Guy S. Boggs

Biophysical correlates of relative abundances of marine megafauna at Ningaloo Reef, Western Australia

Changes in the relative abundance of marine megafauna (whales, dolphins, sharks, turtles, manta rays, dugongs) from aerial survey sightings in the waters adjacent to Ningaloo Reef between June 2000 and April 2002 are described. Generalised linear models were used to explore relationships between different trophic guilds of animals (based on animal sighting biomass estimates) and biophysical features of the oceanscape that were likely to indicate foraging habitats (regions of primary/secondary production) including sea surface temperature (SST), SST gradient, chlorophyll-a (Chl-a), bathymetry (BTH) and bathymetry gradient (BTHg). Relative biomass of krill feeders (i.e. minke whales, whale sharks, manta rays) were related to SST, Chl-a and bathymetry (model [AICc] weight = 0.45) and the model combining these variables explained a relatively large amount (32.3%) of the variation in relative biomass. Relative biomass of fish/cephalopod feeders (dolphins, sharks) were weakly correlated with changes in SST, whereas that of other invertebrate/macroalgal feeders (turtles, dugong) was weakly correlated with changes in steepness of the shelf (bathymetry gradient). Our results indicate that biophysical variables describe only a small proportion of the variance in the relative abundance and biomass of marine megafauna at Ningaloo reef.

Assessment of SPOT 5 and QuickBird remotely sensed imagery for mapping tree cover in savannas.

The relative abundance and distribution of trees in savannas has important implications for ecosystem function. High spatial resolution satellite sensors, including QuickBird and IKONOS, have been successfully used to map tree cover patterns in savannas. SPOT 5, with a 2.5 m panchromatic band and 10 m multispectral bands, represents a relatively coarse resolution sensor within this context, but has the advantage of being relatively inexpensive and more widely available. This study evaluates the performance of NDVI threshold and object based image analysis techniques for mapping tree canopies from QuickBird and SPOT 5 imagery in two savanna systems in southern Africa. High thematic mapping accuracies were obtained with the QuickBird imagery, independent of mapping technique. Geometric properties of the mapping indicated that the NDVI threshold produced smaller patch sizes, but that overall patch size distributions were similar. Tree canopy mapping using SPOT 5 imagery and an NDVI threshold approach performed poorly, however acceptable thematic accuracies were obtained from the object based image analysis. Although patch sizes were generally larger than those mapped from the QuickBird image data, patch size distributions mapped with object based image analysis of SPOT 5 have a similar form to the QuickBird mapping. This indicates that SPOT 5 imagery is suitable for regional studies of tree canopy cover patterns.

Measuring fragmentation of seagrass landscapes: Which indices are most appropriate for detecting change?

Many indices are available for assessment of spatial patterns in landscape ecology, yet there is presently no consensus about which ones effectively quantify habitat fragmentation. Research that has been carried out to date has evaluated indices primarily using computer-simulated models of terrestrial environments, but how they perform when applied to real landscapes, particularly in the marine environment, has received little attention. Eleven indices that are commonly used for quantifying habitat fragmentation were assessed for their abilities to measure different levels of fragmentation in 16-ha landscape windows of mapped seagrass. The landscape windows were grouped into five categories, from highly fragmented to continuous seagrass landscapes. Nested within the fragmentation categories were high and low levels of seagrass cover. Hierarchical analysis of variance techniques were used to differentiate between the different fragmentation categories and levels of seagrass cover within the fragmentation categories. Principal component analysis was also employed to determine strong correlations between the indices. The results suggest that (1) landscape division and (2) area-weighted mean perimeter to area ratio were the most appropriate indices for differentiating between independent levels of seagrass fragmentation. The splitting index may also be useful when the detection of small differences in cover is important.

