THE NATURAL ENVIRONMENT

Geography 101

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Animals

Biogeography is the study of the distribution of living things. Biogeographers map the distributions of species, groups of species, and whole ecosystems, and then try to explain how and why that distribution developed. Within biogeography are two branches, the study of the distribution of vegetation and the study of the distribution of animals, also called zoogeography.

We begin with broadly recognized Zoogeographic Realms. Because animals are mobile, there are many thousands of species common to two or more Realms. Nonetheless, each of the Realms shown on the map has a core of quickly recognizable species that distinguish it. If I asked you which Realm contained giraffes and zebras, you would immediately think of sub-Saharan Africa, the Ethiopian Realm. How about kangaroos? The Australian Realm. Grizzly bears and pronghorn antelope? Nearctic. Giant sloths and llamas? Neotropical.

The distinctions between Realms are based largely on the mammals they contain. The oddest collection of mammals is found in the Australian Realm, dominated by marsupials and including Earth's only remaining egg-laying mammals. These two groups diverged from the placental mammals that dominate all other Realms over 100 million years ago. The three mammalian groups are distinguished by different strategies for raising their young. Marsupials give birth to very tiny babies, about the size of an American nickel, who then crawl into their mother's pouch to finish developing. A nursing pouch is the distinguishing feature of all marsupials. This strategy provided better protection for their young than simply laying eggs. It was not as effective, however, as allowing the fetus to develop much longer inside the mother's womb. The young of placental mammals share nutrients and oxygen with the mother through a placenta organ, which allows birth to be delayed until they are much more capable of survival.

Having a placenta turned out be a big evolutionary advantage and wherever the two groups lived together marsupials died out, with a few exceptions like the opossums of North and South America. Australia, however, became detached from the other continents and drifted in isolation across the Southern Ocean; placental mammals did not have the opportunity to colonize it. Thus, a unique and ancient collection of marsupial mammals survived as a kind of living museum for tens of million of years.

Marsupials are not Australia's only living relic. Two representatives of an even earlier mammalian group, the egg-laying monotremes, survive there as well: the platypus and the echidna, or spiny anteater. These animals are truly living fossils whose continued existence is a marvelous, random accident of geology and biology.

Interestingly, marsupials developed similar body shapes to placentals to fill similar niches in their environments. The most obvious example is the Tasmanian wolf, the last of which died in a cage in 1936. It looks identical to placental canine carnivores, but is, in fact, a marsupial mammal complete with nursing pouch.

Of course today, Australia is overrun with placental mammals and the splendid isolation and preservation of the marsupials and monotremes is over. The continent teems with horses, cows, rabbits, cats, dogs, sheep, rats, pigs, deer, goats, ferrets, camels, and, of course, humans. Indeed, one half of the native marsupials of Australia have become extinct in the past 200 years.

Marsupials originally populated the Neotropical Realm of South and Central America, but successive waves of placental mammals gradually replaced all of them except the opossum and opossum rat. About 60 million years ago the primitive edentate group arrived represented today by the armadillo, sloth, and giant anteater. More recently, 20-40 million years ago, rodents and the New World monkeys joined them, somehow migrating from Africa. About one half of the New World monkey species are distinguished by having prehensile tails, which can grip tree branches. These original placentals have been somewhat displaced by a recent flood of mammals from the north, such as llamas and jaguars, which were able to enter the continent when the land bridge of Central America formed about two million years ago.

The Oriental and Ethiopian Realms have many similar species that have diverged in recent geologic time because of the formation of the wide, impassable Sahara desert. Earlier, however, a great deal of migration took place between the two Realms. They contain the world's only rhinoceroses, elephants, and apes. Placental mammals originated here and marsupials became completely extinct.

The Palearctic and Nearctic Realms also contain similar fauna, as animals have been able to migrate between the two Realms on land bridges formed when sea levels were lower than today. Harsh climate characterizes these areas where only hardy animals survive, such as bison, mountain goat, bear, and the horse.

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