Ants as seed dispersers in a Mediterranean old-field succession

Seed dispersal by ants was studied in three Mediterranean old-fields (6, 15 and 43 years after abandonment) and a Quercus pubescens forest that represented the last successional stage. In each plant community, we investigated 1) the occurrence of both seed dispersal strategies involving ants, namely myrmecochory and dyszoochory, relatively to plant species frequency and cover, 2) the abundance of disperser species likely to perform one dispersal type or the other, 3) the relative effectiveness of myrmecochory and dyszoochory, by experimentally offering elaiosome-bearing seeds of Viola alba and Euphorbia nicaeensis, and seeds of the same species deprived of their elaiosome. The proportion of dyszoochorous plants and the abundance of granivorous ants followed the same trend along the successional gradient. The youngest stage had the highest proportion of dyszoochorous plants and the highest abundance of granivorous ants, including the harvester ant Messor structor, whereas the oak forest showed the lowest proportion of dyszoochorous plants and held no granivorous ants. In contrast, there was no coincidence during the succession between the number of myrmecochorous plants and the number and abundance of ant species collecting elaiosomes. More generally, myrmecochorous plants were rare while ant species collecting elaiosomes were present from old-field to Forest. Seed removal experiments showed an overall significant increase in the removal rate of elaiosome-bearing seeds over that of seeds without elaiosome, but in the youngest old-field the difference was not significant for V. alba and was attenuated for E. nicaeensis seeds, due to high harvester ant activity. We suggest that dyszoochory by ants could be a major mechanism influencing plant dynamics in open and grassy vegetation in the Mediterranean region. We also suggest that, though apparently efficient, myrmecochory is underrepresented because of biogeographical features (rarity of the medio-European flora) and lack of strong selective pressure. We emphasize that dispersal strategies involving animals, be it ants, birds or mammals, all share the same dichotomous pattern that shows the evolutionary strength of two mechanisms, one directly derived from predation (dyszoochory), the other generally more deeply shaped by mutualistic processes (e.g. myrmecochory).