Megafauna extinction and phenotypic lag
The Age of Great Mammals ended long before chainsaws and internal combustion engines evolved. In Europe and nontropical regions of Asia, it petered out in steps between 50 and 15 thousand years ago, when the straight-tusked elephants, woolly mammoths, rhinos, and other great beasts of the Pleistocene epoch vanished. Throughout that vast continental mass, a quarter of all genera of animals regarded as megafauna -- those weighing more than a 100 pounds, or 45 kilograms -- were lost to extinction. Europe lost all 6 species of herbivores weighing more than 1000 kilograms. In Australia, the Age of Great Mammals ended sometime between 40,000 and 30,000 years ago, when giant kangaroos, enormous wombats, and rhino-like marsupials (as well as the most formidable crocodiles, lizards, and snakes) were purged from the landscape. This extinction catastrophe stripped Australia of all but one of its 16 genera of megafauna. In the Western Hemisphere, the Age of Great Mammals came to an abrupt end 13,000 years ago, when the mastodons and mammoths, the ground sloths and glyptodonts, the native horses and large camels, and a beaver and an armadillo both as big as a bear all disappeared forever. North America lost 68 percent of its generic richness of Pleistocene megafauna (32 of 47 genera), and South America lost 80 percent (47 of 59 genera). Outlying islands were hit even harder, though several thousand years later, following advances in sailing technologies. Not until 7000 years ago, for example, did Cuba lose its half dozen species of sloth, including a ground dweller as big as a black bear. Just 4000 years ago, while the Egyptians were building pyramids, the last mammoths on Earth expired on an island off Siberia. Madagascar lost all of its biggest lemurs within the past 2000 years, along with both native species of hippopotamus, a strange carnivore, and the giant elephant birds unique to that island...
The Age of Great Mammals may be over, but the plants have not yet caught
on. Those that depended upon mammals to swallow big fruits, as well as
those that deployed armaments to deter soft snouts from stripping foliage,
are still doing what they have always done. Fruit rotting on the ground
is the most obvious sign... The big beasts are gone, but the fruits remain.
Year after year in the American tropics (and temperate climes too), trees
and vines produce fruits that make little sense today. Some fruits simply
rot on the ground beneath the parent plant. Others are raided by seed
predators or plundered by pulp thieves. Whether rotted, raided, or plundered,
viable seeds are rarely dispersed. The plants not only remember the great
mammals of the Pleistocene and before, they expect gomphotheres, ground
sloths, toxodons, and their ilk to show up any day now. Thirteen thousand
years is not enough time for plants to notice and genetically respond
to the loss (p. 3).
Maintained by Francis F. Steen, Communication Studies, University of California Los Angeles