The role of fire in the ecology of Leichhardt's grasshopper (Petasida ephippigera) and its food plants, Pityrodia spp.

Leichhardt's grasshopper (Petasida ephippigera White) is endemic to the sandstone heath communities of the ‘Top End’ of the Northern Territory (NT), where it feeds almost exclusively on a few species of shrubs within the genus Pityrodia. There is some evidence that the distribution of Petasida is in decline and it has been suggested that adverse fire regimes are responsible. The major aim of this study was to investigate the impacts of fire regimes on populations of the grasshoppers using the tools of Population Viability Analysis, including population and fire simulation models, and to develop management recommendations based on these results. To this end, studies were conducted to investigate and describe the habitat of the grasshoppers, the population biology of Pityrodia and the grasshoppers, and the patterns of past and present sandstone fire regimes in the Top End. A review of the literature reveals that information on traditional Aboriginal fire regimes in the sandstone heaths is sparse, but what evidence there is strongly indicates that under contemporary regimes fires are later, more intense and larger in extent. Many of the relevant studies, however, are based on the interpretation of satellite imagery at a resolution too coarse to show the fine scale burning patterns that are crucial to understanding the fire ecology of Petasida. Fine-scale transect data collected within sandstone heath firescars and analysed by Price et al. (2003) was re-examined in order to describe the spatial patterns of the burnt areas, in ways which were not attempted in the previous study. Additional transect data were collected within areas supporting populations of Pityrodia. The results provide a good description of the internal spatial structure (patchiness) of fires upon which to base fire models, and suggest that in some circumstances entire populations of Petasida might fall within unbroken burnt areas. Assessment of fire history variables extracted from satellite-based firescar maps revealed no significant differences between sites with and without Petasida populations present. Habitat studies focussed on the presence of Pityrodia as a indicator of grasshopper habitat, and used environmental and floristic data in an existing database for Nitmiluk National Park and from field studies in Kakadu and Nitmiluk National Parks. Environmental relations were investigated using Generalized Linear Modelling (GLM) and floristic relations using Non-Metric Multidimensional Scaling (NMDS) ordination and Indicator Species Analysis. The results support previous observations that the Pityrodia species upon which Petasida feeds are confined to sandstone habitats and that their distribution is distinctly patchy at a range of scales. Pityrodia presence is associated with rock cover, particularly of large rocks and boulders, with open vegetation and with shallow, sandy soils. The floristic associations of Pityrodia are dominated by sandstone heath species, particularly short-lived obligate seeder shrubs. The results suggest that Pityrodia habitat is subject to fire regimes of intermediate frequency and some patchiness. Pityrodia population studies were primarily aimed at answering the question of whether or not fire affected the amount or extent of resources available to grasshoppers. Quadrat based studies were used to assess the impact of both fire and the absence of fire on the density of Pityrodia stems, and to investigate mortality and recruitment. Surveys for Petasida were conducted at several locations across the 'Top End' of the Northern Territory. Mark-recapture studies were conducted over four seasons, mostly in the Gubara area of Kakadu. The density of Pityrodia stems increased after fire and decreased in the absence of fire. Two examples of mass mortality of stems in the absence of fire, in different species, are reported. Most Petasida populations were very low and sparsely distributed. One population approximately doubled annually for two years until it was reduced after half the Pityrodia patch it occupied was burnt. Several local extinctions of Petasida are reported, some of which were clearly not caused by direct mortality due to fire. The dispersal ability of Petasida is relatively low, but a 'fattailed' movement distribution indicates occasional longer-distance dispersal, possibly by flying rather than walking. Two cellular models were created using Microsoft Excel© 2002 and the programming language VBA. The models are simple, discrete-time, count-based (unstructured), stochastic population growth models coupled with cellular landscape models in which fires with spatially realistic characteristics operate on the grasshopper populations. The first, fine scale, model was used to model populations in a landscape consisting of a single habitat patch of 1 ha and explicitly modelled the internal spatial structure (patchiness) of fires. The second modelled several habitat patches and incorporated dispersal of grasshoppers between patches and fires of varying sizes. In simulation results the probability of quasi-extinction of grasshopper populations was very sensitive to mean fire interval, total unburnt area within firescars and to fire size. The sensitivity of the results to several estimated or arbitrarily set parameters was investigated. The results suggest that all the changes to fire regimes since the transition from traditional Aboriginal to contemporary burning patterns are detrimental to populations of grasshoppers at the scale of the model landscapes. Management recommendations for the grasshoppers are precisely consistent with those for the conservation of obligate seeding sandstone heath shrubs. It is recommended that a strategic and targeted program of on-ground and aerial ignition be used by land managers to create a pattern of firebreaks in the early dry in order to reduce the annual area of intense, late dry season burning. Recommendations for ongoing research and monitoring are presented.