Plant growth rates and seed size: a re-evaluation

Small-seeded plant species are often reported to have high relative growth rate or RGR. However, because RGR declines as plants grow larger, small-seeded species could achieve higher RGR simply by virtue of their small size. In contrast, size-standardized growth rate or SGR factors out these size effects. Differences in SGR can thus only be due to differences in morphology, allocation, or physiology. We used nonlinear regression to calculate SGR for comparison with RGR for 10 groups of species spanning a wide range of life forms. We found that RGR was negatively correlated with seed mass in nearly all groups, but the relationship between SGR and seed mass was highly variable. We conclude that small-seeded species only sometimes possess additional adaptations for rapid growth over and above their general size advantage.     

Autumn migratory fuelling: a response to simulated magnetic displacements in juvenile wheatears, Oenanthe oenanthe

Recent experiments exposing migratory birds to altered magnetic fields simulating geographical displacements have shown that the geomagnetic field acts as an external cue affecting migratory fuelling behaviour. This is the first study investigating fuel deposition in relation to geomagnetic cues in long-distance migrants using the western passage of the Mediterranean region. Juvenile wheatears (Oenanthe oenanthe) were exposed to a magnetically simulated autumn migration from southern Sweden to West Africa. Birds displaced parallel to the west of their natural migration route, simulating an unnatural flight over the Atlantic Ocean, increased their fuel deposition compared to birds experiencing a simulated migration along the natural route. These birds, on the other hand, showed relatively low fuel loads in agreement with earlier data on wheatears trapped during stopover. The experimental displacement to the west, corresponding to novel sites in the Atlantic Ocean, led to a simulated longer distance to the wintering area, probably explaining the observed larger fuel loads. Our data verify previous results suggesting that migratory birds use geomagnetic cues for fuelling decisions and, for the first time, show that birds, on their first migration, can use geomagnetic cues to compensate for a displacement outside their normal migratory route, by adjusting fuel deposition.     

Habitat-mediated variation in predation risk by the American marten

The probability of prey encounter, attack, capture, and kill are often hypothesized to depend on habitat structure, but field evidence in terrestrial systems is rare. We tested whether predation efficiency by the American marten (Martes americana) and fear of predation by their primary prey, the red-backed vole (Clethrionomys gapperi), differed between 20- to 50-year-old regenerating forest stands and older uncut stands. Our results showed that the frequency of prey encounter, prey attack, and prey kill were higher in old uncut forests, despite the fact that small-mammal density was similar to that in younger logged forests. These differences in predation efficiency were linked to higher abundance of coarse woody debris, which seems to offer sensory cues to martens, thereby increasing the odds of hunting success. Red-backed voles in regenerating forest stands exhibited increased wariness compared to voles living in old uncut forest, suggestive of a behavioral response to habitat-mediated variation in predation risk.     

Resource availability and the abundance of an N-based defense in Australian tropical rain forests

Plant defense theories predict that relatively resource-rich environments (those with more fertile soil) will support a greater abundance of plants with nitrogen-based chemical defense, but this has yet to be adequately tested. We tested this prediction by measuring the diversity and contribution to total biomass of cyanogenic plants (those that release hydrogen cyanide from endogenous cyanide-containing compounds) in the Australian tropical rain forest. We examined 401 species in thirty 200-m2 plots, six at each of five sites, for cyanogenesis. In upland/highland rain forest, two pairs of sites similar in rainfall and altitude, but differing in soil nutrients, were selected, as well as one site in lowland rain forest. Sites differed markedly in species composition and foliar N was positively related to soil fertility. Holding altitude constant, we did not detect significant differences in the proportion of cyanogenic species with soil fertility, nor did we consistently detect significant increases in the contribution of cyanogenic species to total biomass on higher nutrient sites. Thus we found no clear evidence that soil fertility affects the distribution and prevalence of species investing in a constitutive N-based defense at the community level.     

Spatial distribution of defense chemicals and markers and the maintenance of chemical variation

Exploring the spatial distribution of variation in plant secondary metabolites is critical for understanding the evolutionary ecology of biochemical diversity in wild organisms. In the present study, concentrations of foliar sideroxylonal, an important and highly heritable defense chemical of Eucalyptus melliodora, displayed strong, fine-scale spatial autocorrelation. The spatial patterns observed could promote associational effects on herbivore foraging decisions, which may influence the selection pressures exerted on sideroxylonal content. Multiple chemical traits have roles in certain eucalypt-herbivore interactions, and the spatial characteristics of the herbivore foraging environment are therefore determined by these different factors. We used a model of E. melliodora intake by common brushtail possums (Trichosurus vulpecula), based on the combined effects of two chemical traits, to explore this idea and found that the spatial patterns were different to those of sideroxylonal alone. Spatial genetic autocorrelation, examined using microsatellites, was strong and occurred at a fine scale, implying that restricted gene flow might allow genetic patches to respond to selection relatively independently. Local two-dimensional genetic autocorrelation, explored using a new heuristic method, was highly congruent with the pattern of local phenotypic variation observed for sideroxylonal, suggesting that the genetic variance underlying the sideroxylonal variation is similarly structured. Our results suggest that the spatial distribution of genetic and phenotypic variation could influence both the selective pressure imposed by herbivores on eucalypt defenses and the potential of populations to respond to natural selection. Spatial context should be considered in future studies of plant-herbivore interactions.     

