Spatio-temporal variation in pre-dispersal reproductive losses of a Mediterranean shrub, Euphorbia dendroides L.

The factors that reduce the pre-dispersal reproductive potential of Euphorbia dendroides are identified and the magnitude and variability of their effects are examined, both on a spatial and on a temporal scale. The aims of the study were: (1) to assess whether such variation was related to plant attributes describing size and/or fecundity, and (2) to determine the consistency of those effects in plant reproductive success. Pre-dispersal losses were measured over 3 years in a total of 45 plants from two populations in Cabrera island (Balearic Islands, western Mediterranean). Two types of insect-plant interactions (a moth that preys on the inflorescences and a wasp that feeds on the seeds) significantly reduced the potential number of seeds of E. dendroides, whereas lack of ovary in the cyathium, ovary abortion or seed abortion were the causes of reproductive loss that can be considered "intrinsic" to the plant. Significant variation was found both spatially and temporally in their effects, though such variation could not be attributed to the plant traits measured. Moths and wasps were not found to exert any opposing selective pressure on the plant, and their effect was additive. Key factor analyses performed with the data obtained over three seasons showed that the influence of a factor on among-plant variation in total reproductive losses cannot be predicted by the magnitude of the loss caused by such a factor; thus, seed abortion, while representing a low reproductive loss (< 20% of the potential seed production) accounted for the greatest among-plant variation in total losses. The analyses also showed that the contribution of the plant-insect interactions to the variation in total losses varied significantly both spatially and temporally. This lack of consistency, together with the lack of association with the plant traits measured, suggests that the demographic changes produced by these interactions are unlikely to translate into adaptive changes.