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The impacts of environmental change on dispersal of large-seeded forest species by the emu

Nield, Andrew (2014) The impacts of environmental change on dispersal of large-seeded forest species by the emu. PhD thesis, Murdoch University.

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Seed dispersal is a critical stage in the life-cycle of plants. It is the process by which seeds escape the potentially deleterious density-dependent effects of deposition beneath conspecifics and other individuals. Long distance dispersal (LDD) – the movement of seeds considerable distances away from the parent plant – strongly influences plant population structures and dynamics. In response to global environmental change, LDD is a key mechanism by which species may be able to establish in new habitats and climatic ranges.

The emu (Dromaius novaehollandiae) is an important LDD vector in Australian ecosystems, capable of traversing many kilometres over a few days, and retaining large amounts of seed within the gut for up to a few weeks. In this thesis, the population dynamics of four large-seeded species within the jarrah (Eucalyptus marginata) forests of southwestern Australia was investigated within areas of contrasting high/low emu abundance. The focal plant species could all be dispersed by the emu and represented different life histories; Leucopogon nutans (Ericaceae), a fire-killed, short-lived seeder shrub, Persoonia elliptica (Proteaceae), an epicormic resprouting tree, Macrozamia riedlei (Zamiaceae), a long-lived, apical resprouting cycad, and Podocarpus drouynianus (Podocarpaceae), a long-lived, basal resprouting coniferous shrub.

Plant demographic assessment revealed that there was no difference in population structure, demographic rates or spatial structure for any of the species in contrasting sites of high/low emu abundance. A spatial simulation model and Bayesian change point analysis highlighted the difficulties in detecting a change in spatial structure following dispersal agent loss. For the three larger species, extreme plant longevity and environmental heterogeneity make it difficult to detect a change in plant spatial pattern following dispersal agent loss. For the shorter-lived L. nutans, high seed production and low frequency of removal masks any effect of whether emus are present or not in terms of population structure.

Visitation and fruit removal rates were generally low for each species, irrespective of emu abundance. The dispersal agent community was depauperate, with only a few vertebrates acting as local dispersal agents for each focal species. The emu was observed to ingest all of the focal species except P. elliptica. Telemetric tagging of M. riedlei seeds revealed that the Australia Raven (Corvus coronoides) acts as a novel LDD vector, regularly dispersing individual seeds > 300 m from the parent plant.

Spatially-explicit simulation modelling of emu movement, informed from the Global Positioning System (GPS) tracking of birds within jarrah forest showed that the emu is a highly mobile and effective LDD vector, and could disperse many seeds > 1500 m from the parent plant and up to a maximum dispersal distance of ca. 7000 m during a 1000-hr simulated period. Modelling also revealed that the emu (and subsequent seed dispersal) is highly sensitive to changes to landscape configuration and composition. Simulated increases in habitat fragmentation and disconnection substantially reduced seed dispersal distances and collapsed LDD.

The emu is a highly effective seed dispersal agent (in terms of both quantity and quality of seeds dispersed), yet remains directly persecuted in some agricultural areas, and faces the same population pressures from habitat loss as other large organisms. The emu has shown a remarkable resilience to these pressures, and provides a pathway for plant species to expand their ranges under global environmental change. However, in order to fulfil this function, the emu requires a relatively contiguous landscape. Future work should focus on understanding the long-term (seasonal) movements of the emu to provide a complete picture of how this species contributes to seed dispersal in a changing landscape.

Publication Type: Thesis (PhD)
Murdoch Affiliation: School of Veterinary and Life Sciences
Supervisor: Enright, Neal and Ladd, Phil
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