78 BOTANICAL FEATURES OF THE ALGERIAN SAHARA. 



THE BIOTIC FACTOR. 



It will be well to summarize some of the main facts regarding another 

 and important environmental feature of the Saharan plants, namely, the 

 relation to herbivorous animals. In the western Sahara, wherever there is 

 any forage, animals which subsist on it are to be found. Of the wild animals 

 the gazelle is probably the most numerous and the most destructive. All 

 travelers across the desert have noted the presence of this animal. In the 

 northern Sahara Tristram remarked its abundance fifty or more years ago, 

 and it may be frequently seen by the traveler at the present time. In 

 addition to the native animals, the domestic animals, especially the sheep, 

 goats, and camels, are very numerous, very destructive of plants, and range 

 great distances for food. As a result, an area around every well or oasis, 

 extending as far as 40 kilometers or even much more than this,* is repeat- 

 edly grazed over and has been utilized in this manner for centuries. As a 

 result only the poisonous or the distasteful species, or the plants especially 

 well armed, are left undisturbed to grow and reproduce, while the balance 

 are more or less consumed, frequently so much so as to be quite unrecog- 

 nizable. From the large number of camels, sheep, and goats which range 

 the desert pastures it might be concluded that the leading types of plants 

 to be found would be such as are not eaten by them, but this is not the 

 case. On the other hand, possibly the most generally consumed form is 

 Haloxylon, which grows on the oued flood-plain, the reg, and the hamada. 

 Thus, so far as. this type is concerned, there is probably little or no diminu- 

 tion in numbers because of the attacks of animals. 



A similar conclusion would doubtless be drawn after study of other forms, 

 but there lies at least one notable exception, namely, the influence of animal 

 grazing on the distribution of the betoum {Pistacia atlantica) . The betoum, 

 which is the largest arboreal species in the Sahara, is confined to the region 

 of the Dayas; that is, to the country immediately south of Laghouat. The 

 tree is unarmed and is eagerly sought after by all herbivorous animals for 

 its foliage and tender twigs. Owing to the presence of such animals, wild 

 and domesticated, the young tree would have no chance to survive were 

 it not that, growing in association with it, is the jujube (Zizyphus lotus), 

 which is armed and is not eaten by any animals. The jujube affords safe 

 protection for the seedling betoum, and in its capacity as nurse prevents 

 predatory attacks by animals during the critical period. The survival (and 

 probably the distribution as well) of the betoum is mainly conditioned on 

 the presence of its protector. 



When I first visited southern Algeria it seemed improbable that any por- 

 tion of it, or at least any portion that I should be likely to see, would exhibit 

 the possibilities of plant growth as unaffected by herbivorous animals. 



* Hayward, loc. cit., p. 320, says that at In Salah camels are driven 200 kilometers 

 before finding suitable grazing-grounds. 



