Can differences in the structure of larval,juvenile and adult coral‐reef fish assemblages be detected at the family level?

Processes occurring at the end of the larval stage are of major importance in shaping spatial structure of fish assemblages in coral reefs. However, because of the difficulty in identifying larvae to species, many studies dealing with these stages are limited to the family level. It remains unknown if variation in the spatial structure of coral‐reef fish assemblages across life stages can be detected at such a coarse taxonomic level. Two different surveys conducted in a similar area of New Caledonia, Southwest Pacific, provided the opportunity to compare the structure of coral‐reef fish assemblages collected as pre‐settlement larvae, juveniles and adults along a coast to barrier reef gradient. Adult and juvenile fish were sampled using underwater visual counts (UVC) during the warm seasons of 2004 and 2005. Pre‐settlement larvae were sampled with light‐traps during the same seasons. In order to standardize data between sampling methods, analyses were conducted on the relative abundance (for larvae) and density (for juveniles and adults) of 21 families commonly collected with both methods. Relative abundances/densities of families were analysed as a function of life stage (larvae, juveniles or adults), large‐scale spatial location (coast, lagoon or barrier) and years (2004, 2005) using non‐parametric multidimensional scaling (nMDS) and permutational multivariate analysis of variance (permanova ). Kruskal–Wallis tests were then used to examine differences among life stages and locations for individual families. Different levels of spatial and temporal variability characterized fish assemblages from different life stages, and differences among life stages were detected at all locations and years. Differences among life stages were also significant at the level of individual families. Overall results indicate that studies conducted at the family level may efficiently reveal changes in coral‐reef fish spatial structure among successive life stages when large spatial scales are considered.     

Invertebrate bioturbation can reduce the clogging of sediment: an experimental study using infiltration sediment columns

1. Invertebrate bioturbation can strongly affect water‐sediment exchanges in aquatic ecosystems. The objective of this study was to quantify the influence of invertebrates on the physical characteristics of an infiltration system clogged with fine sediment. 2. Two taxa (chironomids and tubificids) with different bioturbation activities were studied in experimental slow infiltration columns filled with sand and gravel and clogged with a 2 cm layer of fine sediment at the surface. We measured the effects of each taxon separately and combined on hydraulic head, water mobility and sediment reworking. 3. The results showed that invertebrates could reduce sediment clogging and this effect was linked to the functional mode of bioturbation of each group. Tubificid worms dug networks of galleries in the fine sediment, creating pathways for water flow, which reduced the clogging of sediment. In contrast, the U‐shaped tubes of chironomids were restricted to the superficial layer of fine sediments and did not modify the hydraulic conductivity of experimental columns. The combination of invertebrates did not show any interactive effects between tubificids and chironomids. The occurrence of 80 tubificids in the combination was enough to maintain the same hydraulic conductivity that 160 worms did in monospecific treatment. 4. The invertebrates like tubificid worms can have a great benefit on functioning of clogged interfaces by maintaining high hydraulic conductivity, which contributes to increased water‐sediment exchanges and stimulates biogeochemical and microbial processes occurring in river sediments.     

Linking the occurrence of brook trout with isolation and extinction in small Boreal Shield lakes

1. We surveyed 62 Canadian Shield lakes (<50 ha) to determine the relationship between factors related to isolation and extinction and the occurrence of brook trout (BT) (Salvelinus fontinalis), for which local extinctions have been documented over the last century in half of the lakes. 2. Logistic regression and information–theoretic model selection were used to determine the importance for the occupancy of BT of (i) isolation factors (degree of lake connectivity and the proximity of source populations of BT in neighbouring bodies of water) and (ii) extinction factors (lake morphometry and trophic status, as proxies of the risk of lake anoxia; predation and competition; and flooding caused by beaver (Castor canadensis) dams, which could potentially increase the risk of anoxia). 3. Isolation factors were the best predictors of the absence of BT in these lakes. Among extinction factors, only the impact of beaver dams (as measured by an index of increased water level and mortality of shrubs and trees in the littoral zone) improved model fits. Beaver dams were present at the outlets of all study lakes, but extensive mortality of riparian trees and shrubs was more common in lakes where BT populations were extinct. 4. Taken together, these results suggest that recolonization is a major factor determining the occurrence of BT while flooding caused by beaver dams might contribute to BT extinction by increasing the likelihood of winterkill in these small lakes.     

