The diet, attributes of feeding sites and patterns of seasonal movements of a
population of the Gouldian finch, Erythrura gouldiae,
were studied in the Yinberrie Hills area north of Katherine in the Northern
Territory. In the dry season (April–November) Gouldian finches foraged
mostly on burnt ground and fed on exposed seed of annual grasses, especially
seed of spear-grass, Sorghum spp. In the wet season
(December–March) Gouldian finches fed on seed of a sequence of perennial
grass species, including Themeda triandra,
Alloteropsis semialata,
Chrysopogon fallax and
Heteropogon triticeus. Gouldian finches undertake
regular seasonal shifts in habitat, from breeding areas in hill woodland in
the dry season to adjacent lowlands throughout much of the wet season, in
response to seasonal changes in food availability. There is an annual pulse in
abundance of fallen seed in the early dry season that is depleted to near zero
levels by germination of annual grasses early in the wet season. Thereafter,
finches depend on seed from other sources, principally ripe and ripening seed
of perennial grasses. Observations over three successive wet seasons suggest
that Gouldian finches track seed resources provided by seeding perennial
grasses over an extensive area of lowland grassy woodland adjacent to the
breeding area, favouring small patches of grassy woodland for brief periods
until seed fall. There were subtle differences between years in the types of
resources used. Management of Gouldian finch populations will entail
protection and management of the full range of grassland habitats used
throughout the annual cycle, and will require predictive knowledge of the
causes of patterning of seed resources and probably an ability to exert
control over the timing and extent of fires in fire-prone seasonal savanna
landscapes.
In this paper, the dynamics of primary production in the Daly River in tropical Australia are investigated. We used the diurnal-curve method for both oxygen and pH to calculate photosynthesis and respiration rates as indicators of whole-river productivity. The Daly River has maximum discharges during the summer, monsoonal season. Flow during the dry season is maintained by groundwater discharge via springs. The study investigated how primary production and respiration evolve during the period of low flow in the river (April–November). The relationship between primary production and the availability of light and nutrients enabled the role of these factors to be assessed in a clear, oligotrophic tropical river. The measured rate of photosynthesis was broadly consistent with the estimated mass of chlorophyll associated with the main primary producers in the river (phytoplankton, epibenthic algae, macroalgae, macrophytes). A significant result of the analysis is that during the time that plant biomass re-established after recession of the flows, net primary production proved to be ~4% of the rate of photosynthesis. This result and the observed low-nutrient concentrations in the river suggest a tight coupling between photosynthetic fixation of carbon and the microbial degradation of photosynthetic products comprising plant material and exudates.
As part of investigations into strategies for biological monitoring of mining impacts in the vicinity of the Kakadu Conservation Zone, statistical procedures were evaluated in nearby Rockhole Mine Creek, a site of past mining activities. The BACI design and associated statistical test is based on temporal replication of some measure of difference between paired control and impact areas, and it requires that the difference values meet certain statistical requirements while providing adequate statistical power. In investigating extensions of the BACI design to community-level data, a simple gradient-based model of control and impact sites is described that yields predictions concerning the relative performance of different community dissimilarity measures. Abundance data for benthic macroinvertebrates based on the first 2 years of our study at Rockhole Mine Creek are then used to evaluate the potential statistical power of these alternative dissimilarity measures. The measures vary widely in their statistical power, with the Bray-Curtis measure providing high power over both years.
1. Periodic collecting of floating cast pupal cuticles of chironomids (exuviae) in two tropical northern Australian streams demonstrates (i) spatial heterogeneity in species composition across a wide stream, (ii) temporal heterogeneity in the maximum abundance of each species, and that (iii) species accumulate as a function of sample size and duration of sampling. 2. Spatial heterogeneity is ascribed to variation in larval microhabitat across the wide stream, combined with short exuvial drift duration and restricted upstream mixing. 3. Temporal heterogeneity is ascribed to diel periodicity in adult emergence and, as with spatial heterogeneity, to the short floating life. 4. The consequences of spatial and temporal variation for the sampling of exuvial drift are discussed in relation to the objectives of particular programmes. Thus, if the objective is assessment of chironomid species composition for inventory purposes such as faunistics or conservation, the large sample sizes attained by 24-h sampling are necessary and appropriate. However, for rapid assessment that requires comparable samples at different sites, species numbers may be optimized by temporally and spatially restricted sampling of the maximal emergence period, which in this study is at dusk, or by subsampling from a 24-h sample.
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