relatively hot every day. Several plant types (Senecio,Lobelia) show gigantism – plant
genera which are small herbs in temperate regions become large trees in this environ-
ment. The leaves are fleshy and store water. Few animal species are adapted to these
conditions, but one is the hill chat (Cercomela sordida).
Warm semi-desert scrub is most extensive in a band surrounding the Sahara and extend-
ing through Arabia, Iran, and to India. The Somali horn of Africa and the Namibian
zone of southwest Africa have, in prehistory, been joined to the Sahara. The vegeta-
tion is scattered thorn bush (Acacia) and succulents, with a sparse herb layer. Several
of the antelopes in the Somali–Sahara area are browsers with convergent adaptations
of long necks and the ability to stand up on their back legs (the dama gazelle (Gazella
dama), dibatag (Ammodorcas clarkei), and gerenuk (Litocranius walleri)). In both Asia
and Africa, the main arid-adapted small mammals are the gerbils (Gerbillus, Tatera)
and jerboas (Jaculus, Allactaga).
North American semi-desert scrub surrounds the Sonoran and Mojave deserts.
Creosote bush (Larrea divaricata) is common and there is a wide variety of other
spiny and succulent plants such as prickly pear (Opuntia). A number of arid-adapted
small mammals such as pocket mice (Perognathus) and kangaroo rats (Dipodomys)
live on seeds. Ground-feeding birds such as doves, new world sparrows, and juncos
are characteristic. The equivalent Australian vegetation is dominated by shrubs of
the family Chenopodiaceae. Small mammals include hopping mice (Notomys) and
the marsupial jerboa pouched mouse (Antechinomys). However, most of the mam-
mals and birds are derived from the temperate woodlands and are recent invaders.
These areas are known for the large flocks and nomadic movements of Australian
finches (Ploceidae) and budgerigars (Melopsittacus undulatus) following the unpre-
dictable pattern of rainfall.
At higher latitudes in the rain shadow of the Rocky Mountains and the Himalayas,
a cool semi-desert vegetation is characterized by low, aromatic shrubs such as
sagebrush (Artemisia) and perennial tussock grasses. Small mammals and birds are
similar to those in the warm semi-deserts. Ground squirrels (Spermophilus) are com-
mon in this type of vegetation in North America.
Deserts tend to occupy the mid-latitudes and extend from the west towards the
middle of continents – the Sahara in Africa, the Gobi in Asia, and the deserts of Australia,
southern California, and Arizona are examples. They receive on average less than
250 mm of rain per year. Smaller ones include the Namib desert of southern Africa,
the Sonoran and others in southwest USA, and the Atacama of Chile. Below 20 mm
annual rainfall there is no vegetation, and from 20 to 100 mm it is very sparse: plants
have typically xeric adaptations – many species lie dormant as seeds for periods
of several years, but germinate, flower, and set seed again in quick succession
after a rain storm. At this time the desert comes to life as insects breed and
nomadic birds move in to take advantage of the high seedset. Few large mammals
are adapted to this environment but the addax (Addax nasomaculatus) in the Sahara,
the camel (Camelus) in Asia, and the red kangaroo (Macropus rufus) of Australia are
examples.
Marine biomes can be divided into open ocean (pelagic), sea floor (benthos), and
continental shelf.
BIOMES 17
2.6 Semi-desert scrub
2.7 Deserts
2.8 Marine biomes
