A stochastic model of seed dispersal pattern to assess seed predation by ants in annual dry grasslands

Abstract

A combined field experiment and modelling approach has been used to provide evidence that ants may be responsible for an observed lower patchiness and higher plant diversity in the neighbourhood of ant nests, within Mediterranean dry grasslands belonging to the phytosociological class Tuberarietea guttatae. The hypothesis was that seeds occurring in clumps may have an higher probability to be harvested than seeds having a scattered distribution. In order to test this hypothesis, four analysis steps have been performed. First, the seed productivity and dispersal pattern was recorded for four plant species found, either more abundant beside the ant nests (Tuberaria guttata, Euphorbia exigua) or away from the ant nests (Bromus scoparius and Plantago bellardi). Second, a stochastic model was developed to simulate the observed dispersal patterns of each studied species. Third, ten seed arranges in accordance to the distribution patterns created by the model were offered to the ants and the positions of the predated seeds was recorded. Finally, the observed pattern of seed predation was matched to models performed by different distributions of probability. Results confirmed the initial hypothesis, showing that the probability of being predated decreases with the increase of the reciprocal distance of seeds. It was concluded that the preference of ants for high concentrations of food items holds down the dominant species sufficiently to allow the subordinates to survive. The observed higher frequency of small-seeded, small-sized, or creeping therophytes close to the ant nests can be therefore seen as an example of indirect mirmecophily.