Shrubs are well-known facilitators of both other plants and animals in arid ecosystems. The amelioration of harsh conditions by shrubs creates islands of fertility in otherwise desolate areas. These foundation species shape communities by interacting directly and by indirectly mediating interactions of its beneficiary species. Shrubs are known to mediate the interactions between plants and herbivores; however, it is plausible that the effect of shrubs on herbivory also impacts pollination. Herbivory can change the abundance and quality of floral resources available to pollinators and so if shrubs allow plants to better compensate for these impacts, shrubs can mediate this interaction. In this study, we ask whether shrubs mediate the interaction between herbivory and pollination by increasing the ability of plants to compensate for damage. The response of the annual Malacothrix glabrata to two types of artificial herbivory in the presence and absence of the shrub Larrea tridentata and how herbivory affected pollen limitation was tested in the Mojave Desert and in a greenhouse. Plants received artificial clipping of leaves and flowers as well as hand pollination in shrub and open mesosites in a fully crossed design. The resulting number of flowers and seed mass were measured as response variables.
Results/Conclusions
Both florivory (floral herbivory) and folivory (leaf herbivory) had some impact on plant fitness. Plants that experienced florivory consistently had fewer flowers set seed than those that did not; however, they also had a higher total seed set. Plants also had more seeds per flower when associated with shrubs when flowers received supplemental pollination, folivory, or florivory treatments; however, more flowers set seed in the open when they received supplemental pollination than under shrubs. Overall, the compensation strategy for M. glabrata differed depending on whether plants were in the open or under shrubs, although the strategy under shrubs better increased overall seed set. Plants in the open were more likely to produce more flowers while those under shrubs were more likely to invest in increased seed set per flower. However, M. glabrata is pollen limited and so these strategies only occurred in the presence of supplemental pollination. While M. glabrata was no more or less pollen limited in the presence of L. tridata, the shrub still mediated the impact of herbivory. These results help us to better understand how foundation species shape communities and influence direct and indirect interactions.