Unit bar architecture in a highly‐variable fluvial discharge regime: Examples from the Burdekin River, Australia

Unit bars are relatively large bedforms that develop in rivers over a wide range of climatic regimes. Unit bars formed within the highly-variable discharge Burdekin River in Queensland, Australia, were examined over three field campaigns between 2015 and 2017. These bars had complex internal structures, dominated by co-sets of cross-stratified and planar-stratified sets. The cross-stratified sets tended to down-climb. The development of complex internal structures was primarily a result of three processes: (i) superimposed bedforms reworking the unit bar avalanche face; (ii) variable discharge triggering reactivation surfaces; and (iii) changes in bar growth direction induced by stage change. Internal structures varied along the length and across the width of unit bars. For the former, down-climbing cross-stratified sets tended to pass into single planar cross-stratified deposits at the downstream end of emergent bars; such variation related to changes in fluvial conditions whilst bars were active. A hierarchy of six categories of fluvial unsteadiness is proposed, with these discussed in relation to their effects on unit bar (and dune) internal structure. Across-deposit variation was caused by changes in superimposed bedform and bar character along bar crests; such changes related to the three-dimensionality of the channel and bar geometry when bars were active. Variation in internal structure is likely to be more pronounced in unit bar deposits than in smaller bedform (for example, dune) deposits formed in the same river. This is because smaller bedforms are more easily washed out or modified by changing discharge conditions and their smaller dimensions restrict the variation in flow conditions that occur over their width. In regimes where unit bar deposits are well-preserved, their architectural variability is a potential aid to their identification. This complex architecture also allows greater resolution in interpreting the conditions before and during bar initiation and development

Effects Of Prescribed Fire On An Ant Community In Florida Pine Savanna

1. The effects of prescribed fire on ant community structure were examined in a regenerating longleaf pine savanna in Florida, U.S.A. The presence of ants on 20, 10 × 10 m plots was determined by baiting every 1-3 months from 18 months before a fire until 6 months afterwards. 2. Expected species richness (based on rarefaction) and species density 6 months post-fire were significantly lower than for the same month (September) 6 months before the fire. 3. Cluster analysis revealed that the effects of fire were far less important predictors of ant community structure than seasonality and unexplained interannual variation. Thus, overall, the impacts of fire were relatively minor and short term at the community level. 4. Different functional groups of ants (as defined by Andersen, 1997) responded to fire in strikingly different ways. Generalised Myrmicinae (e.g. Pheidole spp., Monomorium viride) were affected more severely by fire than were the other functional groups. In contrast, the dominant Dolichoderinae (Forelius pruinosus) exhibited a large increase after the fire and seemed to be responsible for the decline in abundance of several species. 5. A strong negative correlation between F. pruinosus and other groups of ants immediately after the fire suggested more intense competition among ants at that time. Six months post-fire, the abundance of F. pruinosus decreased markedly and the abundance of other species rebounded. 6. 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Loss of animal seed dispersal increases extinction risk in a tropical tree species due to pervasive negative density dependence across life stages

Overhunting in tropical forests reduces populations of vertebrate seed dispersers. If reduced seed dispersal has a negative impact on tree population viability, overhunting could lead to altered forest structure and dynamics, including decreased biodiversity. However, empirical data showing decreased animal-dispersed tree abundance in overhunted forests contradict demographic models which predict minimal sensitivity of tree population growth rate to early life stages. One resolution to this discrepancy is that seed dispersal determines spatial aggregation, which could have demographic consequences for all life stages. We tested the impact of dispersal loss on population viability of a tropical tree species, Miliusa horsfieldii, currently dispersed by an intact community of large mammals in a Thai forest. We evaluated the effect of spatial aggregation for all tree life stages, from seeds to adult trees, and constructed simulation models to compare population viability with and without animal-mediated seed dispersal. In simulated populations, disperser loss increased spatial aggregation by fourfold, leading to increased negative density dependence across the life cycle and a 10-fold increase in the probability of extinction. Given that the majority of tree species in tropical forests are animal-dispersed, overhunting will potentially result in forests that are fundamentally different from those existing now

Discrimination factors (∆15N and ∆13C) in an omnivorous consumer: effect of diet isotopic ratio

