Diversity, patterns and processes of vegetation and invertebrate orders in natural temperate grassland in the Australian Capital Territory

Abstract

The study aimed to gain understanding about diversity, patterns and processes in natural temperate grassland sites in the ACT. Thirty natural grassland sites were surveyed for plant composition, soil invertebrate abundance and site attributes. Floristic associations were identified using multivariate analysis. These were compared to the other site attributes to determine related characteristics.
Total plant species richness was 191 species, of which 41% were exotics. Forbs comprised 56% of the total species, with 69 native forb species. There were 10 native grass species and 12 sub-shrubs recorded. The number of species found within ten 1 m2 quadrats measured in each site ranged from 23 to 56 species.
Three floristic associations were identified through classification analysis using frequency data. The Danthonia association occurred in well drained sites on soils with high clay content and low nutrient levels and a greater percentage of bare ground. The sites were assessed as having been subject to moderate to high disturbance in the past and 46% were grazed and 46% mown. Species richness of both natives and exotics was high (quadrat richness averaged 37 species, with 32.8% exotic cover in spring). The Wet Themeda association occurred in poorly drained sites and had also been subject to moderate to high disturbance; 67% of sites were grazed and 25% mown. Phosphorus levels were higher in these sites and the acidity of the soil was also greater. Quadrat richness averaged 39 species, with 35.5% exotic cover in spring. The Dry Themeda association was well drained. These sites had a lower native and exotic diversity, with a mean quadrat richness of 30 species, and 11.1% exotic species cover in spring, but a high species richness in the sites (mean 65.4 species) and a high litter cover of 39.9%. All have been subject to low disturbance and none were grazed. Phosphorus levels were low (7.9 ppm). Wet Themeda grassland sites were more similar to Danthonia grassland sites in terms of species occurrence and frequency than Dry Themeda grasslands, despite both associations being dominated by Themeda australis. While trends emerged regarding management and disturbance levels in the three associations, these were not major differentiating attributes.
Invertebrates were collected from soil samples from the sites and identified to order level. Twenty two orders were found, but only three orders, Acarina, Collembola and Coleoptera, were sufficiently abundant to enable statistical tests to be undertaken. Invertebrate order richness and abundance showed strong relationships with vegetation attributes that measured or reflected vegetation structure, with higher abundance in sites dominated by T. australis and where wet soil colour was darker. Both order richness and total abundance of invertebrates were highest in mown sites and lowest in grazed sites. The invertebrate order richness and abundance did not have significant relationships with the floristic associations.
A field experiment was conducted in which above- and below-ground gaps were created using the herbicide, Glyphosate, to remove interspecific competition and comparing responses between the Dry Themeda and Danthonia associations. Litter load and soil disturbance were also manipulated. The subsequent establishment of species was studied over two growing seasons, a period of 18 months. Both native and exotic forb richness and cover increased as a result of gap formation. Exotic grass cover and richness showed a strong increase in the first season, but after 18 months had decreased to levels similar to those prior to the application of treatments. Native grasses had not recovered to pre-treatment levels after 18 months. Native forb richness and cover were not inhibited by litter retention, but exotic forb richness and cover were higher in treatments with litter removal. Soil disturbance to a depth of 20 mm had no significant effect on the recruitment of the species.
Recommendations for protection and management of the sites were developed from the study. Conservation recommendations are based on the aim of maintaining or enhancing native species diversity and habitat, while protecting threatened species and their habitat. Management plans should recognise requirements based on the floristic associations, diversity of native species, drainage conditions and retention of invertebrate habitat, including structural requirements. Grazing should be retained as a viable management regime, particularly in sites where small species occur. Future research should aim towards a greater understanding of the effects of management on species diversity, including invertebrates, and include a monitoring component with which to determine if management actions should be modified.

Date of Award	1997
Original language	English
Supervisor	David Williams (Supervisor)