Seed Predation and Seedling Mortality in the Recruitment of a Shrub, Haplopappus Venetus (Asteraceae), along a Climatic Gradient

Haplopappus ventus is a characteristic shrub of the coastal sage scrub of southern California. Its relative frequency decreases rapidly and significantly from the coast to the central valleys of southern California. Insect damage to developing flowers and seeds is high (35-61%). Experimental exclusion of predispersal flower and seed predators caused a significant increase in the number of viable seeds released because of: (1) a decrease in insect-caused abortion of flower heads ( 11%), (2) an increase in pollination success (19%). and (3) a decrease in damage to developing seeds (104%). A significant and proportionate increase in the number of seedlings established at sites over the climatic gradient from coast to inland valleys followed the exclusion of seed predators. In the inland area, however, extremely high mortality among established seedlings prevented recruitment of juveniles. Mortality of established adult plants, in contrast, was higher at the coast than inland (41 vs. 31%. respectively), suggesting that the geographic difference in the abundance of juveniles would persist. These results differ from those for a parallel experiment on H. squarrosus in which juvenile recruitment was determined by differential predispersal seed predation along the gradient and was not altered by differential mortality among seedlings. Consequently, I conclude that insect seed predation can be the critical factor limiting population recruitment. as in the case of H. squarrosus, but it need not be. as in the case of H. venetus. In the latter case, seedling mortality patterns, superimposed on seed predation, were critical to an explanation of plant recruitment and occurrence over the climatic gradient.