1. Naturally occurring stable isotopes in resources and their consumer allow the estimation of nutritional flows between the two and have been much used to improve our understanding of the nutritional ecology of free-living animals. 2. The difference in isotopic composition between an animal and its diet is represented by a discrimination factor. Carbon and nitrogen flows are estimated by calculating the discrimination factors in stable isotope ratios (δ15N and δ13C), which are presumed to be c. 3‰ and 1‰ heavier in the consumer tissues than those in their resources, respectively. 3. The discrimination factor is known to vary according to species, tissue, age, growth rates and food quality, but the estimation of discrimination factors is difficult and a fixed discrimination factor is usually used in diet reconstruction. It has also been suggested that discrimination factors could vary linearly with the diet isotopic ratio. If this linear relationship could be demonstrated using regression, this would provide an adequate method for the estimation of discrimination factors. In order to understand how diet isotopic ratios affect the discrimination factor, we investigated the pattern of its change in nitrogen (∆15N) and carbon (∆13C) in different tissues (liver, muscle and hair) of an omnivore species, the rat Rattus rattus. We fed captive rats with diets of the same nutritional quality but on different isotopic ratios. 4. First, discrimination factors for ∆15N and ∆13C showed great variability, ranging from –1•46‰ to 4•59‰ and from –8•79‰ to 0•64‰, respectively. Discrimination factors depended on both diet isotopic ratio and tissue. 5. We also show that isotope ratios in shaved hairs showed a turnover during the first month, and then stabilized during the second month. Using shaved hairs has the potential to be an effective non-lethal method for determining resource shifts in non-specialist consumers. 6. Finally, we demonstrated, for all tissues, a decrease of ∆15N and ∆13C with an increased values of δ15N and δ13C, respectively. These relationships allow us to propose a framework to estimate discrimination factors from diet isotopic ratios by means of regression models.Peer reviewe

Nutrients in fruits as determinants of resource tracking by birds

Fruit pulp is an important source of nutrients for many bird species. Fruit-eating birds use a variety of strategies to cope with changes in the availability of fruits, exhibiting a remarkable ability to track resources. We assessed the role of nutrient availability in the fruiting environment as a factor driving resource tracking by fruit-eating birds. Fruit consumptionby the four most common frugivorous species in a 6-ha plot in the Southern Yungas montane forest of Argentina was assessed. We determined the content of selected nutrients (soluble carbohydrates, proteins, phenols, ascorbic acid and essential minerals) in 22 fruiting plant species eaten by birds, and measured fruit-frugivore interactions and the availability of nutrients and dry fruit pulp mass over 2 years. There was strong temporal covariation in the availability of the selected nutrients in fruits across the study period. Similarly, the availability of nutrients in the fruiting environment co-varied with pulp mass. Fruit consumption by the four commonest bird species and theabundance of most species were positively associated with nutrient availability and dry pulp mass. Nutrient availability was a good predictor of temporal fruit tracking by three of the four commonest frugivores. Despite large differences in particular nutrient concentrations in fruits, overall nutrient (and pulp) quantity in the fruiting environment played a greater role in fruit tracking than did the nutritional quality of individual fruits.While overall nutrient availability (i.e. across fruit) and total pulp mass were important determinants of fruit tracking, we suggest that plant species-specific differences in fruit nutrient concentration may be important in short-term foraging decisions involved in fruit choice and nutritional balance of birds.Fil: Blendinger, Pedro Gerardo. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Instituto de Ecología Regional; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Giannini, Norberto Pedro. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo; ArgentinaFil: Zampini, Iris Catiana. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales E Instituto Miguel Lillo; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Tucumán. Instituto de Quimica del Noroeste; ArgentinaFil: Ordóñez, Roxana Mabel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Tucumán. Instituto de Quimica del Noroeste; Argentina. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo; ArgentinaFil: Torres, Sebastián. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales E Instituto Miguel Lillo; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Tucumán. Instituto de Quimica del Noroeste; ArgentinaFil: Sayago, Jorge E.. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales E Instituto Miguel Lillo; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Tucumán. Instituto de Quimica del Noroeste; ArgentinaFil: Ruggera, Román Alberto. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Instituto de Ecología Regional; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Isla, Maria Ines. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Tucumán. Instituto de Quimica del Noroeste; Argentina. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo; Argentin