The ecology of mutualism in Acacia myrtifolia

A popular and thoroughly studied case of an interdependent mutualistic relationship is that of the American swollen-thorn acacias and their ant inhabitants (Janzen, 1966). Whether such interactions exist amongst Australian acacias remains unknown, despite the fact that they represent one of the most abundant and speciose groups of vascular plants on the continent. Ants are remarkably diverse in Australia, widespread, can act as generalist predators, and are frequently seen feeding on acacia’s extrafloral nectaries. In this thesis I address gaps in our understanding of multi-species interactions of the Australian angiosperm Acacia myrtifolia, under field conditions. I conducted ant- and pollinator-exclusion experiments on 60 A. myrtifolia plants at four sites varying in moisture, temperature and fragmentation from western to eastern Victoria. The main field seasons (five) were carried out during 2010 with sampling periods at the end of 2011 and 2012, asking 1) whether the presence of ants produces favorable outcomes for the plants across a longitudinal gradient and 2) what is the combined impact of ants, herbivores, pollinators and seed dispersers on the plant’s fitness. The exclusion of ants did not enhance herbivory damage or increase fruit set. Ants have significant short temporal-scale effects on diaspore characteristics and seedling survival, and long temporal-scale effects on growth. There is a strong suggestion of a geographical and environmental gradient in ant community composition, seed-dispersing ant assemblages and some of the variables related to plant fitness (bud number, growth, seed weight, and seedling growth). The composition of ant guilds attending A. myrtifolia appears to affect the degree of benefit plants derive from this diffuse mutualism (dependence on multiple partners). In relating multiple interactions we gain an insight into the evolution of complex net interactions and the result of different trade-off decisions.