The potential impacts of climate change on plant species are predicted to be extensive, and those with small populations and low dispersal are predicted to be among the most vulnerable to climate change. Many studies have modelled the potential impacts of climate change on plant population persistence. However, the majority are based on geographical and a-biotic factors and seldom take into account the potential effects on species population dynamics. Triunia robusta is a long-lived, endangered subtropical rainforest shrub which was reported to have less than 400 adult plants across 11 small populations in Southeast Queensland of Australia. This research investigated the potential impacts of climate change on the population viability of T. robusta by integrating species distribution models (SDM) and population models using a coupled model approach. This study re-censused all known populations from a previous 1999 survey and six newly discovered populations. Surveys were undertaken to identify potential dispersers, and dispersal and predation dynamics in both natural and green house environments. The potential impacts of climate change on the species habitat distribution were investigated at both continental and regional scales using a species distribution modelling approach. Finally, the potential effects of climate change on population viability of the species were examined over a 90-year period using integrated metapopulation models. Dramatic contraction of the species habitat range was predicted within the species distribution range, with the habitat areas showing southward contraction toward higher elevations into the Sunshine Coast hinterland. Some populations are already in decline particularly in the northern range, where populations showed much lower growth and reproductive rates than those in the southern range, possibly early signs of climate change impacts. In contrast, larger populations (approximately 100 or more plants including seedlings) in the southern range have increased over a ten year period, highlighting the importance of maintaining larger population sizes for a long-term survival. The population viability analyses revealed high vulnerability of small populations (<30 plants including seedlings) to local extinction regardless of geographical location, whilst larger populations in the southern range are likely to show persistence in-situ for at least the next 90 years under the effects of climate change. The models predicted greater impacts due to changed temperature and rainfall on the species population viability than habitat loss. Only short-distance seed dispersal (maximum 10.3m) by local small mammals was documented within the study time frame, indicating the species limited potential for migration or rescue effects under climate change, thus populations are more likely to be acting independently. It is suggested that priority should be primarily given to small and isolated populations in the northern part of the currently known species distribution range, which were predicted to be under high risk of extinction. Conservation efforts may involve seed collection and propagation, and assisted revegetation to ensure the populations to maintain viable population size under climate change. On the other hand, larger populations located in the southern part of the species distribution range showed relative resilience within the modelled 90 year period under all climate change scenarios tested in this study. Conservation efforts should be also placed to these populations as these may become the potential refugia or translocation sites for T. robusta in the future. This study demonstrated that using coupled models to integrate complex interactions between life history, disturbance regime and distribution pattern could provide informative predictions on extinction risk of endangered, vulnerable and rare rainforest plant species under climate change, and suggest potential implications for future conservation and management planning for the species.
Submitted in the fulfilment of the requirements of the degree of Doctor of Philosophy, University of the Sunshine Coast, 2015.