Seed and fruit selection by harvester ants, Messor barbarus, in Mediterranean grassland and scrubland

1. We analyse diet and propagule selection by the harvester ant Messor barbarus (L.) in Mediterranean grassland and scrubland in central Spain. 2. Diet was estimated by the identification of worker-transported prey in 34 colonies per system type, and compared with seed and fruit availability in the foraging areas. Propagules were characterized by six morphological traits: total weight; weight of seed content; the three main dimensions; and shape. 3. The effect of propagule attributes on selectivity was analysed after transforming data into phylogenetically independent contrasts. Propagules from a small number of species dominate the diet of M. barbarus in the study area, in terms of both frequency and contribution in seed weight. In grassland, prey selection depends on ln(prey length) and ln(prey weight) (R-2 = 0.57). In scrubland, ln(prey length) explains 64% of selection. Long and heavy propagules are preferred. 4. This pattern of selection can be a mere effect of a time-saving foraging strategy, as apparent preference for long propagules can be expected even if workers forage in a non-selective way. 5. Messor ants are likely to play a role in the plant composition of Mediterranean grassland and scrubland, limiting the abundance of long propagules and thus indirectly favouring small-seeded species without dispersal appendages.