| Parasitic plants provide a unique opportunity to test the generality of ecological models intended for herbivores. Both types of consumers display preferences for hosts, reduce host biomass and allocation patterns, interact directly and indirectly with other organisms, and can alter community dynamics. My goal was to understand how abiotic stress, host constraints, diet mixing, and interactions with below-ground symbionts affected foraging by the parasitic plant Cuscuta indecora in the salt marsh. First, I examined how abiotic stress affecting host plants mediated Cuscuta performance on a variety of hosts (Batis maritima, Salicornia virginica, Suaeda linearis, and Borrichia frutescens). Salinity had a severely negative affected host and Cuscuta performance, while flooding was better tolerated by both host and Cuscuta. In the salt marsh, plants that tolerate high salinities may avoid Cuscuta parasitism. Second, I looked at three possible constraints (phenology, height, and sexual reproduction) that could make some hosts (Batis maritima, Salicornia virginica, Salicornia bigloveii, and Suaeda linearis ) less susceptible to parasitism, thereby affecting Cuscuta 's host range in the field. All three constraints influenced Cuscuta. Cuscuta only parasitized plants that shared the same phenological schedule. Cuscuta attacked taller host plants and the upper canopy portions of hosts. Instead of reproducing sexually by seeds, Cuscuta reproduced asexually by vegetatively overwintering in hosts. Constraints on host use eliminated half of Cuscuta's potential hosts. Third, because Cuscuta can parasitize many hosts simultaneously, I examined if Cuscuta selectively foraged to obtain a mixed diet. Cuscuta did not prefer or perform better on a mixed diet comprised of Iva frutescens and Borrichia frutescens than a diet of either host alone. Host preference for Iva or Borrichia was not significant but trends showed that Cuscuta preferred hosts that were more abundant. Diet mixing is a result of lack of mobility of Cuscuta . Finally, I examined the relationship between Cuscuta and arbuscular mycorrhizal (AM) fungi that share the same host plant Iva frutescens. Cuscuta did not have a strong effect on AM fungi. It is possible that pathogenic fungi decreased host and Cuscuta biomass, perhaps by disrupting the plant-mycorrhizal symbiosis. Interactions between Cuscuta and AM fungi seem to be weak. Overall, my results show that a variety of factors influence the foraging of Cuscuta indecora. Although there are fundamental differences between plants and herbivores such as mobility, foraging by holoparasitic plants can be understood within the framework of existing plant-herbivore theory. |