Estimating landscape-scale vegetation carbon stocks using airborne multi-frequency polarimetric synthetic aperture radar (SAR) in the savannahs of north Australia

This study investigates the use of polarimetric AirSAR (TopSAR) data for estimating biomass and carbon storage of Eucalyptus miniata (Darwin Woolly Butt) and E. tetrodonta (Stringybark) dominated open‐forest savannah in the Northern Territory, Australia. Radar backscatter intensity was correlated with basal area for 30 plots within the Wildman River Reserve, Northern Territory. Published allometric relationships were used to convert tree basal area to estimates of above‐ground biomass for each of the measured plots. Below‐ground biomass was also estimated for these plots, using additional published allometric relationships between below‐ground and above‐ground biomass. Backscatter of the L‐HV channel had the highest regression co‐efficient (r2 = 0.92) with ground‐based tree basal area measurements. Using a linear regression equation of backscatter intensity for the L‐HV channel versus above‐ground biomass gave a mean above‐ground biomass of 94 t DM (dry mass) ha−1 for the eucalypt dominated vegetation in the Wildman River Reserve, equivalent to 47 t C ha−1 stored in this biomass pool. Estimated below‐ground biomass was 28 t C ha−1, giving a total carbon biomass storage for this savannah ecosystem of 75 t C ha−1. The results of this study indicate that the L‐HV channel of polarimetric SAR is best suited to model biomass of the tropical savannahs of northern Australia. Given the vast spatial extent of savannah woodlands across north Australia, SAR has the potential to be a major tool in carbon stock assessment, critical for carbon accounting, as well as to contribute to gaining a better understanding of the role the tropical savannahs of northern Australia play in the biochemical cycles of Australia.

Dynamics of Acacia aneura—Triodia boundaries using carbon (14C and δ 13C) and nitrogen (δ15N) signatures in soil organic matter in central Australia

Soil organic matter (SOM) was sampled from soil profiles on a near level sandsheet at the southern limit of the Tanami Desert in central Australia to determine if boundaries of Triodia hummock grassland—Acacia aneura shrublands had changed in the Holocene. Accelerator Mass Spectrometry (AMS) 14C dating of 16 soil profiles showed that SOM that had accumulated at 100 to 140 cm depth, (near the base of most profiles) had ages between 1175 and 2630 14C years, averaging 1906 14C years. The stable carbon isotopic (δ13C) composition of SOM from the upper 50 cm soil profiles in the A. aneura shrubland (inhabited by plants with predominantly C3 photosynthetic pathway) was significantly more 13C-depleted than the comparable soil interval beneath a Triodia grassland (predominantly C4 photosynthetic pathway). Mean age of SOM at 50 cm depth was 830 14C years, suggesting the vegetation has been stable for about 1000 years. However, soil profiles in Triodia grassland adjacent to the shrubland boundary had slightly more depleted δ 13C relative to sites >0.5 km from the boundary. With respect to stable nitrogen isotopic values, only surface soils in the Acacia shrublands were found to be 15N-enriched relative to all other soil depths. Although there were no obvious environmental discontinuities, such as change in soil type or slope angle, associated with the ecosystem boundaries, the Acacia shrublands were found to occur on more clay-rich soils with higher concentrations of total phosphorus, nitrogen and potassium compared with the surrounding grasslands, and these trends became more pronounced with increasing distance from the ecotone: it is unclear if these differences are a cause or an effect of the vegetation mosaic. The concordance of the vegetation boundaries with the δ13C and δ15N and of soil nutrients are consistent with only minor attrition of the A. aneura shrublands in the late Holocene at this site.

Broad-scale environmental influences on the abundance of saltwater crocodiles (Crocodylus porosus) in Australia

Saltwater crocodile (Crocodylus porosus) populations have recovered strongly across northern Australia over the 30 years since the species was protected from hunting. However, monitoring studies show large geographical variations in abundance across the Northern Territory, Queensland and Western Australia. The Northern Territory has considerably higher densities, raising questions about constraints on recovery in the other states. We examined broad-scale environmental influences on population abundance by modelling the species–environment relationships across northern Australia. The hypothesis-based models showed strong support for the linkage to (1) the ratio of total area of favourable wetland vegetation types (Melaleuca, grass and sedge) to total catchment area, (2) a measure of rainfall seasonality, namely the ratio of total precipitation in the coldest quarter to total precipitation in the warmest quarter of a year, and (3) the mean temperature in the coldest quarter of a year. On the other hand, we were unable to show any clear negative association with landscape modification, as indicated by the extent of high-impact land uses or human population density in catchments. We conclude that geographical variations in crocodile density are mostly attributable to differences in habitat quality rather than the management regimes adopted in the respective jurisdictions.

