Morphological changes in Camponotus punctulatus (Mayr) anthills of different ages

In north-eastern Argentina, Camponotus punctulatus builds large numbers of big and coherent anthills after abandonment of rice cultivation. These anthills easily reach 1 m in height and 2 m in diameter, and a density of 1800 nests ha(-1). We studied the internal morphology of C punctulatus aged anthills of 4, 6 and 15 years, respectively, by describing and quantifying, meso- and macroporosity of undisturbed soil samples using image analysis. Four different parts were distinguished on these cone-shaped anthills: the loose granular cortex, the summit, the core of the anthill and the base of the anthill at ground level. The percentage of macroporosity in anthills significantly differed between the parts of the anthill, but changed little with age except for the 15 year old anthill that had increased percentages of mesopores and macropores with rounded and irregular shapes. Walls of the chambers and galleries were highly compacted, highlighting intense ant activity in the anthills, although it was localised mainly in the upper central part. After 6 years the anthills became surrounded by a discontinuous peripheral depression, which likely affects water drainage and infiltration. In 15 year old anthills the lateral depressions became a continuous ditch where water accumulates giving place to a constant wetted zone inside the anthill. Our results support the previous consideration of C punctulatus as an ecosystem engineer, although in this case related to the changes produced on the soil surrounding the anthills which may affect the survival and distribution of other soil organisms. (c) 2005 Elsevier B.V. All rights reserved

Soil organic matter dynamics along a rice chronosequence in north-eastern Argentina : evidence from natural C-13 abundance and particle size fractionation

We studied the consequences of rice cultivation and its subsequent abandonment for soil organic matter (SOM) dynamics in northeastern Argentina. Two chronosequences, which included a pristine grassland with C4 vegetation as a control, and several stages of rice (C3) fields abandoned for 1, 2, 4, 6 and 15 years were selected, and soil samples from the first 10 cm were gathered from each plot. Natural C-13 abundance coupled with particle-size fractionation were employed to characterize SOM changes through time discriminated by SOM origin. Soil samples up to 50 cm were also collected throughout one chronosequence. Most changes in SOM occurred on the first 20 cm layer and, bulk density, carbon and nitrogen content, as well as delta C-13 remained similar at greater depths. After the rice cropping, the bulk density was slightly greater than in the natural grassland, and remained stable after the abandonment. Carbon and nitrogen contents remained almost stable in the surface layer during the cultivation. delta C-13 varied accordingly with the changes in vegetation cover with a C4 signature in the natural grassland and mainly a C3 signature in the rice fields. The abandonment of the rice cropping induced a decrease of the soil organic matter content, mainly of natural grassland origin, during the first 4 years. When the abandonment extended, the SOM content (from C4 origin) increased slowly and after 15 years, was almost the same as that of the natural grassland. The carbon turnover was greater in the coarser fractions than in the finer ones, confirming that soil organic carbon in the sand fraction was relatively labile. However, all the fractions were affected by inputs and outputs of C derived from rice and natural grassland. This fact could indicate that the former protected carbon could become less stable due to cultivation. (c) 2006 Elsevier Ltd. All rights reserved

Camponotus punctulatus ant's demography: a temporal study across land-use types and spatial scales

Agricultural activities promote the explosion of diverse pest populations. In Argentina, the ant Camponotus punctulatus invades agricultural fields after production ceases. The temporal demography and spatial distribution of colonies of C. punctulatus were studied over a five year period using replicated plots of different land use types representing a gradient of increasing agricultural disturbance. We experimentally tested the hypothesis that the increase in C. punctulatus colony density was related to increasing levels of agricultural disturbance. Abandoned rice fields represented the situation with greatest disturbance. Sown pastures were intermediate. Natural grasslands represented no agricultural disturbance. The predictions were (1) the greater the soil disturbance produced by agriculture, the greater the susceptibility for invasion by C. punctulatus, (2) rice fields offers greater opportunities for establishment of colonizing species than sown pastures, and (3) disturbed land use areas that were more recently colonized as well as land use areas with greater soil disturbance will exhibit patterns of colony aggregation at a small scale but with time the patterns will become uniform. Initially, colonies in the abandoned rice fields had a higher annual mortality and larger turnover than in sown pastures. Over five years, abandoned rice fields sustained higher densities of colonies than sown pastures. The colonies were the largest and had the longest lifespans in abandoned ricefields. Natural grasslands had the lowest colony density, survivorship, and size but had variable levels of colonization. More than one type of spatial distribution was found in field replicates. At small spatial scales across disturbed land use types, replicates exhibited regular distributions. At greater spatial scales, spatial distributions were mostly random in sown pastures, there were many cases of aggregation in rice fields, although some cases of uniform distributions were also found in all disturbed land uses. These results highlight significant intraspecific variation in ant demography across types of land use, space, and time, and show a clear predisposition of C. punctulatus to invade and successfully establish in the most disturbed land use types. Hypotheses that can account for the changes in demography across land use types are discusse

Tree diversity reduces herbivory by forest insects

Biodiversity loss from plant communities is often acknowledged to affect primary production but little is known about effects on herbivores. We conducted a meta-analysis of a worldwide data set of 119 studies to compare herbivory in single-species and mixed forests. This showed a significant reduction of herbivory in more diverse forests but this varied with the host specificity of insects. In diverse forests, herbivory by oligophagous species was virtually always reduced, whereas the response of polyphagous species was variable. Further analyses revealed that the composition of tree mixtures may be more important than species richness per se because diversity effects on herbivory were greater when mixed forests comprised taxonomically more distant tree species, and when the proportion of non-host trees was greater than that of host trees. These findings provide new support for the role of biodiversity in ecosystem functioning across trophic level