distance between zoos in a metapopulation network for each taxonomic class of terrestrial vertebrates.

Class	Metapopulation threshold m	Number of species in cluster	Radial distance (km)	Probability of reaching m	Average number of zoos in cluster	Average number of zoos with species in cluster
Mammalia	>50	124	1,300	0.252	126.6	7.1
	>100	90	1,500	0.221	159.9	11.5
	>250	47	2,200	0.182	239.8	24.6
Aves	>50	85	1,600	0.240	175.2	6.6
	>100	56	1,500	0.158	160.1	8.5
	>250	18	1,700	0.231	187.2	16.9
Reptilia	>50	77	1,700	0.248	186.5	6.9
	>100	48	1,900	0.197	212.1	10.8
	>250	20	2,000	0.121	224.1	19.6
Amphibia	>50	15	900	0.239	69.8	2.5
	>100	9	1,600	0.421	174.6	8.1
	>250	8	2,000	0.245	224.1	11.3

Optimal radial distance between zoos, average probability of reaching the metapopulation size threshold within that radial distance, average number of zoos within radial distance and average number of zoos within radial distance that hold the threatened species. The optimal radial distance represents the distance needed to optimize the probability of reaching an average metapopulation size within the shortest possible distance between zoos (the metapopulation thresholds m are >50, >100 and >250 individuals). The column for “number of species in cluster” indicates the number of threatened species for which all ISIS zoos have at least m individuals.