Extinction of Megaherbivores

EXTINCTION OF MEGAHERBIVORES About 3 million years ago, the uplift of the Panamanian Isthmus separating North and South America created an effective barrier to isolate Atlantic and Pacific marine species, which began to pursue their separate evolutionary paths. Simultaneously, extinction impoverished the once rich western Atlantic fauna. Meanwhile, South America, which had been isolated from the rest of the world for 80 million years, witnessed a strong exchange of land species with North America across the Panama land bridge, causing many species to be unable to compete with newcomers to extinction. The connecting landform between North and South America stops the flow of cold water currents from the Atlantic to the Pacific. In addition, the closure of the Arctic Ocean from the warm Pacific current helped start the Pleistocene glaciation. Unlike past ice ages, the Pleistocene was unusual because its low extinction rate may have been because the species was already well adapted to colder conditions. The species living today are particularly hardy, having survived extreme environmental changes over the past 3 million years, when glaciers stretched across much of the Northern Hemisphere. As the last ice age drew to a close between 12,000 and 10,000 years ago, an unusual extinction killed off large land plant-eating mammals called megaherbivores. These animals grow to enormous sizes and roam ice-free areas in many parts of the Northern Hemisphere. Giantism probably resulted from similar circumstances that led to the large size of some dinosaurs, including abundant food supplies and lack of predation. Woolly rhinos, mammoths, and Irish reindeer disappeared in Eurasia.Great buffalo, giant hartebeest, and giant horses disappeared in Africa. More than 80 percent of large mammals and large numbers of bird species disappeared from Australia. Meanwhile, giant ground sloths (Fig. 197), mastodons, and woolly mammoths (Fig. 198) disappeared from North America. A possible exception is the woolly dwarf mammoth, which probably survived in the Arctic until about 4,000 years ago. The disappearance of these animals also caused their main predators, American lions, saber-toothed tigers, and monstrous wolves, to extinction. About 8,000 to 9,000 years ago, conditions were much hotter and drier than today. The global environment is reacting to climate change by declining forests and expanding grasslands. Climate change is disrupting the food chains of many large animals. When they lose their food source, they simply disappear. Also at this time, man became a proficient hunter and wandered north on the backward heels of glaciers. On their journey, they encounter a lot of wildlife, many species that they might have hunted to extinction. In North America, 35 classes of mammals and 10 classes of birds are extinct. The extinction occurred between 13,000 and 10,000 years ago, with the largest mortality peaking about 11,000 years ago. Most of the badly affected mammals are large herbivores weighing more than 100 pounds, with many weighing up to a ton or more. Unlike previous episodes of mass extinction, this event did not significantly affect small mammals, amphibians, reptiles, and marine invertebrates. Surprisingly, having survived several previous glaciation periods over the past 2 to 3 million years, these large mammals succumbed to extinction after the Ice Age. During this time, Ice Age people occupied many parts of North America. Their spear points have been found resting among the remains of giant mammals, including mammoths, mastodons, tapirs, native horses, and camels. These people crossed into North America from Asia via a land bridge formed by the drying of the Bering Sea and moved through an ice-free corridor east of the Canadian Rockies.Instead of migrating to North America in a few waves, however, small groups of nomadic hunters probably crossed ancient land bridges in pursuit of big game and ended up in the New World purely by accident. Human hunters arriving from Asia drove across the virgin continent, following migrating herds of large herbivores, which they apparently hunted to extinction.