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Notes: Abstract  Every year large proportions of northern Australia's tropical savanna landscapes are burnt, resulting in high fire frequencies and short intervals between fires. The dominant fire management paradigm in these regions is the use of low-intensity prescribed fire early in the dry season, to reduce the incidence of higher-intensity, more extensive wildfire later in the year. This use of frequent prescribed fire to mitigate against high-intensity wildfire has parallels with fire management in temperate forests of southern Australia. However, unlike in southern Australia, the ecological implications of high fire frequency have received little attention in the north. CSIRO and collaborators recently completed a landscape-scale fire experiment at Kapalga in Kakadu National Park, Northern Territory, Australia, and here we provide a synthesis of the effects of experimental fire regimes on biodiversity, with particular consideration of fire frequency and, more specifically, time-since-fire. Two recurring themes emerged from Kapalga. First, much of the savanna biota is remarkably resilient to fire, even of high intensity. Over the 5-year experimental period, the abundance of most invertebrate groups remained unaffected by fire treatment, as did the abundance of most vertebrate species, and we were unable to detect any effect of fire on floristic composition of the grass-layer. Riparian vegetation and associated stream biota, as well as small mammals, were notable exceptions to this general resilience. Second, the occurrence of fire, independent of its intensity, was often the major factor influencing fire-sensitive species. This was especially the case for extinction-prone small mammals, which have suffered serious population declines across northern Australia in recent decades. Results from Kapalga indicate that key components of the savanna biota of northern Australia favour habitat that has remained unburnt for at least several years. This raises a serious conservation concern, given that very little relatively long unburnt habitat currently occurs in conservation reserves, with most sites being burnt at least once every 2 years. We propose a conservation objective of increasing the area that remains relatively long unburnt. This could be achieved either by reducing the proportion of the landscape burnt each year, or by setting prescribed fires more strategically. The provision of appropriately long unburnt habitat is a conservation challenge for Australia's tropical savanna landscapes, just as it is for its temperate forests.

Notes: Abstract  Taken literally, the aim of biodiversity monitoring is to track changes in the biological integrity of ecosystems. Given the overwhelmingly dominant contribution of invertebrates to biodiversity, no biodiversity monitoring programme can be considered credible if invertebrates are not addressed effectively. Here we review the use of terrestrial invertebrates, with a particular focus on ants, as bioindicators in Australia in the context of monitoring biodiversity in Australia's rangelands. Ant monitoring systems in Australia were initially developed for assessing restoration success following mining, and have since been applied to a wide range of other land-use situations, including grazing impacts in rangelands. The use of ants as bioindicators in Australia is supported by an extensive portfolio of studies of the responses of ant communities to disturbance, as well as by a global model of ant community dynamics based on functional groups in relation to environmental stress and disturbance. Available data from mining studies suggest that ants reflect changes in other invertebrate groups, but this remains largely undocumented in rangelands. The feasibility of using ants as indicators in land management remains a key issue, given the large numbers of taxonomically challenging specimens in samples, and a lack of invertebrate expertise within most land-management agencies. However, recent work has shown that major efficiencies can be achieved by simplifying the ant sorting process, and such efficiencies can actually enhance rather than compromise indicator performance.

Notes: Ants are widely used as bioindicators in Australian land assessment and monitoring programs, particularly in relation to ecosystem restoration following mining. Little is known, however, about the relationship between ant community development and key ecological processes such as nutrient cycling. We have examined the relationship between ant species richness and soil microbial biomass at 17 sites subject to disturbance by mining in the Kakadu region of Australia's Northern Territory. The number of ant species recorded ranged from 7 at an unvegetated site undergoing restoration to 43 at a site that was undisturbed except for edge effects. Soil microbial biomass ranged from 19.3 to 134.3 μgC/g. Ant species richness was positively correlated with soil microbial biomass (r= 0.638), more so than was plant species richness (r= 0.342 for total plant species, r= 0.499 for woody species only). Our findings demonstrate a correlation between aboveground ant activity and belowground decomposition processes at disturbed sites, thereby providing support for the use of ants as indicators of restoration success following disturbance. Interestingly, when a range of undisturbed sites in the region was considered, a negative rather than positive relationship between ant richness and soil microbial biomass was found. This illustrates the importance of distinguishing between variation within a habitat due to disturbance and variation across different habitats when searching for indicators of ecological change.

