Biology Reference
In-Depth Information
35 million. It seems inconceivable that only sixty years ago the world popu-
lation was 2.5 billion; within the lifespan of most of us living today it will
reach 9 billion people, and it is expected to crest at midcentury at around
10 billion. The Earth is clearly approaching the limits of its economic, po-
litical, religious, social, civil, and environmental tolerances, and momen-
tous events are surely in store in the forthcoming decades. The beginning
of James Hutton's famous statement is no longer true, and the latter part is
no longer certain.
Between its ancient beginning and its present state, a convenient start-
ing point for tracing the natural history of the modern New World is around
100 Ma. This is when outlines of its present-day physical features, climates,
lineages, plant communities, and ecosystems can just be recognized, and
when they began the latter stages of modernization. The angiosperms, or
fl owering plants, today constituting the principal vegetation cover of the
Earth, were in the early stages of their diversifi cation and radiation. The
great western cordilleras of the Alaska Range, Brooks Range, Rocky Moun-
tains, Sierra Madre of Mexico, and the Andes of South America were just
being uplifted. The Sierra Nevada, Transvolcanic Belt, the islands of the
Antilles, and the land bridge connecting the New World continents through
Central America had not yet appeared. An epicontinental sea extended
from the Arctic Ocean south through the interior lowlands, and across
Mexico, inundating the Central Plateau, the Isthmus of Tehuantepec, and
the Yucatán Peninsula. In South America, shallow Cretaceous seas covered
parts of the present-day Amazon Basin and other low-lying regions from the
base of the Mato Grosso Plateau in Brazil, south through the pampas, Gran
Chaco, Patagonia, and the Paraguay-Uruguay-Paraná inlet. The sea was be-
ginning to retreat about 100 Ma, preserving along its margin some of the
most extensive fossil plant and animal assemblages in the world. Climates
were gradually cooling from mid-Cretaceous highs, putting pressure on the
giant reptilian herbivores at the top of the food chain that had dominated
the Earth's fauna since the Triassic period at 250 Ma. Toward the end of
this Mesozoic cooling trend at 65 Ma, an asteroid landed on the Yucatán
Peninsula. The combined effect of climate and impact was to end the age of
reptiles and begin the age of mammals and, ultimately, the anthropogene,
or the age of humans. Geological, environmental, and biological histories
are a continuum, but 100 Ma is a suitable time to begin.
Several assumptions have guided the writing, selection of subjects, or-
ganization, and sequence of topics in this work. One is that the biological
units under consideration must be the Earth's ecosystems. As defi ned here,
ecosystems are the extensive, recognizable, natural units of the Earth's sur-
