Seed predation of the tussock-grass Stipa tenacissima L. by ants (Messor spp.) in south-eastern Spain: the adaptive value of trypanocarpy

Seed predation by animals has been shown to be an important factor in the evolution of dispersal mechanisms and diaspore morphology of plants. We studied seed production and predation of the perennial tussock-grass Stipa tenacissima L. in “El Desierto”, a semi-desert area in south-eastern Spain. As almost all other species in the genus, this plant produces diaspores with a long awn capable of hygroscopic movements that may lead to a drilling of the basal part of the diaspore into the soil substrate, to inserting into crevices or to lodging under rock fragments (trypanocarpy). Three species of granivorous ants (Messor barbarus, M. timidus, M. bouvieri) consume a substantial proportion of the seed production of S. tenacissima. We show in detail that the presence of the awn and the drilling mechanism of S. tenacissima reduce the intake rate of diaspores by Messor spp. dramatically. The long awn (mean overall length 56.8 mm) makes the diaspore cumbersome and heavier, so that it can only be retrieved much more slowly than a diaspore without the awn. Cutting of the awn by an ant requires a long time (mean 9.35 min) and small workers (head width < 1.3 mm) are incapable of it altogether. Diaspores drilled into the ground can still be excavated by workers. However, once the awn breaks off by weathering at the distinct preformed breaking point at the base of the awn, diaspores have practically escaped predation because ants cannot locate them underneath the soil surface. These results are consistent with the hypothesis that trypanocarpy is a very effective mechanism against seed predation in S. tenacissima. We also evaluate how seed predation by Messor spp. affects the population structure of S. tenacissima and caution against categorizing this grass as a mast species as previously suggested. Finally, we discuss the implications of pre-dispersal seed predation by Messor spp. for the interaction with S. tenacissima and other plants.