In Part 1, we touched on some of the plants that first brought scientists’ attention to the Eastern Asian – Eastern North American Disjunct.
When we look at what vertebrate animals also show this pattern – living in East Asia and Eastern North America, but no where else – we find that most of them are poor disperses. That is, they don’t easily colonize new areas due to either environmental requirements (seen in the American Alligator) or habitat barriers (seen in aquatic salamanders).
There are only two species of Alligator alive today – A. mississippiensis (American Alligator) and A. sinensis (Chinese Alligator). Alligators are the most cold-tolerant of the crocodylians, so much so that there’s a fairly credible story of an American Alligator surviving 6 or 7 winters in Pennsylvania (Barton, 1955)! However, they aren’t likely to cross great distances between bodies of water and they tend to stay where it’s relatively warmer. That’s why we don’t get Alligators in the Midwest or the northern Atlantic Seaboard.
Chinese Alligators are critically endangered. They only live in a portion of the Yangtze River near the coast. In 1999 it was estimated that only 130-150 individuals remained in the wild; however, they have been successfully bred in captivity, with over 10,000 individuals housed in a preserve in Anhui, China, alone.
This is a family of salamanders with a rich fossil history, but is survived by only three living species: two Asian species known as Giant Salamanders (Andrias japonicus in Japan and Andrias davidianus in China) and the North American Hellbender (Cryptobranchus alleganiensis). The Hellbender, in turn, has two subspecies: one in the Ozarks and one in the Appalachians. All three species are threatened to some degree because they require cool, clear running water in order to breath through their skin. This makes them particularly poor disperses, limited not only by their aquatic lifestyle but also by water quality.
Cryptobranchids are some of my favorite animals. The Chinese Giant Salamander is the largest living amphibian in the world. They’re also critically endangered. Most modern individuals are about 1 meter in length, but the one that made international news in December 2015 was about 1.4 meters (4’6″) in length. (That’s about the average height of a 10-year-old boy in the U.S.)
Of course, this pattern is seen in other organisms as well. This is still an active area of biogeographic research and new discoveries surely await!
To understand why these plants and animals exist where they do today – and why they seem to have ‘disappeared’ in between – we need to understand their fossil history as well as some big-scale changes that have occurred in Earth’s history. We’ll tie it all together in the final post of this series!
*Barton, A. J. 1955. Prolonged survival of a released alligator in Pennsylvania. Herpetologica: 210-210