Concordance between dispersal and mitochondrial gene flow: isolation by distance in a tropical teleost, Lates calcarifer (Australian barramundi)

Patterns of population subdivision and the relationship between gene flow and geographical distance in the tropical estuarine fish Lares calcarifer (Centropomidae) were investigated using mtDNA control region sequences. Sixty-three putative haplotypes were resolved from a total of 270 individuals from nine localities within three geographical regions spanning the north Australian coastline. Despite a continuous estuarine distribution throughout the sampled range, no haplotypes were shared among regions. However, within regions, common haplotypes were often shared among localities. Both sequence-based (average Phi(ST)=0.328) and haplotype-based (average Phi(ST)=0.182) population subdivision analyses indicated strong geographical structuring. Depending on the method of calculation, geographical distance explained either 79 per cent (sequence-based) or 23 per cent (haplotype-based) of the variation in mitochondrial gene flow. Such relationships suggest that genetic differentiation of L. calcarifer has been generated via isolation-by-distance, possibly in a stepping-stone fashion. This pattern of genetic structure is concordant with expectations based on the life history of L. calcarifer and direct studies of its dispersal patterns. Mitochondrial DNA variation, although generally in agreement with patterns of allozyme variation, detected population subdivision at smaller spatial scales. Our analysis of mtDNA variation in L. calcarifer confirms that population genetic models can detect population structure of not only evolutionary significance but also of demographic significance. Further, it demonstrates the power of inferring such structure from hypervariable markers, which correspond to small effective population sizes.

Transport, storage and mobilization of nitrogen by trees and shrubs in the wet/dry tropics of northern Australia

Xylem sap from woody species in the wet/dry tropics of northern Australia was analyzed for N compounds. At the peak of the dry season, arginine was the main N compound in sap of most species of woodlands and deciduous monsoon forest. In the wet season, a marked change occurred with amides becoming the main sap N constituents of most species. Species from an evergreen monsoon forest, with a permanent water source, transported amides in the dry season. In the dry season, nitrate accounted for 7 and 12% of total xylem sap N in species of deciduous and evergreen monsoon forests, respectively In the wet season, the proportion of N present as nitrate increased to 22% in deciduous monsoon forest species. These results suggest that N is taken up and assimilated mainly in the wet season and that this newly assimilated N is mostly transported as amide-N (woodland species, monsoon forest species) and nitrate (monsoon forest species). Arginine is the form in which stored N is remobilized and transported by woodland and deciduous monsoon forest species in the dry season. Several proteins, which may represent bark storage proteins, were detected in inner bark tissue from a range of trees in the dry season, indicating that, although N uptake appears to be limited in the dry season, the many tree and shrub species that produce flowers, fruit or leaves in the dry season use stored N to support growth. Nitrogen characteristics of the studied species are discussed in relation to the tropical environment.

The haliclonacyclamines, cytotoxic tertiary alkaloids from the tropical marine sponge Haliclona sp

2D-NMR spectroscopic data is reported for the haliclonacyclamines A - D (1)-(4) and for two bismethiodide adducts (5) and (6). The structures of two new alkaloids, haliclonacyclamines C (3) and D (4), which are the 15,16-dihydro analogues of the haliclonacyclamines A (1) and B (2) are described. Revised assignments deduced by 2D-INADEQUATE spectroscopy are presented for (1) and (2). The alkene substituent in the C,, spacer group of (2) and (4) is positioned between C27-C28 by NMR, and confirmed by x-ray structural analysis for (2). Metabolite (3) has a C25-C26 double bond. (C) 1998 Elsevier Science Ltd. All rights reserved.

Using Lindenmayer systems to model morphogenesis in a tropical pasture legume Stylosanthes scabra

This paper summarizes the processes involved in designing a mathematical model of a growing pasture plant, Stylosanthes scabra Vog. cv. Fitzroy. The model is based on the mathematical formalism of Lindenmayer systems and yields realistic computer-generated images of progressive plant geometry through time. The processes involved in attaining growth data, retrieving useful growth rules, and constructing a virtual plant model are outlined. Progressive output morphological data proved useful for predicting total leaf area and allowed for easier quantification of plant canopy size in terms of biomass and total leaf area.

Effects of habitat modification in mangroves on the structure of mollusc and crab assemblages

