Seed fate and distribution of ant-dispersed plant species in Australia
thesisposted on 2022-03-28, 17:01 authored by Mary Louise Rodgerson
Plant species adapted for ant dispersal occur in many different types of habitats, but are most common in sclerophyll vegetation (which occurs on infertile soils) in Australia and South Africa. The investigations presented in this thesis were centred around the potential selective forces imposed by ant behaviour on seeds adapted for ant dispersal in sclerophyll vegetation around Sydney, Australia. Two complementary questions were posed: 1) How does variation in behaviour between ant species affect the fate of elaiosome-bearing seeds? and 2) Can differences in the behaviour of ant species on infertile compared to fertile soils provide an explanation for the fact that plant species adapted for ant dispersal are much more common on infertile than fertile soils, in Australia? -- Ant behaviour may affect the fate of seeds in a number of different ways including predation, dispersal distance and burial depth. Some of the ways in which a plant may respond over evolutionary time to ant predation and the chance of burial were assessed. The factors examined were seed release by the plant (specifically the quantity and duration of seed release), the composition of the ant diet (in terms of the ratio of insects to seeds) and the presence of larvae within the ant colony. The rationale behind these two last factors were that they both vary seasonally, therefore, a plant has the evolutionary potential to adjust its seed release to periods when seed predation is less likely to occur. -- Seed predation assays were performed on laboratory colonies of Rhytidoponera metallica, which were kept in artificial nests. R. metallica is a common seed-removing species in sclerophyll vegetation. These nests enabled both the number of seeds eaten and the number of seeds remaining buried within the nest to be unambiguously determined. To examine how seed quantity may affect both the chance of seed predation and seed burial, four species were used, namely Bauera rubioides, Boronia pinnata, Dillwynia juniperina and Poranthera ericifolia. The quantity of seeds which entered the nest did not affect either the level of seed predation or the incidence of seed burial for B. pinnata, D. juniperina and P. ericifolia, however, for B. rubioides the number of seeds eaten increased with the quantity of seeds. Variation in the duration over which seeds entered the nest was assessed for D. juniperina and P. ericifolia, and was found to have no effect on the level of either seed predation or seed burial. -- The effect of dietary composition for R. metallica, was investigated by varying the ratio of insects:seeds in the diet and was found to have no effect on the level of either seed predation or seed burial. Similarly, the presence/absence of larvae in the colony did not affect the level of seed predation or the chance of seed burial. -- A common view (Morton 1985, Andersen 1988, Davidson and Morton 1988, Hughes and Westoby 1992a and b) has been that seed predation by ants may be an important selective force in Australia. The results from the set of experiments described above were contrary to this view and so an explanation was sought. The mechanical properties of a range of species adapted for ant dispersal were assessed in terms of defence strategies against seed predators. Seed strength was measured using two techniques, a Universal Force Tester and pressure applied with forceps. Most species tested produce relatively strong seeds, though a significant minority had relatively soft seeds. Laboratory seed predation assays revealed that the strength of the seed appeared to be related to the level of seed predation in that strong seeds were rarely eaten by R. metallica and Pheidole sp. 4. In contrast, species with relatively soft seeds experienced much higher levels of seed predation, up to 100%.