Effect of size, taxonomic affiliation and geographic origin of dandelion (Taraxacum agg.) seeds on predation by ground beetles (Carabidae, Coleoptera)

Ground beetles are important predators of herb seeds that are scattered on the ground after dispersal. Beetles prefer particular species of seed, but the causes of this selectivity are not clear because the seeds of different herb species vary in many characteristics. To reveal which factors determine carabid preference, we offered seeds of the multispecies complex of Taraxacum agg. (dandelion) to 19 carabid species. The seeds were similar in shape and morphology, differed in taxonomic affiliation (sections Ruderalia and Erythrospermae) and geographic origin (Czech Republic and Italy, c. 800km apart) and ranged in size between 0.27 and 0.74mg. The seeds were offered in cafeteria experiments where beetles selected between two (dual-choice experiments) or four (multi-choice experiments) types of seed. As expected, the size of the seed and the size of the carabid species determined the number of seeds consumed and the preference for the seed (the proportion of the total number of seeds consumed). In dual-choice experiments providing Ruderalia seeds, the preference for large seeds increased with carabid size, while in multi-choice experiments, seed size was the factor that explained the most variation in seed preference. In dual-choice experiments using seeds of a similar size, some species of carabid had a marked preference for Ruderalia (nine species) or Erythrospermae (four species) and were also able to distinguish between Ruderalia of Czech origin (preferred by eight species of carabid) and Italia (no preference). Preference for seed size could be enforced by matching the morphology of carabid mouthparts and the size of the seed. Effects of taxonomic affiliation and geographic origin on preference indicate that stimuli on seed coat and surface are also important for carabid choice, but underlying mechanisms of seed selection remain to be tested.