Edge and disturbance effects on forest floor invertebrates in two Hobart urban reserves

This paper describes a study being conducted on forest floor invertebrates in two adjacent bush reserves in Sandy Bay, Hobart. The Hobart City Council (HCC) manages approximately 3000 hectares of bushland and included in this area are Lambert Park (4.8 ha) and Bicentennial Park (51.7 ha), 3.5 km southeast of the centre of Hobart in the suburbs of Sandy Bay and Mt Nelson. In 1948 the HCC reserved bush as the Skyline Reserve‚ÄövÑvp along the Mt Nelson skyline ridge and the upper and middle Lambert Rivulet catchment. In 2004 this was expanded by 45 ha and the entire 180 ha reserve, which sits above Churchill Avenue, became Bicentennial Park. On the lower side of Churchill Avenue, 10 acres was donated to the City over 100 years ago, forming the wet gully forest reserve of Lambert Park. Both parks retain much original bush vegetation, ranging from wet closed forests to dry sclerophyll woodland surrounded by rocky outcrops, and wet streamside vegetation. Soils vary from shallow and sandy to clay soils ‚Äö- siltstone and mudstone derived soils in Lambert Park and dolerite derived soils on the upper Mt Nelson slopes. Wet gully forests surround Lambert Creek and its two tributaries, with drier forests bordering the urban edges characterised by residential properties along the upper slope margins, typically with exotic to semi-exotic gardens and generally open fence lines. These are associated with a range of introduced grasses and weeds found along park margins, as well as woody weeds like Cotoneaster. Both parks are a focus for recreational and commuter walking. Two publications (Hird 1995; AVK 1998) describe their general context, vegetation, and vertebrate fauna. There are no publications that describe the invertebrate fauna of either reserve. In this article we describe a study being conducted on forest floor invertebrates, which forms a project under the new Student Directed Inquiry (SDI) syllabus subject for the TCE students at Hutchins School, Hobart. The school is adjacent to Lambert Park. The effectiveness of urban reserves for the conservation of invertebrate diversity is partially dependent on the degree to which exotic species are prevented from invading a range of habitats, which is in turn dependent on the size of the reserve and the level of disturbance from clearing, fire, vegetation/weed invasion and maintenance, track development and management, etc. (Gibb & Hochuli 2002). A primary aim of the study is to assess the influence of reserve width and distance to urban edges on the presence of exotic invertebrate species. The two reserves are typified by having sharp boundaries between native bushland and the adjacent suburban blocks. We are making a series of measurements to rate the level of disturbance at each study site and relate it to distance from the urban edge‚ÄövÑvp and the composition of pitfall trap catches. The size and cover of reserved forest fragments and soil moisture are also important factors that influence arthropod diversity in urban reserves (Watts & Larivi‚àö¬Ære 2004), and these are also being measured. Our study is being conducted during autumn-spring of 2008 in Lambert and Bicentennial Parks, with pitfall trapping of invertebrates and collecting data on habitat features. Pitfall trap data is being used to compare the diversity and abundance of forest floor invertebrates at ten sites selected along a gradient of reserve width and proximity to the urban edges of the reserves. In this article we present the study methodology and some preliminary results.