The abundance and species richness of mollusc and crab assemblages were examined in a subtropical mangrove forest in Moreton Bay, Queensland, Australia, which has been disturbed and damaged by the construction of a wooden boardwalk and a path. Sections of the forest immediately adjacent to the boardwalk and path were compared with reference areas to determine whether changes to the small-scale structural complexity within the forest affected the benthic fauna. The disturbed area was characterised by having 65-80% fewer pneumatophores, significantly fewer species and individuals of molluscs, but significantly more species and individuals of crabs than the reference areas. The abundance of mangrove pneumatophores and the attached epiphytic algae were manipulated at two sites to determine whether observed differences in these features could account for the differences in the assemblage of molluscs in the disturbed area of the forest compared with reference areas. Five experimental treatments were used: undisturbed controls, pneumatophore removals (abundance reduced by ca. 65%), epiphytic algal removals (algae removed from ca. 65% of pneumatophores), pneumatophore disturbance controls and algal disturbance controls. The experimental reduction of the abundance of mangrove pneumatophores and the associated epiphytic algae led to significant declines (by as much as 83%) in the number of molluscs utilising the substratum in the modified plots. There was no significant difference in the abundance of molluscs in the pneumatophore and algal removal plots suggesting any effect was primarily related to removal of the epiphytic algae from the surface of the pneumatophores. The responses by the biota to the changes in the physical environment demonstrate that even relatively small-scale modifications to the physical structure of subtropical mangrove forests can lead to significant effects on the diversity and abundance of macrobenthic organisms in these habitats. Such modifications have the potential to cause cascading effects at higher trophic levels with a deterioration in the value of these habitats as nursery and feeding grounds. Future efforts at conservation of these estuarine environments must focus on the prevention or reduction of modifications to the physical structure and integrity of the system, rather than just on the prevention of loss of entire patches of habitat. (C) 2000 Elsevier Science B.V. All rights reserved.

Determining forest structural attributes using an inverted geometric-optical model in mixed eucalypt forests, Southeast Queensland, Australia

The Montreal Process indicators are intended to provide a common framework for assessing and reviewing progress toward sustainable forest management. The potential of a combined geometrical-optical/spectral mixture analysis model was assessed for mapping the Montreal Process age class and successional age indicators at a regional scale using Landsat Thematic data. The project location is an area of eucalyptus forest in Emu Creek State Forest, Southeast Queensland, Australia. A quantitative model relating the spectral reflectance of a forest to the illumination geometry, slope, and aspect of the terrain surface and the size, shape, and density, and canopy size. Inversion of this model necessitated the use of spectral mixture analysis to recover subpixel information on the fractional extent of ground scene elements (such as sunlit canopy, shaded canopy, sunlit background, and shaded background). Results obtained fron a sensitivity analysis allowed improved allocation of resources to maximize the predictive accuracy of the model. It was found that modeled estimates of crown cover projection, canopy size, and tree densities had significant agreement with field and air photo-interpreted estimates. However, the accuracy of the successional stage classification was limited. The results obtained highlight the potential for future integration of high and moderate spatial resolution-imaging sensors for monitoring forest structure and condition. (C) Elsevier Science Inc., 2000.

Returns to farm-level soil conservation on tropical steep slopes: The case of the Eritrean highlands

This study conducts an economic analysis of investment in simple soil conservation technologies in the highlands of Eritrea. The data used in the analysis were obtained from a farm survey and supplemented with data from secondary sources. Risk analysis techniques are used to take account of the uncertainties regarding the relationship between soil erosion and crop yield. The financial analysis reveals negative net present values (NPVs) and internal rates of return (IRRs) below 12 per cent for various slope categories. On the other hand, the economic analysis returns positive NPVs and IRRs of over 20 per cent. The results clearly indicate that in-vestment in soil conservation technology may not be a viable short-term proposition from the farmer's point of view and yet the net social benefits are positive. There is a strong case for government to provide incentives for soil conservation in view of the economic benefits.

Expression of POU, Sox, and Pax genes in the brain ganglia of the tropical abalone Haliotis asinina

In gastropod mollusks, neuroendocrine cells in the anterior ganglia have been shown to regulate growth and reproduction. As a first step toward understanding the molecular mechanisms underlying the regulation of these physiological processes in the tropical abalone Haliotis asinina, ive have identified sets of POU, Sox, and Pax transcription factor genes that are expressed in these ganglia. Using highly degenerate oligonucleotide primers designed to anneal to conserved codons in each of these gene families, we have amplified by reverse transcriptase polymerase chain reaction 2 POU genes (HasPOU-III and HasPOU-IV), 2 Sox genes (HasSox-B and HasSox-C), and two Pax genes (HasPax-258 and HaxPax-6). Analyses with gene-specific primers indicated that the 6 genes are expressed in the cerebral and pleuropedal ganglia of both reproductively active and spent adults, in a number of sensory structures, and in a subset of other adult tissues.

Synchronous oogenesis during the semilunar spawning cycle of the tropical abalone Haliotis asinina

On the southern Great Barrier Reef, Haliotis asinina (Vetigastropoda: Pleurotomarioidea) synchronously spawn every 2 wk in a predictable fashion. allowing detailed analysis of reproduction, gametogenesis, and gonad development. Histological examination of the ovaries of members of the Heron Reef population during this semilunar cycle reveals that oogenesis is also synchronous and predictable, and requires more than two spawning cycles (i.e. >28 days) to complete. Shortly after a spawning event the ovary comprises two cohorts of primary oocytes, one of which will be released at the next spawning event, and clusters of oogonia. At this time there is a rapid proliferation and expansion of trabeculae, germinal epithelial, and oogonia, and a dramatic increase in the size of the vitellogenic oocytes to be: spawned at the next spawning event. Within 4 days these oocytes have filled the ovary. On the day of the next spawning a lumen forms in the ovary as a result of localized degradation of trabeculae. The large primary oocytes dissociate from the receding trabeculae. initiate maturation, and accumulate in the lumen; these oocytes become embedded in a jelly coat layer. The next cohort of oocytes remain attached to the trabeculae. The jelly coat appears to be completely dissolved within 30 min of spawning. Comparison of the oogenesis and ovary development in II. asinina with other abalone species indicates that these processes are very similar in tropical and temperate abalone. This suggests that insights into the regulation of reproduction and spawning in H. asinina are likely to be applicable to other haliotids.

