A note on combining two methods of dispersal-for-distance



We ask under what circumstances two methods of dispersal-for-distance should be combined, given that the second method may carry the diaspore back towards its point of origin. The combination is made possible when the morphological adaptations of the diaspore are compatible. It is advantageous when the return on investment in the first method of dispersal declines sharply beyond some level of investment. The median seed achieves the best net distance when the two methods achieve similar distances; the upper decile of seeds achieve nearly a simple sum of the two distances. The first two conditions apply to the combination of ballistic with ant-dispersal, which is widespread in Australian sclerophyll shrubs. Mother plants’fitnesses could well be determined by the upper decile of distances their seeds achieve. It remains an open question whether the addition of ant-dispersal to ballistic dispersal achieves the selective advantage of distance or of placement.