Herbivory drives a genetic bottleneck but not a demographic one in the rare plant Calochortus tiburonensis
Heavy herbivory on racemes that reduces or eliminates seed production will have a negative effect on individual fitness but may not have demographic consequences. We explored this possibility in the rare serpentine-endemic Calochortus tiburonensis (Liliaceae) at the only location where it grows (Ring Mt, Marin County, CA). C. tiburonensis plants live for 10 – 12 years, but do not flower until they are at least 3 years old and typically do not flower every year, presumably because the low availability of essential nutrients in serpentine soils. It is not uncommon for a plant to flower only 3 or 4 times in its lifetime. The loss of the entire raceme to herbivory in any given year therefore represents a substantial loss in lifetime reproductive output. To determine whether heavy herbivory has demographic consequences, we used demographic matrix models parameterized with empirically estimated vital rates from a 4 year period. We coupled this with elasticity analysis and projections to estimate whether eliminating herbivory would increase population growth.
Herbivory rates were variable but generally high. Forty percent of small flowering plants (1-2 flowers per plant) lost their racemes to herbivores while 75% of large flowering plants (3-10 flowers per plant) were herbivorized. Matrix models show that this population was stable or declined slightly (lambda = 0.98-1.01) over the 4 year study period. The substantial reduction in seed production due to raceme herbivory had minimal demographic effects because of the low elasticity of lambda to the seed-seedling transition (elasticity = 0.002-0.01). Projections showed that if herbivory were eliminated, lambda would have changed minimally (0.99-1.01). Although herbivory had no appreciable effect on lambda, it may have long-term evolutionary consequences. We estimated effective population size from demographic data and found that it was very low (Ne/Nc = 0.08), suggesting that raceme herbivory may drive a genetic bottleneck even when it does not drive a demographic one. This has important implications for this plant’s ability to evolve as the climate becomes hotter and drier. As a serpentine endemic with limited dispersal ability, C. tiburonensis cannot migrate with its preferred climate and so must adapt in situ (or go extinct) and this genetic bottleneck may limit its ability to do so.