IMPORTANCE OF DIRECT AND INDIRECT EFFECTS IN THE INTERACTION BETWEEN A PARASITIC ANGIOSPERM (CUSCUTA-EPITHYMUM) AND ITS HOST-PLANT (HORMATHOPHYLLA-SPINOSA)

In this paper, I analyse the interaction between a parasitic angiosperm (Cuscuta epithymum, Cuscutaceae) and its host plant (Hormathophylla spinosa, Cruciferae), taking into account the other species which directly affect the fitness of the host plant. I studied the spatial pattern of Cuscuta infestation in H. spinosa in the Sierra Nevada, Spain, for 6 years (1988-1993). In addition, I assessed the composition and abundance of the pollinator assemblage of Cuscuta during 1991. I determined the effect of dodder on H. spinosa reproductive success by comparing the fruit and seed production of both parasitized and non-parasitized host plants, and determining the mortality factors affecting reproductive potential. Cuscuta epithymum was found to parasitize the labelled host plants only in three of the six years of study, and only in two of the three host plant populations. The maximum percentage of infestation ocurred in 1990, in which 20% of the labelled host plants were parasitized by dodders in one population and 15% in the other population. No labelled host plant had more than 35% of its canopy surface covered by dodders. Parasitized plants were visited by fewer pollinators than were non-parasitized plants. In addition, the diversity of the pollinator assemblage also differed between these two groups of plants. The dodder did not significatively affect the fruit or seed production of its host plant, either during the study years or the years following the strong infestation. The dodder causes little harm mainly because it is scarce. However, even the relationship between an individual dodder and an individual host plant is neutral, due to the array of direct and indirect, positive and negative effects appearing between Cuscuta and H. spinosa during the sucessives phases of the reproductive cycle of the host plant.