Notes: The composition and structure of ant communities were used to assess the success of the preliminary restoration program at Ranger uranium mine in the seasonal tropics of northern Australia. Ants were surveyed at eight sites, including two relatively undisturbed control sits, within the Ranger lease. The revegetated sites represented a range of variables likely to influence restoration success: revegetation age (two, four, and eight years), proximity to undisturbed sites (which act as potential sources of recolonization), and burning treatment. Revegetation at most sites was dominated by fast-growing species of Acacia. There was a clear succession of ant species across revegetated sites. Initial colonization was by species of Iridomyrmex, but as plant cover and litter development increased these were replaced by broadly adapted, opportunist species, especially the introduced Paratrechina longicornis. Ant recolonization was very slow at isolated sites, with only 12 species present after eight years (the oldest site available). This compares with 21 species after only four years at a site located close to potential sources of recolonization. The ant community at this site, however, was very similar to that at another site located close to colonization sources, but eight years old. Ant succession therefore appeared to have stalled at this point, with species richness and composition bearing little resemblance to that at control sites. The heavy shade and litter produced by acacias were considered to be the major impediment to further change. Results from a site that had undergone a prescribed burn after two years, thereby breaking dominance by acacias and allowing for the establishment of a wide variety of plant taxa, suggest that such management practices may promote further colonization by ant species.

Notes: Ant communities have been widely used as indicators of minesite rehabilitation in Australia and are beginning to play a similar role in other parts of the world. Here we examine ant communities on rehabilitated ash dams associated with a coal-fired power station on the highveld of South Africa, to improve our understanding of ecosystem development on these substrates. Ants were sampled using pitfall traps at 11 ash-dam sites, ranging from unrehabilitated to 9-year-old rehabilitated sites, as well as two adjacent natural grassland sites. Sampling was conducted on 12 occasions from March 1997 to January 1999. Forty-nine ant species from 19 genera were recorded during the study. Site species richness was positively correlated with rehabilitation age, ranging from 10 to 25 at ash-dam sites, compared with 28 and 34 at the two natural grassland sites. There was a humped relationship between total ant abundance and rehabilitation age, with abundance peaking after 5–7 years at levels far higher than those at natural sites. Ordination analysis showed clear separation between ash-dam and natural sites along the first axis. The unrehabilitated ash-dam site was also separated from rehabilitated sites along the first axis. Sites of different rehabilitation age were separated along the second axis. Individual ant species showed clear successional patterns across the rehabilitation gradient. Although there was a clear successional trend for the development of ant communities on rehabilitated ash dams, this trend was not toward natural grassland. The lack of convergence toward ant communities of natural grasslands reflects the markedly different substrate and plant composition on ash dams and supports the widely held view that restoration of natural grassland communities is not a realistic goal of ash-dam rehabilitation. However, the development of species-rich ant communities, containing at least some late-successional species, indicates the potential for rehabilitated ash dams to support diverse and complex ecosystems.

