Professor of Biology Emeritus San Francisco State University Pacifica, California, United States
Abstract: Background/Question/
Methods: Scatter-hoarding rodents are involved in burying seed for multiple plant species, forming soil seed banks. In California chaparral, some of these are persistent, long-term seed banks that germinate based on some infrequent cue, such as fire. Long-term seed banks must persist despite rodent predation. Several hypotheses suggest how these seed banks survive predation by their dispersal agents. Marginal value theorem as extended by giving-up-density (GUD) among several ideas suggests rodents may give up recovering seed once they reach some energy balance between effort and reward as modified by their own fear of predation. Another hypothesis is that the seeds emit no odors until the time they initiate germination. This ‘crypsis’ hypothesis suggests seeds are hidden from rodents’ olfactory searches. In this study, I tested whether seed recovery would stop at a density balancing energetic costs and rewards. I assessed seed banks of multiple Arctostaphylos species and estimated their caloric content with a null hypotheses that all species had equivalent values. A prediction for the crypsis hypothesis would be significant differences between fresh fruit versus those buried for multiple years. This was tested using fresh manzanita fruit and old fruit/seed extracted from the soil to create field caches for rodents to discover and pilfer.
Results/
Conclusions: Arctostaphylos species range in fruit and seed size. Rodents generally cache whole fruit, but the dry pericarp rapidly deteriorates leaving fused or partially fused seed encased in hard endocarps (nutlets). Seed among different species are calorically equivalent by weight, so seed size, seed bank density, and % viability were determined to estimate seed bank energy values. The seed bank energy values for 19 taxa were not similar and unexpectedly statistically correlated with more energy remaining in the lowest density seed banks of the largest seeded taxa indicating that there is no GUD from the perspective of total caloric energy. In the field cache experiment, rodents successfully found and removed fruit from all the fresh fruit caches but rarely found caches made from older fruit extracted from the soil. These results indicate that as fruit pericarps deteriorate and fresh fruit loses odor with aging, that crypsis is supported at that point.
