COMMUNITIES IN DRY, OPEN LANDS 465 



Desert communities. — The desert forms the extreme of aridity 

 in the open lands. Permanent or temporary flowing water is entirely 

 absent. Rain occurs seldom and irregularly, and in extreme cases may 

 fail for several years in succession. The yearly rainfall is subject to 

 wide fluctuations, and rains when they do come may be torrential. In 

 many regions there is not even dew. The clearness of the unclouded sky 

 results in a very decided warming during the day, a quick cooling by 

 night, so that the daily temperature variations may amount to 50° or 

 more. In regions like the southwestern United States, where full desert 

 conditions are realized in limited areas, the adjacent territory should 

 be referred to as semi-desert or desert-steppe; but a sharp distinction 

 is not possible; all transitional conditions can be found from grass- 

 land through scrub to desert. 



Only a limited number of forms are able to survive the unfavorable 

 environment of deserts, and have become adjusted to it. In Brehm's 

 words, "The desert is poor, but not dead." Since the vegetation is very 

 sparse, only a small amount of animal life can develop. In general, only 

 small animals occur; the desert fox is one of the largest in the Sahara; 

 the gazelle (G. loderi) stays in the semi-desert; it disappears where 

 the vegetation is too sparse. The lion is not a desert animal; it does not 

 find enough food there. Where vegetation disappears entirely, in the 

 stony desert south of Biskra or in parts of the Libyan sandy desert, at 

 most only a few dung-eating insects are able to survive, by feeding on 

 the dung of camels traveling through the desert, and a few birds and 

 lizards, which feed on such insects. 



In spite of the extraordinary dryness, a number of animals whose 

 taxonomic relations are all with inhabitants of regions of moist air are 

 able to survive in the desert. There are no desert amphibians; they 

 occur only in oases and extend into semi-arid lands from more humid 

 regions. As has been stated previously, a number of snails and isopods 

 have been able to enter the desert. In North Africa, where not a trace 

 of vegetation is present and where the temperature at noon rises to 

 43°, one may find the ground at times thick with the shells of Helix 

 lactea, and Helix pisana occurs in clumps as large as two fists. 73 The 

 success of snails in these dry areas is shown also by the fact that the 

 individuals of H. (Leucochros) candidissima attain twice the size of 

 the European individuals. 74 H. desertorum seems to extend as far in 

 the Sahara as night dew falls: 20 km. inward from the seacoast, and 

 4 km. around the oases. 75 



The shells of desert snails in widely separated regions may possess 

 a striking similarity, without close relationship of the species. There 



