Shifts in species and functional diversity along environmental gradients in a threatened Australian annual plant community

Background/Question/Methods

Recent research has highlighted the emergence of novel species assemblages resulting from changing environmental conditions, however little is known about the function and significance of these novel systems. To address this deficiency, we are exploring novel annual plant assemblages within York Gum (Eucalyptus loxophleba) woodlands throughout the Western Australian Wheatbelt. Most woodlands in the wheatbelt were cleared during the 20^th Century and converted to wheat cropping. Thus the remaining small areas of woodland have high conservation value and are subject to ongoing degrading processes, i.e. fragmentation, grazing, fertiliser drift and weed invasion. In addition, the region’s climate is predicted to dry in coming decades, thereby increasing uncertainty about the future composition and function of these communities. We aim to answer the following questions: (1) Does annual plant species richness and composition change in relation to climatic, edaphic and land-use factors? (2) How do plant trait distributions shift along environmental and land-use gradients? and (3) How does functional diversity relate to species diversity along these gradients? A survey of annual plant assemblages was undertaken in September – October 2010 across existing environmental gradients, with sites stratified into edge and core categories. The sampling design was hierarchical with communities recorded at the local scale (0.09 m²) within multiple swathes within multiple remnants across the study region. A number of traits were measured from more than 3000 plants. Hierarchical Bayesian models were used to explore relationships between trait distributions and environmental variables. 

Results/Conclusions

Richness in edge and core assemblages was similar, but tended to be more variable between quadrats within edge communities. The number of exotic species was greater in edge situations, and this was most neatly explained by soil Phosphorus levels. Higher rainfall areas also supported more exotic species. Community composition shifted along climatic gradients, but these shifts were subtle compared to shifts associated with soil nutrient increases. We identified a number of disconnects between trait distributions and species richness along the gradients explored. For example, plant height declined with declining annual rainfall, while species richness did not.  In conclusion, environmental changes associated with agricultural development in the Wheatbelt are strong drivers of the assembly of novel communities and associated shifts in functional diversity. Our results suggest that landuse changes interact with existing climatic gradients by producing more pronounced shifts in functional diversity than species diversity, particularly in higher rainfall areas.