Ecosystems and Extinctions
cycles were not the only geological and climatic characteristics of
the Pleistocene. Volcanic activity was also occurring in the rift
valleys of Africa and in western North and South America. In southwestern
North America, the Colorado River began to carve out the Grand Canyon.
Although the Pleistocene represents a brief portion of geologic time,
it includes detailed records of profound changes in climate and landscape.
Most Pleistocene plants and animals were quite similar to modern forms.
Many species of flowering plants, conifers, mosses, insects, mollusks,
birds, and mammals still exist today. In many places of the world,
the Pleistocene landscape would have looked very similar to that of
today. However, glaciation cycles resulted in the evolution of cold-adapted
mammals in northern continents, as well as fragmentation and geographic
shifts of existing ecosystems. During these periods, familiar organisms
were often found in novel environmental combinations.
The Pleistocene was also a time of extinction. By the end of this
epoch, many species of mammals had gone extinct in North America,
including llamas and camels, tapirs and horses, and musk oxen. In
addition, other large mammals such as mammoths and mastodons, saber-tooth
cats, and ground sloths went completely extinct. Similar extinctions
of large mammals occurred in Australia and South America as well.
These extinctions are a source of active research and controversy.
Many scientists believe that human migrations were an important factor
in the extinctions of large mammals, especially in North America and
Australia. However, the dramatic climate changes that were also occurring
may have been a factor as well.