A Space-time Model of Carbon Translocation along a Shoot Bearing Fruits

A carbon-based model is described of the source-sink relationshipsof a stem bearing fruits in space and time and focusing on growthvariability along the branch. The novelty of the model comesfrom the aggregation of physiological processes taking intoaccount spatial aspects. The stem is represented as a set ofcompartments (metamers) connected to source (leafy shoots) andsink (fruits) compartments. Each leafy shoot forms one compartment.The fruit consists of three compartments involved in translocation(cytoplasm), structure (cell wall) and storage (vacuole). Physiologicalprocesses considered are photosynthesis, respiration of fruitsand leaves, translocation of assimilates and fruit growth. Assimilateproduction is regulated by sink strength. Carbon translocationbetween two compartments depends on the gradient of assimilateconcentration. The gradient induces carbon translocation fromthe most to the least concentrated compartment, except for thevacuole compartment into which translocation is possible whateverthe concentration gradient. Fruit growth, in terms of freshweight, results from the phloem water supplied to the fruitaccording to the concentration gradient between the fruit andthe stem. The model is calibrated for peach trees by comparingobserved and simulated fruit dry and fresh weights for a shootwith normal fruit load. The model simulates variability betweenpeach fruits and the effect of contrasting fruit loads. Accordingto this model, photosynthesis increases and assimilate concentrationsin leaves and phloem decrease with decreasing leaf:fruit ratioas reported in the literature. Simulated concentrations of assimilatesin the phloem range from 2 to 14%. Simulated concentration gradientsand specific mass transfer for peach trees range from 0.05 to0.17 g cm^-3m^-1and from 0 to 3 g cm^-2h^-1, respectively, and areof the same order of magnitude as those reported for variousother tree species. The model is used to analyse the effectof fruit position relative to the leaves. Copyright 1999 Annalsof Botany Company Peach tree, Prunus persica (L.) Batsch, model, carbohydrates, translocation, source-sink, fruit.     

The interaction of soil biota and soil structure under global change

The structural framework of soil mediates all soil processes, at all relevant scales. The spatio-temporal heterogeneity prevalent in most soils underpins the majority of biological diversity in soil, providing refuge sites for prey against predator, flow paths for biota to move, or be moved, and localized pools of substrate for biota to multiply. Just as importantly, soil biota play a crucial role in mediating soil structure: bacteria and fungi aggregate and stabilize structure at small scales (μm–cm) and earthworms and termites stabilize and create larger-scale structures (mm–m). The stability of this two-way interaction of structure and biota relations is crucial to the sustainability of the ecosystem. Soil is constantly reacting to changes in microclimates, and many of the soil–plant–microbe processes rely on the functioning of subtle chemical and physical gradients. The effect of global change on soil structure–biota interactions may be significant, through alterations in precipitation, temperature events, or land-use. Nonetheless, because of the complexity and the ubiquitous heterogeneity of these interactions, it is difficult to extrapolate from general qualitative predictions of the effects of perturbations to specific reactions. This paper reviews some of the main soil structure–biota interactions, particularly focusing on soil stability, and the role of biota mediating soil structures. The effect of alterations in climate and land-use on these interactions is investigated. Several case studies of the effect of land-use change are presented.     

A Comparative Study of 14 Strains of Naegleria australiensis Demonstrates the Existence of a Highly Virulent Subspecies: N. australiensis italica n. spp.1

Fourteen strains of Naegleria australiensis, including the type strain, were compared for virulence for mice, maximum growth temperature, lectin agglutination, isoenzyme pattern, and total protein banding pattern. Their relation to other species of Naegleria also was compared by immunoelectrophoretic analysis. Strains with high virulence, comparable to that of N. fowleri, were found to be different in concanavalin A agglutination as well as with regard to zymograms and total protein patterns. Although serologically different from N. fowleri and reacting with N. australiensis antiserum in the fluorescent antibody test, these high-virulence strains differed in number of immunoelectrophoretic precipitin bands. Because of these results, the high-virulence strains are considered to be a subspecies of N. australiensis. The low-virulence strains showed minor differences from the type strain. Thus, N. australiensis does not appear to be as homogenous a species as N. fowleri. Pathogenic N. australiensis also seems to be more widespread than previously thought.     

Sexual behaviour and evolution of sexual dimorphism in body size in Jaera (Isopoda Asellota)

Individuals of the genus Jaera do not mate at random. In the species from the Mediterranean group, J. italica and. J. nordmanni, large males and medium sized females are at an advantage and their sizes are positively assorted. These effects are attributable to sexual competition between males. In the Ponlo-caspian species J. istri, no advantage of large males exists, but sexual selection could be the cause for a long passive phase prior to copulation and for normalizing selection upon female size at pairing. In the Atlantic species, J. albifrons, no selection can be ascertained.
Differential mating success in males appears as one of the causes of the evolution of sexual dimorphism in body size, which makes males larger, of equal size, or smaller than females according to the species. The reason for this reversal in dimorphism seems to differ in the two sexes. Sexual selection provides an explanation for the evolution of male size, while the interspecific changes in female length are more likely due to ecological factors.