Seed predation of two alien Opuntia species invading Mediterranean communities

Invasibility depends on the interaction of the introduced species with the abiotic and biotic factors of the recipient community. In particular, the biotic resistance posed by native herbivores has been claimed to be of great importance in controlling plant invasion. We investigated fruit and seed predation of two exotic Opuntia species within and between Mediterranean communities in order to determine how patterns of predation matched patterns of invasion. Predators were small mammals, presumably mice, which could consume more than 50% of the seeds produced. Predators could be equally effective in consuming fruit and single seeds. O. maxima fruits were slightly preferred to O. stricta fruits, but predators did not distinguish between seeds. Seed predation was more intense in invaded than in non-invaded communities. However, there was a high spatial variation in seed predation that did not always match patterns of invasion, suggesting that seed predation alone is not a good predictor of community invasibility to Opuntia. According to these results invasibility to Opuntia is limited in some (but not all) communities by native mice. Seed losses by predation were high for both species. However, we estimated that more than 75% of seeds dispersed by birds to non-invaded areas are not predated.