Differential impact of vertebrate and invertebrate herbivores on the reproductive output of Hormathophylla spinosa

We analyzed herbivory on the reproductive structures (flowers, fruits, and seeds) of Hormathophylla spinosa (Cruciferae) by highly dissimilar species of herbivores, from ungulates to endophagous seed predators and gall-formers, in three different populations from 1988 to 1992. Specific objectives of the study were to quantify (i) the intensity of herbivory on reproductive structures of H. spinosa and the consequent loss of reproductive output; (ii) the frequency of herbivory, and how herbivory varied between years on the same individuals; (iii) spatial, among-population variation in herbivory intensity; and (iv) the relative importance of each herbivore species with respect to the losses in plant reproductive output. All plants of H. spinosa (n = 80) were heavily attacked by herbivores in the high mountains of the Sierra Nevada, where most plants were attacked by different types of herbivores during each year of the study. Plants lost about 50% of their reproductive structures during the study period. However, there was a pronounced difference in the magnitude of damage produced by each herbivore since ungulates accounted for most of the damage, whereas other herbivores caused much lighter damage. Mammalian herbivores appear to be a major ecological factor determining seed production in populations of H, spinosa.