384 THE FUTURE OF ARID LANDS 
Siberia, the species of grasshoppers which occur commonly over 
the steppe zone, become persistent pests mainly on its northern 
fringe where it comes in contact with forests and a mosaic of 
grassland and forest is formed. In Australia, the areas particu- 
larly favorable to grasshoppers are characterized by a transitional 
mosaic of low shrubs and bunchgrasses. 
In Central Asia and in tropical Africa, the migratory locust 
thrives on the edges of river flood-plains overgrown with dense 
tall reeds and grasses but bordering on semi-desert grassland with 
sandy bare patches. The mosaic structure of vegetation is, on the 
other hand, often directly due to human activities, or very much 
accentuated by them. 
In Siberia, the clearance of forests for settlement and cultiva- 
tion has created favorable conditions for the extension of steppe 
grasshoppers into the former forest zone, while the fallow system 
of agriculture, with disturbed land lying idle for several years, 
has produced a mosaic of weeds, grass, and bare patches emi- 
nently suitable for mass outbreaks of these pests. 
In the dry hill pastures of Mediterranean countries, excessive 
grazing and destruction of perennial shrubs for fuel and by goats 
have resulted in a cover of short bunchgrasses, with abundant 
bare spots, and here the Moroccan locust is enabled to thrive. 
Similar examples of human activities favoring grasshoppers and 
locusts are found in South Africa, Australia, and North America. 
In the Philippines and Indonesia, deforestation and seasonal 
burning have resulted in forest-grassland mosaic, enabling the 
migratory locust to live and multiply where it could not even 
exist before. In the Sudan, mechanized cultivation in the transi- 
tion zone between the tall grass and short grass savanna has 
caused several native grasshoppers to become important pests. 
Effects of Human Activities 
In all these cases, the general effect of human interference has 
been to destroy the original uniform structure of vegetation and 
to create a mosaic in which the more arid habitat was particu- 
larly encouraged to expand, together with its grasshopper popu- 
lation. The importance of such increase in the ecological aridity 
