Doctor of Philosophy
School of Biological Sciences
Gooden, Ben William, Native community responses to alien grass invasion: role of landscape and native plant functional traits in mediating invader impacts, Doctor of Philosophy thesis, School of Biological Sciences, University of Wollongong, 2014. http://ro.uow.edu.au/theses/4204
It is well-known that alien plant invaders significantly threaten the diversity and ecological function of native plant communities. Recent research has shown that impacts of invasion are dependent upon invader attributes, yet it is poorly known whether landscape context and functional attributes of resident native plants also mediate invader impacts on the recipient community. I used the model system of invasion by the alien, stoloniferous grass Stenotaphrum secundatum (Walter) Kuntze into an endangered and highly-fragmented coastal swamp forest community along the southern coastline of New South Wales, Australia, to determine whether impacts of invasion are dependent upon extrinsic landscape processes, such as anthropogenic habitat modification and nutrient enrichment, and/or functional attributes of the resident native plants. Knowledge of this will improve our capacity to predict the types of landscapes and community contexts most likely to be impacted by alien plant invasion.
I first used a spatially-extensive field survey of S. secundatum-invaded and noninvaded forest sites to examine native plant community response to invasion across a gradient of anthropogenic landscape modification. I also extracted soil and used a seedling emergence experiment to examine impacts of invasion on the seed bank community. At each site I measured local disturbance and environmental attributes of both the community (e.g. fire severity, litter abundance, canopy openness, vegetation structure) and adjacent landscape matrix (e.g. cover of forest, urban and agricultural land). I predicted that invasion and landscape modification would synergistically impact the forest community and its seed bank, such that the rate of native plant species loss in response to invasion would increase with the extent of urbanisation in the matrix. Invasion by S. secundatum was associated with substantial reductions (~85%) in native species richness, density of recruits and altered community compositions within the standing vegetation. Invasion caused only moderate reductions (~35%), however, in seed bank species richness. Importantly, impacts of invasion on the seed bank were nonrandom and varied across functional groups. Native herb and graminoid species (which share similar growth forms and root morphologies with S. secundatum) as well as short distance and animal dispersed species had reduced species richness in invaded sites, whilst the number of woody and wind and water dispersed species were unaffected by invasion. Standing vegetation and seed bank species richness were unaffected by local disturbances and landscape context. Furthermore, anthropogenic landscape modification did not moderate the effects of invasion on the community, such that species losses in response to invasion were high regardless of the condition of the adjacent matrix. Invasion had no effect on soil nutrient concentrations but caused a two-fold increase in litter biomass, which may be the driver of native plant recruitment limitation.
Next, in order to determine the likely mechanism by which S. secundatum invasion limits native plant recruitment, I intensively sampled the (1) abundance and frequency of occurrence, (2) reproductive effort (flowering) and output (fruit production) and (3) seed bank densities for three focal native plants from the invaded forest. Invasion reduced the biomass (but not likelihood of occurrence), flowering effort and reproductive output (~75%) of each species. However, invasion had no effect on the species’ seed bank densities, despite the substantial reduction in their reproductive output. Coupled with results from the community-scale seed bank study, this indicates that S. secundatum invasion disrupts native plant recruitment by limiting post-settlement emergence of propagules from the seed bank rather than their supply and storage at invaded sites.
Finally, I contextualised these three correlative studies by performing a community-scale, invader-addition experiment. I tested wether impacts of invasion are dependent upon functional attributes of the resident native plants and/or anthropogenically-driven nutrient enrichment of the coastal forest. Experimental plots contained 18 species (drawn without replacement from a pool of 31 species) with either runner, tufted or woody growth forms. Species growth (% cover), reproductive output, soil temperature and light availability were monitored for two growing seasons in response to S. secundatum invasion and nutrient enrichment. Species richness, community composition, reproductive output, soil temperature and light penetration were unaffected by invasion and nutrient enrichment. Invasion reduced community productivity, but this effect was not moderated by nutrient availability. Furthermore, the impact of invasion on community productivity was non-random and driven only by reduced biomass of functionally-similar native runner species.
My research has shown that impacts of invasive plants on native communities are non-random and strongly mediated by functional attributes of the resident native plants, rather than either landscape context or local community disturbances. My results support the hypothesis that native species functionally similar to invaders are more likely to be displaced from invaded communities than functionally dissimilar ones. The likely mechanism by which invasion disrupts native populations is post-settlement recruitment limitation from the seed bank. Management of invaded communities must consider differential impacts of invasion in the community and actively reintroduce native species, such as those with short distance modes of dispersal, which may be able to spontaneously regenerate following invader control.