Dispersal adaptations of some acacia species in the Australian arid zone

Most Australian representatives of the genus Acacia have diaspores with arillate appendages indicative of adaptation for active dispersal by animals. Based on physical and chemical characteristics of these arils and mechanisms of diaspore presentation, a number of arid zone acacias can be distinguished as probable ornithochores, myrmecochores, or species lacking active dispersal by animals. Two factors suggest that dispersal adaptations are evolutionarily labile in the face of changing selection pressures. Both myrmecochores and ornithochores are prevalent in each of the three largest taxonomic sections of the Australian subgenus Phyllodineae. Second, diaspores of at least one species, Acacia ligulata, exhibit geographic variation, resembling those of myrmecochores in one population and ornithochores in a second population. The colorful lipid-rich arils of avian-dispersed species are nutritionally more valuable than the relatively small, white appendages of ant-dispersed species with similar-sized propagules. While ants often collect the diaspores of ornithochorous acacias, we have no evidence that birds use the arils of myrmecochores. Patterns of seedling establishment on ant mounds, and under the canopies of trees where birds perch and defecate, are consistent with these observations. Exploitation of diaspores by birds may help to reduce the destruction of seeds by parasitoids and, in part, compensate for the higher aril expenditures (per milligram of propagule) in avian-dispersed species. Dispersal of seeds by both ants and birds directs seeds to microhabitats where nutrients are concentrated and water resources are more plentiful and/or are used more efficiently. Similarities between mound and subcanopy microhabitats may have facilitated evolutionary transitions between myrmecochory and ornithochory. The nutrient poverty that generally characterizes Australian soils has possibly played a role in selecting for seed dispersal by ants and birds in this genus as well as in many other Australian plants.