Austro-boreal disjunctions: a remarkable biogeographical pattern illustrated by Nan Ao Island, Guangdong, China
To document disjunct, insular populations of animals separated from similar conspecifics distributed to the north, inland and upland, and compare this pattern with that historically described for northern Florida and the Florida Keys islands. Location
Nan Ao Island is on the Tropic of Cancer 117° E in the South China Sea, eastern Guangdong Province, China. There are archipelagos of islets extending south-east, and several coastal islets around Nan Ao. Methods
Five field surveys were made in 1999–2001. Voucher specimens were tagged, catalogued, and deposited in major museum collections. The comparable Floridian disjunct pattern was researched and described in overview. Results
Five species – a toad, two lizards and two snakes – collected on Nan Ao and three satellite islands were added to the two previously documented species – a lizard and a snake – that fit the austro-boreal disjunct pattern. One lizard and one snake are minor range extensions of a hundred or few km. One snake's Nan Ao population is disjunct by about 450 km and one lizard's by about 550 km. The small, semi-fossorial, short-legged toad is the most notable, separated from the main area of its range by about 800 km. This is similar to the greatest distance between north Floridian and Keys disjunct populations. Main conclusions
(1) Seven Nan Ao species of terrestrial vertebrates are isolated and disjunct from conspecifics found to the north, upland and inland; this compares to five species showing a similar north Florida to Florida Keys pattern. (2) The Nan Ao species have no conspecifics known from the intervening area whereas four of the five Floridian species have conspecific, but dissimilar, populations in the gap. (3) Nan Ao and its satellite islets were high points on a large coastal plain at glacial maximum. (4) Holocene sea level rise and concomitant global warming may have isolated the Nan Ao relicts that had continuous distributions at glacial maximum. (5) The intervening mainland conspecific populations may have been replaced by tropical species that moved north along the mainland but failed to colonize the islands. (6) If so, significant adaptation to a warmer climate must have occurred to enable survival of the Nan Ao relicts to the present.