untitled

extinct in North America. Typical American mammals are deer (Odocoileus), moun-

tain goat (Oreamnos), and pronghorn (Antilocapra). Most of the others are Eurasian

forms.

During the last glaciation (12,000 years ago) a few areas within the northern ice-

sheets were free of ice and some animals survived and evolved in these “refuge” areas.

The northern end of Vancouver Island in Canada was a refuge for elk and marmots

which differentiated into new races.

The climatic fluctuations causing the ice ages also caused the expansion and retreat

of the tropical forests of South America and Africa. The South American forests con-

tain the highest diversity of bird species anywhere. The centers of endemism within

these forests match fairly closely the forest refuge patches left by ice ages (Fig. 3.5).

In general, the ice ages have accounted for many present-day distributions of mam-

mals and birds.

The invasion of people

There is one other historical influence that determined the distribution of the larger

mammals and birds: the spread of people over the world. They spread into Eurasia

from Africa some 200,000 years before present (BP), reaching Australia some 35,000

years BP, North America during the last ice age at 12,000 years BP, and New Zealand,

26 Chapter 3

High bird species endemism

Forest refuges

1000 km

Fig. 3.5Patches of high
species endemism in
birds of the Amazon
rainforest (hatched)
coincide with the areas
that were forest refuges
in the past during
periods of maximum
aridity (stippled). (After
Brown 1987.)