WHAT RESOURCES ARE AVAILABLE TO DESERT GRANIVORES - SEED RAIN OR SOILSEED BANK

Patterns of resource availability mold many ecological processes, but we know little about the availability of resources to consumers in nat ure, even for well-studied systems such as the granivorous animals of North American deserts. What we do know about seed resources in desert s is based primarily on seeds extracted from soil samples, but this mi ght present a distorted view of resource availability if animals mostl y harvest newly produced seeds before they enter the soil seed bank. I n order to assess how large the distortion might be, we simultaneously monitored the seed bank and ''seed rain'' over a 19-mo period in the eastern Mojave Desert of California. The seed bank averaged approximat e to 106 000 seeds/m(2) and 38 g/m(2), much higher than values reporte d for other North American desert sites. This corresponds roughly to t he seed production of a single year, since daily seed rain averaged 26 2 seeds/m(2) and 0.26 g/m(2). However, input from the seed rain did no t accumulate in the soil. Instead, the seed bank decreased by a daily average of 114 seeds/m(2) and 0.007 g/m(2) during our study. This sugg ests that virtually all seeds germinate, die, or are harvested by gran ivores soon after being dispersed. Large seeds comprised a greater fra ction of the seed rain than of the seed bank, suggesting that such see ds are differentially depleted, probably by granivores, before they en ter the soil. Because seed drop was seasonal, temporal variation compr ised a significant component of among-sample variance in the seed rain . Temporal variance in the seed bank was much smaller, presumably beca use granivores harvested most of the seed rain. Conversely, spatial va riance was a significant component for the seed bank, but not the seed rain, perhaps as a result of spatial patterns of seed harvest or seed caching by granivores. By virtue of these variance patterns, as well as other attributes, seeds in the soil present different challenges to granivores than do newly produced seeds. Our understanding of desert granivore foraging and community ecology, and of granivore-seed intera ctions, depends critically on choosing the appropriate measure of seed availability to granivores.