Introduction

Deserts and semideserts are characterized by a striking difference from nearly all other terrestrial environments. Except for brief periods following rare rains, most of the ground is bare, shrubs and some perennial grasses being the only obvious vegetation. This has important implications not only within the producer trophic level but also for the interactions between the producers and primary consumers. Annual plants, which form a nearly negligible fraction of the vegetation in undisturbed parts of better-watered regions, assume a major importance in those deserts that have been investigated ecologically. The reason for the prominence of annuals lies in the resistance of their seeds to the harsh conditions that prevail in deserts most of the time. Thus, the reservoir of seeds left from past brief periods of favorable weather is vital to the maintenance of that ecosystem, and the magnitude of the reservior determines the abundance of the seed-eating animals that are so characteristic of many deserts.

The shrubs and perennials in such areas have deep roots, which spread far beyond the spans of their canopies, and frequently the roots of neighboring shrubs actually touch, even though there is much bare ground between the branches. The distribution usually is attributed to the potential for obtaining as much water as is possible, and competition for water is commonly claimed for arid-land plants as reviewed by Fowler (1986). Thus, at shallow depths of soil, there is space available for occupancy by the shallow-rooted annuals at any time when conditions permit the germination of their seeds.