An important 2013 article in published in Science, “An Update of Wallace’s Zoogeographic Regions of the World,” redivides the world’s faunal regions. The study is based on a sophisticated statistical and phylogenetic analysis of 21,037 species of amphibians, non-pelagic birds and terrestrial mammals. In the end, the authors split the world into 11 zoogeographic realms, almost twice as many as Alfred Russel Wallace demarcated in 1876. They also subdivide some of these “realms” into smaller zoogeographical “regions,” resulting in 20 separate divisions. (See the three maps posted below.)
These updated zoogeographic maps stray further from the continental architecture than Wallace’s map of 1876. Let us begin with the first map, which depicts the larger faunal “realms.” Here the Eurasian Palearctic zone extends across most of the tundra zone of far northern North America, reflecting linkages that were established during glacial periods when the sea level was lower than it is today. But in the Eastern Hemisphere, the Palearctic zone is more restricted than it is in Wallace’s scheme, as North Africa and most the Middle East are placed instead in a separate Saharo-Arabian realm, while Japan, Tibet, and eastern China are also given a realm of their own. The new map retains Wallace’s “Oriental” faunal zone in South and Southeast Asia, but pushes its boundary farther to the southeast, reaching the zoogeographic line of Weber rather than that of Wallace. New Guinea is removed from Australia and appended instead to an “Oceanian” realm, while New Zealand is linked to Australia. Finally, Madagascar is separated from Africa and placed in its own “Madagascaran” realm. All told, these revisions further differentiate the zoogeographic map from that of the continents.
The more finely divided second map deviates further still from the continental model. Here North America is split into four pieces, although the division between the Panamanian and Amazonian regions does come close to the continental border between North and South America. Both South America and Sub-Saharan Africa are divided into two faunal regions. More important, Eurasia vanishes entirely, split among no fewer than eight zoogeographical regions. Intriguingly, the separation of the Indo-Malayan region from the larger Oriental realm almost exactly follows the conventional geographical division of Southeast Asia into Mainland and Insular (island) sub-regions. Note also the “Papua-Melanesian” outliers in northeastern Australia.
It is not surprising that these new maps place Madagascar in its own faunal realm and region. Although Wallace had slotted Madagascar, along with the Mascarene Islands and Seychelles, in a distinctive faunal subregion, he still placed it within thhise larger “Ethiopian” region (Sub-Saharan Africa, essentially). I have always found this classification perplexing, as the fauna of Madagascar is very distinctive. The landmass containing Madagascar, India, and Seychelles rifted away from Africa some 130-160 million years ago, while Madagascar began to split from India around 85 million years ago. As a result of this long separation, Madagascar developed its own faunal assemblage. As another map in the “Update of Wallace’s Zoogeography” article reveals (posted below), Madagascar has the second highest level of “evolutionary uniqueness” among the world’s zoogeographical regions, following only Australia. Such uniqueness is especially pronounced among mammals. As described in a Wikipedia article:
The mammalian fauna of Madagascar is highly distinctive and largely endemic. The extant nonmarine, nonchiropteran [non-bat] taxa constitute (as of June 2014) 168 species, 40 genera and 9 families; of these, besides a probably introduced shrew, endemic taxa make up all the species, all the genera, and all but one of the families. This endemic terrestrial fauna, consisting of lemurs, tenrecs, nesomyine rodents and euplerid carnivorans, is thought to have colonized the island from Africa via four (or five, if aye-ayes arrived separately) rafting events. The other historic terrestrial or semiterrestrial mammal group, the extinct hippopotamuses, is thought to have colonized the island possibly several times, perhaps via swimming
The fauna of Madagascar, moreover, had been far more distinctive than it is today prior to the initial human settlement of the island roughly 2000 years ago. As the Wikipedia article cited above also notes:
Earlier in the Holocene, Madagascar had a number of megafaunal mammals: giant lemurs such as Archaeoindris which at over 200 kg was comparable in mass to the largest gorillas, as well as the hippopotamuses. The island also hosted flightless elephant birds weighing up to 700 kg, the largest known birds of all time. All of these went extinct following the first appearance of humans about 2000 years ago. Today, the largest surviving native mammals of the island, such as the indri and fossa, have weights only approaching 10 kg. Most if not all of the 29 listed extinct species are believed to have died out in prehistoric times; none of these are known to have survived into the post-European contact period.
As a final comment, the “Update of Wallace’s Zoogeographic Regions” article includes separate maps of amphibian, avian, and mammalian regions. Intriguingly, North America (minus the Caribbean) forms its own amphibian region, as do Madagascar, New Guinea, and the Philippines. It is probably not coincidental that North America, and especially the southeastern United States, has by far the world’s greatest salamander diversity (see the map below). The map of mammalian regions is also noteworthy, as it is here that the relationship between continents and zoogeographical regions collapses most completely.
