Long-term consequences of the alteration of the seed dispersal process of Euphorbia characias due to the Argentine ant invasion

The alteration of the seed dispersal process due to the Argentine ant invasion and its consequences on emergence, recruitment, distribution, and survival of seedlings of Euphorbia characias were analyzed. The study was carried out in two zones of Mediterranean cork-oak secondary forest, one invaded by L. humile and the other non-invaded. Two cohorts of E. characias seedlings (those emerged in 2001 and those emerged in 2002) were studied in three study plots in each zone. The level of seed loss due to lack of viability, parasitizing, and vertebrate predation did not differ between the two zones. The mean seed sowing depth was lower in the invaded zone (13.5 mm) than in the non-invaded zone (22.4 mm). This depth difference implies a longer time needed for seedlings to emerge in the non-invaded zone but not a different emergence percent under laboratory conditions. In the field study plots seedling recruitment did not differ between the two zones, probably due to the trade-off between the differences in the initial number of seeds released (higher in the non-invaded zone) and the different emergence proportions (higher in the invaded zone). As for the spatial characteristics of emerged seedlings, no differences in the mean seedling distance to the nearest inflorescence or to the nearest seedling or in the pattern of seedling distribution were found between zones. Seedling survival was assessed once a month until they had reached maturity or until all of them had died. The median seedling survival time was similar between the invaded and the non-invaded zones. Survival curves also did not differ between zones. The present study suggests a functional equivalence of the Argentine ant after the replacement of the native ant species. Despite the initial differences found, the final reproductive success of E. characias was not altered after the invasion. However, the case of E. characias seems unlikely to be the rule, and the seed dynamics of other species may be altered, i.e. increased or decreased, and thus positively or negatively affected by the invasion.