Seed preferences of the harvester ant Messor barbarus in a Mediterranean mosaic grassland (Hymenoptera : Formicidae)

The harvesting behavior of the ant, Messor barbarus, was studied in a mosai c grassland of the Mediterranean area. Among the 78 seed species naturally occurring on the study site, nine species dominated the ants' retrieval in spring and in beginning of summer. Preferences of the ants were estimated f rom natural diet and related to the availability of seeds on areas used for foraging. M. barbarus preferentially harvested seeds of Sanguisorba minor, Hornungia petraea, plantain (Plantago bellardii, P. coronopus), catchfly ( Silene gallica) and wild oat (Avena fatua), of which seedling recruitment m ay be altered by the foraging activity of ants. In contrast, seeds from Cer astium sp., Chrysanthemum myconis, Crepis sp., two clover species (Trifoliu m angustifolium, T. arvense) and four grasses (Aira caryophyllea, Agrostis pallida, Holcus lanatus, Brachypodium distachyon) were collected less frequ ently than expected from their natural availability. Two grasses (Agrostis sp and Radiola linoides) were unused by this harvester ant. Ants' preferenc e for a seed species could not be significantly related to the plant family , to the annual or perennial nature of the plant species or to seed product ivity per crop. Only seed weight seemed to influence ant choices: seeds und er 0.4mg were rarely retrieved, but 4-50mg seeds were harvested preferentia lly. This suggests that herbaceous plants living in this Mediterranean biot ope might escape ant granivory by downsizing their seeds.