Object-based mapping of native vegetation and para grass (Urochloa mutica) on a monsoonal wetland of Kakadu NP using a Landsat 5 TM Dry-season time series

This paper evaluates the use of multi-temporal Landsat 5 TM for object-based classification of native wetland vegetation and the perennial aquatic weed para grass within Kakadu National Park, Northern Territory, Australia. Using identical training data and segmentation, a nearest-neighbour classification produced from a four-image (dry season) time-series was compared with four ‘single-date’ classifications produced from the individual images of the same series. A 15-class vegetation map generated from the multi-date classification produced an overall accuracy of 82 percent (kappa = 0.80). This was an average increase in accuracy of 25 percent (kappa = 0.28) compared to single-date classifications. The multi-date image composite also improved segmentation quality and spectral separability of vegetation classes. Reliable maps of wetland vegetation, potentially useful for strategic conservation, can be produced by integrated, object-based, analysis of multi-temporal Landsat.

Extraction of Tree Crowns from High Resolution Imagery over Eucalypt Dominant Tropical Savannas

High spatial resolution satellite imagery provides data that enable the analysis of detailed landscape information, including tree crowns. The inherent characteristics of Eucalypt crowns are challenging to remotely sensed tree crown extraction. This paper develops and applies an object-based tree crown delineation approach suitable for estimating canopy cover of Eucalypts in tropical savannas. A two level segmentation was undertaken upon QuickBird data. Firstly, a broad segmentation masked out non-Eucalypt dominant communities; and secondly, a finer segmentation and ruleset identified seed objects within crowns and then expanded these objects to cover entire crown extents. Of the 1604 tree crowns manually observed within the scene 84.3% were detected by the automated seed identification process. 75% of tree crowns extracted through the region growing process show strong overlap with their corresponding reference crowns. Results indicate the potential of this method for delineating tree crowns from Eucalypt dominant savanna and the use of this information to estimate canopy cover and tree distribution patterns.

Corrigendum to: Broad-scale environmental influences on the abundance of saltwater crocodiles (Crocodylus porosus) in Australia

Saltwater crocodile (Crocodylus porosus) populations have recovered strongly across northern Australia over the 30 years since the species was protected from hunting. However, monitoring studies show large geographical variations in abundance across the Northern Territory, Queensland and Western Australia. The Northern Territory has considerably higher densities, raising questions about constraints on recovery in the other states. We examined broad-scale environmental influences on population abundance by modelling the species.environment relationships across northern Australia. The hypothesis-based models showed strong support for the linkage to (1) the ratio of total area of favourable wetland vegetation types (Melaleuca, grass and sedge to total catchment area, (2) a measure of rainfall seasonality, namely the ratio of total precipitation in the coldest quarter to total precipitation in the warmest quarter of a year, and (3) the mean temperature in the coldest quarter of a year. On the other hand, we were unable to show any clear negative association with landscape modification, as indicated by the extent of high-impact land uses or human population density in catchments. We conclude that geographical variations in crocodile density are mostly attributable to differences in habitat quality rather than the management regimes adopted in the respective jurisdictions.

Long-term hydrology modelling and analysis in a data limited small catchment in the wet-dry tropics of Australia

Abstract The Ngarradj catchment in the Northern Territory of Australia has high temporal resolution rainfall and runoff data from 1998 to 2002. These data have been used to calibrate the DistFW and HEC-HMS hydrology models. The ability of HEC-HMS to be calibrated using an extended hydrograph provided a better basis for long-term modelling. The calibrated HEC-HMS model was applied to 20 years of rainfall from within the region, providing a more reliable estimate of flood frequencies of higher magnitudes (> 1 in 5 year events). However, before undertaking this type of analysis, the modeller must have a good understanding of the catchment system and the limitations of the chosen hydrology model.