A novel technique for the pre-concentration and extraction of inositol hexakisphosphate from soil extracts with determination by phosphorus-31 nuclear magnetic resonance

Inositol hexakisphosphate (IP6) is often the dominant form of soil organic phosphorus (P), but is rarely investigated because of the analytical difficulties encountered in its extraction, separation, and detection in environmental samples. In particular, recent advances in the study of soil organic P with 31P nuclear magnetic resonance (NMR) have been of limited use for the study of IP6, because the technique does not discriminate between IP6 and other forms of P. This was addressed by developing a novel analytical procedure using the retentive properties of gel-filtration gels for IP6, which allows the combined selective extraction and pre-concentration of IP6 from soil extracts with determination by 31P NMR. While the technique is still in the developmental stage, the results demonstrate that the gel does not interfere with 31P NMR analysis and retains IP6 to concentrations well above those required to give clear spectral signals. The technique has considerable potential for application to the study of IP6 in soil extracts and water samples and, with development, could help to answer fundamental questions regarding the dynamics of organic P in the environment.     

Distance to neighbours influences the trade-off between hiding after disturbance and defending food patches in convict cichlids (<Emphasis Type="Italic">Archocentrus nigrofasciatus</Emphasis>)

Territories are often aggregated. Because of this, distance to neighbours should influence how territory-holders balance safety from predators with the use and defence of resources. I examined the influence of distance to a neighbour on refuge use by pairs of convict cichlids (Archocentrus nigrofasciatus) faced with a conflict between hiding and defending food patches. Neighbours could reduce the rate of intrusions by strangers as a by-product of their own resource defence. This should allow fish with near neighbours to spend more time in the refuge. Neighbours could also steal from patches that are left undefended. This should lead to a reduction in use of the refuge. When one fish was confined to its refuge (so that its patch was undefended), theft by the other increased as inter-patch distance decreased. Distance between patches did not influence the rate of intrusion by non-territorial fish. When both fish defended patches, body mass influenced the effect of inter-patch distance on refuge use. Large fish rarely used the refuge, but small territory-holders spent more time in the refuge when patches were close together, as predicted. However, when one fish was dominant at both patches, distance between patches did not influence refuge use. These results suggest that, despite increased opportunity for theft, there is no realised foraging or defensive benefit to settling near neighbours that are of similar competitive ability.Communicated by J. Krause     

Integrating spatially referenced social and biophysical data to explore landholder responses to dryland salinity in Australia

Researchers attempting to integrate socio-economic data in watershed planning often draw on nationally collected census data. However, there are critical limitations to the usefulness of this type of data for decision makers operating at the watershed scale. In this paper we demonstrate the relevance of spatially referenced socio-economic data collected using mail surveys to random selections of rural landholders. The issue explored was dryland salinity management in two large watersheds in the Murray-Darling Basin of south-eastern Australia. Contrary to the assumptions underlying public policy in Australia, but consistent with the literature on farmer knowledge, comparisons of expert maps and landholder identified salinity sites suggested that landholders in these watersheds had excellent knowledge of the current extent of salinity on their property. Our research also suggested that salinity education was a sound investment by governments. At the same time, the expert maps failed to predict half of the saline affected sites identified by landholders. Accurately mapping the extent of salinity would seem a first step in addressing this nationally significant land degradation issue.     

Compartmentalization of metals within the diverse colloidal matrices comprising activated sludge microbial flocs

Activated sludge floc from a wastewater treatment system was characterized, with regard to principal structural, chemical, and microbiological components and properties, in relation to contaminant-colloid associations and settling. Multiscale analytical microscopies, in conjunction with multimethod sample preparations, were used correlatively to characterize diverse colloidal matrices within microbial floc. Transmission electron microscopy, in conjunction with energy dispersive spectroscopy (EDS), revealed specific associations of contaminant heavy metals with individual bacterial cells and with extracellular polymeric substances (EPS). Floc structure was mapped from the level of gross morphology down to the nano-scale, and flocs were described with respect to settling properties, size, shape, density, porosity, bound water content, and EPS chemical composition; gross surface properties were also measured for correlation with principal floc features. Compartmentalization results based on 171 EDS analyses and representative high-resolution images showed that nano-scale agglomerations of (i) silver (100%) and (ii) zinc (91%) were confined almost entirely to EPS matrices while (iii) Pb (100%) was confined to intracellular granules and (iv) aluminum was partitioned between EPS matrices (41%) and intracellular matrices (59%). The results suggest that engineered changes in microbial physiology and/or in macromolecular EPS composition may influence metal removal efficiencies.