Heterogeneity in inoculum potential and effectiveness of arbuscular mycorrhizal fungi

Arbuscular mycorrhizae are symbiotic associations among glomalean fungi and plant roots that often lead to enhanced water and nutrient uptake and plant growth. We describe experiments to test whether inoculum potential of arbuscular mycorrhizal (AM) fungal communities varies spatially within a broadleaf temperate forest, and also whether there is variability in the effectiveness of AM fungal communities in enhancing seedling growth. Inoculum potential of arbuscular mycorrhizal fungi in a temperate broad-leaved forest did not vary significantly among sites. Inoculum potential, measured as the extent to which the roots of red maple seedlings that had been germinated on sterile sand and then transplanted into the forest, were colonized by AM fungi, was similar in floodplain and higher elevation sites. It was as similar under ectomycorrhizal oaks as it was under red maples and other AM tree species. It was also similar among sites with deciduous understory shrubs with arbuscular mycorrhizae (spicebush, Lindera benzoin) and those with evergreen vegetation with ericoid mycorrhizae (mountain laurel, Kalmia latifolia). Where spicebush was the dominant understory shrub, inoculum potential was greater under gaps in the canopy than within the understory. Survivorship of transplanted red maple seedlings varied significantly over sites but was not strongly correlated with measures of inoculum potential. In a greenhouse growth experiment, arbuscular mycorrhizal fungal communities obtained from tree roots from the forest had different effects on plant growth. Seedlings inoculated with roots of red maple had twice the leaf area after 10 wk of growth compared to the AM community obtained from roots of southern red oaks. Thus, although there appears to be little heterogeneity in inoculum potential in the forest, there are differences in the effectiveness of different inocula. These effects have the potential to affect tree species diversity in forests by modifying patterns of seedling recruitment.

Magnesium in tropical and subtropical soils from north-eastern Australia .1. Magnesium fractions and interrelationships with soil properties

The magnesium (Mg) status of 52 highly weathered, predominantly acidic, surface soils from tropical and subtropical north-eastern Australia was evaluated in a laboratory study. Soils were selected to represent a range of soil types and management histories. Exchangeable Mg concentrations were generally low (median value 0.37 cmol(+)/kg), with deficient levels (<0.3 cmol(+)/kg) being measured in 22 of the soils, highlighting the potential for Mg deficiency as a limitation to plant growth in the region. Furthermore, acid-extractable Mg concentrations, considered a reserve of potentially available Mg, were generally modest (mean and median values, 1.6 and 0.40 cmol(+)/kg, respectively). The total Mg content of the soils studied ranged from 123 to 7894 mg/kg, the majority present in the mineral pool (mean 71%), with smaller proportions in the acid-soluble (mean 11%) and exchangeable (mean 17%) pools, and a negligible amount associated with organic matter (mean 1%). A range of extractant solutions used to displace exchangeable Mg was compared, and found to yield similar results on soils with exchangeable Mg <4 cmol(+)/kg. However, at higher exchangeable Mg concentrations, dilute extractants (0.01 M CaCl2, 0.0125 M BaCl2) displaced less Mg than concentrated extractants (1 M NH4Cl, 1 M NH4OAc, 1 M KCl). The concentrated extractants displaced similar amounts of Mg, thus the choice of extractant is not critical, provided the displacing cation is sufficiently concentrated. Exchangeable Mg was not significantly correlated to organic carbon (P > 0.05), and only 45% of the variation in exchangeable Mg could be explained by a combination of pH(w) and clay content.

Magnesium in tropical and subtropical soils from north-eastern Australia .2. Response by glasshouse-grown maize to applied magnesium

A glasshouse trial, in which maize (Zea mays L. cv. Pioneer 3270) was grown in 35 north-eastern Australian soils of low magnesium (Mg) status, was undertaken to study the response to applied Mg. Of the soils studied, 20 were strongly acidic (pH(1:5 soil:water) <5.4), and in these soils the response to Mg was studied in both the presence and absence of lime. Magnesium application significantly (P < 0.05) increased dry matter yield in 10 soils, all of which were strongly acidic. However, significant Mg responses were recorded in 6 soils in the presence of lime, indicating that, in many situations, liming strategies may need to include consideration of Mg nutrition. Critical soil test values for 90% relative yield were 0.21 cmol(+)/kg of exchangeable Mg or 7% Mg saturation, whilst the critical (90% yield) plant tissue Mg concentration (whole shoots) was 0.15%.