Variation in Seed and Seedling Size: The Effects of Seed Herbivores on Lomatium grayi (Umbelliferae)

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Abstract
The effect of feeding by adult and larval weevils on seed mass and variance, germinability, seedling growth, and competitive ability was studied for Lomatium grayi (Umbelliferae), a perennial herb of steppe in North America. Umbels were paired on plants, and half the umbels were subjected to controlled levels of herbivory by enclosing seed weevils (Smicronyx sp. (cinereus group)) in bags around the umbels. The treatments produced three groups of seeds: control seeds, intact seeds on umbels with weevils, and larval-damaged seeds on umbels with weevils. The mean mass of intact seeds was significantly lower than for control umbels on the same plant, but variance in seed mass was unaffected. A second experiment using adult male Smicronyx and another weevil (Apion oedorhychum), which produces galls in L. grayi ovules, showed a reduction in seed mass but not the variance compared with controls. The two kinds of herbivory, however, appeared to differ in their effects on seed mass, adult Smicronyx feeding reducing seed masses of intact seeds and galls of A. oedorhychum increasing the masses of neighboring intact seeds. Intact seeds exhibited reduced germinability compared with control seeds of equal mass but, when viable, reached seedling masses at 45 d equal to the controls under all environmental conditions tested (greenhouse vs. growth chambers, with or without interspecific competition with Bromus tectorum). Plant mass at 45 d was significantly correlated with initial seed mass in all environments except one (greenhouse, no competition). Few (6%) larval damaged seeds germinated and survived for 45 d; 25% of these seeds were in the highest mass category. The results suggest that these seed parasites affect seed mass and germinability both directly among the seeds in which they oviposit and indirectly through their effects on neighboring seeds. The size structure of seedlings in L. grayi population can depend upon these direct and indirect effects and the correlation between seed mass and plant mass, which varies among environments.