Processes explaining exotic plant occurrence in Australian mountain systems

Mellesa Schroder, February 2014



Mountain systems support diverse ecological communities and species adapted to extreme climatic conditions. They are also important for the ecosystem services they provide such as water resources for hydroelectricity generation and irrigation of lowland areas. Inaccessibility and extreme climate have largely protected mountains from wide scale disturbance. In the last century the richness of exotic plant species in mountains has increased on a world wide scale as these landscapes became more accessible through human activities and as temperatures increase under climate change.

Research on exotic plant species in mountains has focussed on exotic species richness in disturbed areas such as roadsides or human nodes such as ski resorts. Other exotic plant studies in mountains have focused on the strong inverse correlation between elevation and exotic species richness at higher elevations. There has been limited study on the role of abiotic (not including temperature) and biotic factors in explaining exotic species richness or abundance.

A smaller number of exotic plant species occur in undisturbed habitats in the Australian Alps when compared with many other lowland Australian natural systems although richness has increased since the 1950s. Exotic plant programs in the Australian Alps remain largely focused on disturbed areas with the greatest extent of exotic plant invasion. There is an increasing need to develop strategies to identify the potential invaders of undisturbed areas (natural habitats). However in order to detect these species we need to have a greater understanding of the processes which lead to exotic plant occurrence in disturbed and undisturbed habitats and the abiotic and biotic factors which may promote invasive species.

This study focussed on improving our understanding of the processes and had two main objectives to: (1) determine the degree to which elevation filtering can explain exotic plant occurrence in disturbed and undisturbed habitats and (2) establish the degree to which biotic and abiotic factors (other than elevation and temperature) explain differences in exotic plant richness and composition between disturbed and undisturbed habitat types. The research was undertaken in the montane, sub-alpine and alpine areas of Kosciuszko National Park, New South Wales. The park is part of the Australian Alps. Study sites were located at roadsides, human nodes and undisturbed habitats across an elevation gradient of 1000 – 2200 m. Data collection included abiotic and biotic factors such as exotic plant cover, soil samples and elevation.

My results confirmed expectations that disturbed roadsides and human nodes had greater richness and cover of exotic species than undisturbed sites. I found patterns of occurrence between roadsides and human nodes vary and management may need to be approached differently in the two areas. I also found that species composition and soil physical and chemical properties varied between disturbed and undisturbed habitats.

My results suggest that whilst elevation is important in explaining richness in undisturbed habitats, in disturbed habitats exotic plant richness can be explained by understanding interactions between elevation and soil attributes. I also found that exotic species composition in undisturbed and disturbed habitats is explained by a combination of mean temperature and soil attributes. It would appear that elevation (climate) may not prevent exotic plant species establishment at higher elevations and areas where soil disturbance has occurred are more vulnerable to exotic species establishment.

The results from this thesis improve our understanding of the drivers of exotic plant richness and composition in Australian mountain systems. To reduce the potential for increases in exotic species richness and cover anthropogenic disturbances must be minimised. This is even more important in light of predicted changes to climate. Future management should focus on predicting and targeting species which may pose the greatest threat to biodiversity.