Notes: Abstract  Ants are widely used as bioindicators in environmental assessment in Australia, partly because the responses of ant communities to disturbance are relatively well understood. In particular, the use of functional groups has provided a predictive framework for analysing ant community responses to disturbance in the absence of reliable information on the responses of individual species. Here we review 45 studies of the responses of Australian ant communities to disturbance, in order to: (i) identify individual species or species-groups that respond consistently to disturbance; and (ii) examine the usefulness of the functional group scheme as a framework for predicting ant community responses under different disturbance regimes in different biogeographical regions. The most common forms of disturbance in our studies were fire (17 studies), mining (12; mostly studies of minesite restoration) and grazing (7), with other disturbances including clearing, logging, flooding, recreation, urbanization and farming. Responses of individual species were inevitably variable because of differences in vegetation type, severity of disturbance and time since disturbance. However, we identified a range of widespread species that showed predictable responses, including species of the metallica group of Rhytidoponera in temperate Australia (‘increasers’ in relation to disturbance), species of the terebrans and denticulatus groups of Camponotus (increasers), the aeneovirens group of Melophorus (increasers) from the arid zone, and Iridomyrmex pallidus (increaser) from the monsoonal region. The functional group scheme assessed here was not designed specifically in the context of disturbance, but nevertheless in some situations provides a useful framework for analysing ant community responses. Three distinct syndromes of functional group responses can be recognized. First, Dominant Dolichoderinae and Hot Climate Specialists are groups that prefer open environments, and tend to be favoured by low levels of disturbance in well-forested habitats. Second, Opportunists and often also Generalized Myrmicinae are broadly adapted taxa with wide habitat tolerances, but are particularly sensitive to competitive interactions such that their responses oppose those of Dominant Dolichoderinae. Finally, Cryptic Species and Specialist Predators have highly specialized requirements that make them especially sensitive to disturbance. Functional groups are of most use in situations where disturbance causes substantial change in habitat structure, particularly in the ground-layer. Functional groups are of least use in very open habitats, where disturbance merely increases what is already extensive bare ground, and has relatively little impact on microclimate. The ant functional group scheme can play an important role in assessing disturbance in mesic Australia, but may be of more limited use for this in the arid zone.

Notes: Fire is widely used for conservation management in the savannah landscapes of northern Australia, yet there is considerable uncertainty over the ecological effects of different fire regimes. The responses of insects and other arthropods to fire are especially poorly known, despite their dominant roles in the functioning of savannah ecosystems. Fire often appears to have little long-term effect on ordinal-level abundance of arthropods in temperate woodlands and open forests of southern Australia, and this paper addresses the extent to which such ordinal-level resilience also occurs in Australia’s tropical savannahs. The data are from a multidisciplinary, landscape-scale fire experiment at Kapalga in Kakadu National Park. Arthropods were sampled in the two major savannah habitats (woodland and open forest) using pitfall traps and sweep nets, in 15–20 km2 compartments subjected to one of three fire regimes, each with three replicates: ‘early’ (annual fires lit early in the dry season), ‘late’ (annual fires lit late in the dry season), and ‘unburnt’ (fires absent during the five-year experimental period 1990–94). Floristic cover, richness and composition were also measured in each sampling plot, using point quadrats. There were substantial habitat differences in floristic composition, but fire had no measured effect on plant richness, overall composition, or cover of three of the four dominant species. Of the 11 ordinal arthropod taxa considered from pitfall traps, only four were significantly affected by fire according to repeated-measures ANOVA. There was a marked reduction in ant abundance in the absence of fire, and declines in spiders, homopterans and silverfish under late fires. Similarly, the abundances of only four of the 10 ordinal taxa from sweep catches were affected by fire, with crickets and beetles declining in the absence of fire, and caterpillars declining under late fires. Therefore, most of the ordinal taxa from the ground and grass-layer were unaffected by the fire treatments, despite the treatments representing the most extreme fire regimes possible in the region. This indicates that the considerable ordinal-level resilience to fire of arthropod assemblages that has previously been demonstrated in temperate woodlands and open forests of southern Australia, also occurs in tropical savannah woodlands and open forests of northern Australia.

