MECHANICAL DEFENSE IN SEEDS ADAPTED FOR ANT DISPERSAL

Abstract

In this paper I seek an explanation for the low level of seed predation recorded for many plant species adapted for ant dispersal by examining the mechanical defense properties of seeds. Mechanical defense of 115 plant species, in the form of seed strength, was measured using two techniques, a Universal Force Tester and pressure applied with forceps. Most species tested had relatively strong seeds (>20 N), though a significant minority had relatively weak seeds. Preliminary findings of contrasts across independent phylogenetic divergences suggest there was no consistent tendency for seed strength to be greater in species adapted for ant dispersal compared to their relatives that are not. In seed removal assays performed in the field, weaker seeds were more likely to be removed than stronger seeds, whether elaiosomes were present or not. Weaker seeds were also more likely to be eaten than stronger seeds. In laboratory seed predation assays, strong seeds were rarely eaten by two common seed-removing ant species (Rhytidoponera metallica and Pheidole sp. 4). In contrast, species with relatively weak seeds experienced much higher levels of seed predation, up to 100%. In conclusion, the majority of species adapted for ant dispersal produce relatively strong seeds, and this may explain the low level of predation recorded for these species.