Seed harvesting by ants in forests of Eucalyptus regnans F. Muell. in central Victoria

Abstract

In an unburnt, mature forest the germination of E. regnans on undisturbed bare ground is very poor in spite of adequate seed fall, and the presence of conditions suitable for germination over much of the year. No seed storage occurs in the top soil in spite of some temporary seed dormancy and the disturbance of the surface soil by earthworms and lyre birds. Seed is removed from the top soil by several ant species– Prolasius pallidus, P. brunneus, P. flavicornis and Chelaner leae –and is taken into nests. Removal of seed is more rapid and complete in summer than winter and is more vigorous in young forest than old. Observations on artificial nests in the field and laboratory indicate that seed is eaten and not stored for any length of time. The testae may be left intact, fragmented or moulded into crumbs with other material. Certain sugar-like substances, which are extractable in low concentrations from fresh seed, may be involved in the attractivity of seed to ants. The numbers of ant species in the forests vary with the micro-climate of the site, and in the mature understorey of Pomaderris aspera the activities of the three dominant species Iridomyrmex biconvexus, Prolasius pallidus and P, hrunneus are largely separated diurnally, seasonally, and spatially in the foraging areas of the litter layer. The density of the nests of seed-harvesting ants is high, particularly in bare areas of the forest floor. The total number of ants probably exceed 5–6 million/ha and is probably sufficient to effectively remove 60% of the sporadic seed fall of normal years. It is suggested that the success of germination of E. regnans after wild fires is not due to any specific stimulation but rather to a temporary interference of ant foraging activity and then to the saturation of their food requirements by a massive seed fall from canopy-stored capsules.