Parasitism by Cuscuta chilensis and gender affect how the nurse cushion Laretia acaulis increases diversity in Andean alpine communities

Question: Although there is a large amount of information about the anatomy, physiology and auto ecology of parasitic plants, much less is known about how they affect the interaction among other species and if they present gender preferences when the host is dioecious. The little knowledge available is focused on the effects of parasitic plants on negative plant interactions, but their indirect effects on facilitative interactions remains unexplored. We examined whether the parasitic plant Cuscuta chilensis showed gender preference on its host, the dioecious nurse cushion species Laretia acaulis, and how the presence of the Cuscuta affects relationships between Laretia and its community of beneficiary species. Location: Central Chilean Andes. Methods: We randomly sampled 50 L. acaulis cushions infected with Cuscuta, 50 Laretia cushions free of Cuscuta infection, and 50 open sites outside of cushions. We measured the area of each cushion selected, identified cushions as male or female, and identified, counted and recorded the numbers of each other species within cushions and in the open sites. We compared species richness among uninfected Laretia individuals, infected Laretia individuals and open sites, and between infected and uninfected patches within Laretia cushions. Further, we assessed the differences in species assemblage composition between each microhabitat by means of detrended correspondence analysis. Results: Fifteen out of 23 species were more common in Laretia cushions than in the open, non-cushion habitat. Laretia without Cuscuta harboured 3.9 ± 0.2 species per cushion in comparison to 2.5 ± 0.2 species in open habitats. Infection of Laretia cushions by Cuscuta was female-biased and this infection was associated with an increase in the nurse plant effects of Laretia, with seven of 23 species more common in Cuscuta-infected cushions than in cushions without Cuscuta, and a mean of 5.9 ± 0.3 species per Cuscuta-infected cushion. Conclusions: Cuscuta infection showed gender preferences, and this infection increased the facilitative effect of Laretia nurse cushions. This might be related to a decrease in the competitive resistance of the cushion species to colonization by other species (indirect effect) or direct positive effect due to the presence of the parasitic plant (e.g. increases in available soil N). To our knowledge, this is the first documentation of how the indirect effects of a parasitic plant might increase the facilitative effects of a nurse species.