WHAT RESOURCES ARE AVAILABLE TO DESERT GRANIVORES:SEED RAIN OR SOIL SEED BANK?

Abstract

Patterns of resource availability mold many ecological processes, but we know little about the availability of resources to consumers in nature, even for well-studied systems such as the granivorous animals of North American deserts. What we do know about seed resources in deserts is based primarily on seeds extracted from soil samples, but this might present a distorted view of resource availability if animals mostly harvest newly produced seeds before they enter the soil seed bank. In 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 ≈106000 seeds/m2 and 38 g/m2, much higher than values reported for other North American desert sites. This corresponds roughly to the seed production of a single year, since daily seed rain averaged 262 seeds/m2 and 0.26 g/m2. However, input from the seed rain did not accumulate in the soil. Instead, the seed bank decreased by a daily average of 114 seeds/m2 and 0.007 g/m2 during our study. This suggests that virtually all seeds germinate, die, or are harvested by granivores soon after being dispersed. Large seeds comprised a greater fraction of the seed rain than of the seed bank, suggesting that such seeds are differentially depleted, probably by granivores, before they enter the soil. Because seed drop was seasonal, temporal variation comprised a significant component of among-sample variance in the seed rain. Temporal variance in the seed bank was much smaller, presumably because granivores harvested most of the seed rain. Conversely, spatial variance 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 interactions, depends critically on choosing the appropriate measure of seed availability to granivores.