Complicating a complex ecosystem function : the controversial role of gastropods in a myremcochorous seed dispersal mutualism
Several thousand plant species worldwide are adapted to seed dispersal by ants (myrmecochory) and they are a dominant part of many plant communities, as is the case in European beech forests. Ants, however, are often rare in beech forests. The consequence of this paradox might be that the majority of seeds fall prey to seed predators and myrmecochores migrate mostly by vegetative spread. I compared ant, gastropod and myrmecochore abundance data from beech forests and I conducted seed removal experiments. Gastropods contributed most to the seed removal. Therefore I offered seeds to four gastropod species in the laboratory and I collected red slugs (Arion rufus) in beech forests and searched for seeds in their feces. I tested their dispersal potential by assessing germination rates of slug-defecated seeds and distances red slugs move in the field. The cover and species richness of myrmecochores in beech forests was negatively correlated with the abundance of ants but positively with the abundance of gastropods, supporting my assumption of a discrepancy in the presence of plants and their ant-dispersers. Gastropods were most important for seed removal in my removal experiments, while rodents and insects including ants played only a minor role. Laboratory feeding experiments showed that gastropods consumed seeds of all plant species offered. Swallowed seeds were defecated undamaged and germinated as well as control seeds. I also discovered seeds in the feces of wild-caught red slugs. Thus, gastropods may act as seed dispersers. Red slugs moved up to 14.6 m in 15 hours, the median gut passage time of seeds, and thus, might transport seeds even further than ants. The results of my experiments indicate that myrmecochores might lack ants as their seed dispersers in beech forests, but they also suggest that ants could be substituted by gastropods.