Notes: This study aims primarily to assess the response of two invertebrate groups to the effects of pastoralism and military training, at one site in the tropical savanna of north-eastern Queensland. The richness and species composition of ants and terrestrial spiders were examined at two contrasting times of year across three land use treatments (pastoralism, military training and undisturbed) and four landscape positions (upper slope to riparian). Ant species richness was least in the grazed sites, and a high proportion of the ant species recorded varied significantly in frequency between the grazed and the two ungrazed land uses. This variation was generally greater than that associated with landscape position. Although variation in the richness of spiders was significantly related to land-use type, this effect was less pronounced than for ants, was less marked than variation associated with landscape position and was confounded by a strong interaction between land use and landscape position. Quadrat-scale variation in the composition of spider assemblages was influenced most by season of sampling. For both spiders and ants, there were few differences in richness or species composition between undisturbed land and that managed for military use.

Notes: Grasshoppers are a dominant group of herbivorous insects throughout the world, and their high diversity, functional importance, sensitivity to disturbance and ease of sampling makes them potentially useful bioindicators for land management. In Australia, however, the dynamics of grasshopper assemblages are extremely poorly understood. Here we describe the responses of grasshopper (Acridoidea, Eumastacoidea and Tettigonioidea) assemblages in the Kakadu region of the Northern Territory, Australia to disturbance associated with mining. Three questions were addressed in this study: (i) do local grasshopper assemblages show consistent responses to disturbance?; (ii) can particular species or functional groups be identified that are reliable indicators of ecological disturbance?; and (iii) to what extent do the responses of grasshopper assemblages merely reflect those of vegetation? Grasshoppers were sampled at 26 sites located in and around the Ranger uranium mine, representing three habitat types with respect to degree of disturbance: (i) ‘natural’ (10 sites representing a range of ‘undisturbed’ savanna habitats); (ii) ‘disturbed’ (10 sites representing a range of disturbances, but with soil intact); and (iii) ‘waste rock’ (six sites undergoing rehabilitation on a constructed landform). A total of 56 grasshopper species in 46 genera was recorded during the study, with site species richness ranging from five to 20. There were no significant differences between habitat types in site species diversity, but multivariate analysis demonstrated a strong correspondence between grasshopper species composition and degree of habitat disturbance. Using Indicator Species Analysis, six species and one functional group were identified as significant indicators of habitat type in relation to disturbance. Grasshopper responses were correlated with that of vegetation, but grasshopper assemblages showed apparently meaningful differentiation among disturbed sites that was not evident on the basis of floristic data. Our results demonstrate that grasshopper assemblages respond to disturbances associated with human land use and that these responses do not simply reflect those of plants. Grasshoppers are therefore potentially useful bioindicators of ecological disturbance in Australia, but further work is required on the extent to which their responses reflect general ecological change.

Notes: Abstract The impact of dry deposition of SO2 emissions on ant abundance, diversity and composition was investigated at Mount Isa in the semiarid tropics of northern Australia. Forty plots were sampled, stratified at two levels: sulfur deposition zones (high, medium, low, and two control zones) and habitat (Ridge and Plain). The two habitats supported distinctly different ant communities. Ants had clear responses to SO2 emissions. Ant abundance was lowest in the high and medium sulfur zones in both habitats. Species richness in high SO2 plots (up to 5 km from the source) was approximately half that of control plots in Ridge habitat, and was substantially less than controls in the Plain habitat. Ant community composition in the high sulfur zone was clearly separated from those of other zones in ordinations. Vector fitting showed soil SO4 concentration as a primary correlative factor in this separation. Ant abundance and richness were both negatively correlated with soil SO4 concentration, and positively correlated with plant species richness and distance away from the smelters. The abundance of 10 of the 21 most common species showed significant responses to emissions. Five species showed positive responses, and all belong to species-groups known to be abundant at disturbed sites throughout northern Australia. Relative abundance and richness of Eyrean (arid adapted) taxa collectively responded positively to sulfur, and Torresian (tropical) and Widespread species responded negatively. Despite large changes in species composition and abundances, there was relatively little change in the abundance of functional groups that have been widely used in studies of Australian ant communities. Ants are sensitive to SO2 emissions and appear to be good candidates as an indicator group in this context. However, an alternative functional group framework is required for the identification of recurrent responses of arid zone ant communities to disturbance.