Large seeds, extinct vectors and contemporary ecology: testing dispersal in a locally distributed cycad, Macrozamia lucida (Cycadales)

The possible reasons for the localised distribution of Macrozamia lucida Johnson in south-eastern Queensland were investigated through testing and quantifying aspects of seed dispersal, ex situ germination across local substrates and the spatial distribution of plants of different ages. Seeds of cones exposed to animals were moved significantly further (108.4 ± 13 cm, n = 10 cones) than those of cones not exposed to animals (49 ± 11 cm, n = 4). Only two mammal species interacted with seeds, common brushtail possums (Trichosurus vulpecula) and bush rats (Rattus fuscipes), but relatively infrequently. The possums were both seed dispersers and predators, whereas the rats ate only sarcotesta. Seed predation and removal occurred at a constant but low rate at cones exposed to animals, compared with no seed removal at unexposed cones. Seeds with sarcotesta intact and sarcotesta alone suffered significantly higher rates of removal by mammals. Seeds in soil had a significantly higher rate of germination (50%) than those in gravel (24.4%) or leaf litter (25.6%) and partially buried seeds germinated at a significantly higher rate (51.2%) than those on the surface (23.2%) and those buried completely (25.6%). In summary, this population appears to be localised spatially by low rates of dispersal, restricted dispersal distance and limited sites for successful germination. The results are discussed in relation to the possible extinction of the original seed dispersers of these organisms and the limitations this imposes on these plants should they be exposed to an unfavourable climatic regime.