Arnan X., Bosch J., Comas L., Gracia M., Retana J. (2011) Habitat determinants of abundance, structure and composition of flying Hymenoptera communities in mountain old-growth forests. Insect Conservation and Diversity. 4: 200-211.EnllaçDoi: 10.1111/j.1752-4598.2010.00123.x
1.Old-growth forests have features that endow them with an extraordinary ecological value. These forests are sources of habitat diversity and, consequently, biodiversity, which makes them a basic objective of conservation programs. Insects have been traditionally used as indicators of forest condition. 2.The aim of this study is to uncover patterns of Hymenoptera abundance and diversity, and their relationship with structural features in old-growth forests. We use pan traps to sample the community of flying Hymenoptera in two old-growth forest types (silver fir and mountain pine) with important structural differences. 3.Compared to other surveys of local Hymenoptera communities, our sampling yielded an extremely high number of species (630). 4.At the plot level, the two forest types showed important differences in family richness and diversity, but not in abundance or in species richness and diversity. However, variability in species richness was higher among pine than silver fir plots, leading to overall higher species richness in the former. 5.Species composition also differed between pine than silver fir forests, and these differences were related to important structural differences between the two forest types. 6.Canonical correspondence and multiple regression analysis yielded contrasting habitat requirements among Hymenoptera families and functional groups (bees, sawflies, parasitic wasps and predatory wasps). 7.We conclude that flying Hymenoptera communities can be used as good indicators of forest structure, habitat complexity and conservation status. © 2010 The Authors. Insect Conservation and Diversity © 2010 The Royal Entomological Society.
Arnan X., Ferrandiz-Rovira M., Pladevall C., Rodrigo A. (2011) Worker size-related task partitioning in the foraging strategy of a seed-harvesting ant species. Behavioral Ecology and Sociobiology. 65: 1881-1890.EnllaçDoi: 10.1007/s00265-011-1197-z
Messor bouvieri is a seed-harvesting ant species in which workers forage in trails from the nest to a search area. A previous observation of seed transfer events between workers returning to the nest suggested potential task partitioning. In this study, we describe seed transportation and analyze the role of task partitioning in the foraging strategy of this species in terms of seed intake efficiency in relation to costs and benefits based on transport speed and task reliability. We assess the harvesting efficiency of task partitioning by comparing cooperative seed transport (CST) and individual seed transport (IST) events. Our results show task partitioning in the form of a sequence of transfer events among workers going from the search area to the nest. Importantly, and despite the weak worker polymorphism of this species, this sequence involved workers of different sizes, with seeds usually being passed along from smaller to larger workers. In addition, we show that small workers are better at finding seeds (spend less time finding a seed), and large workers are better at transporting them (were faster when walking back to the nest and lost fewer seeds). However, we failed to demonstrate that workers of different sizes are specialized in performing the task in which they excel. Overall, sequential CST in M. bouvieri results in a greater seed intake because seed search time decreases and task reliability increases, compared to IST. The determinants and adaptive benefits of CST are discussed. © 2011 Springer-Verlag.
Arnan X., Gaucherel C., Andersen A.N. (2011) Dominance and species co-occurrence in highly diverse ant communities: A test of the interstitial hypothesis and discovery of a three-tiered competition cascade. Oecologia. 166: 783-794.EnllaçDoi: 10.1007/s00442-011-1919-y
The role of competitive exclusion is problematic in highly diverse ant communities where exceptional species richness occurs in the face of exceptionally high levels of behavioural dominance. A possible non-niche-based explanation is that the abundance of behaviourally dominant ants is highly patchy at fine spatial scales, and subordinate species act as insinuators by preferentially occupying these gaps-we refer to this as the interstitial hypothesis. To test this hypothesis, we examined fine-scale patterns of ant abundance and richness according to a three-tiered competition hierarchy (dominants, subdominants and subordinates) in an Australian tropical savanna using pitfall traps spaced at 2 m intervals. Despite the presence of gaps in the fine-scale abundance of individual species, the combined abundance of dominant ants (species of Iridomyrmex, Papyirus and Oecophylla) was relatively uniform. There was therefore little or no opportunity for subordinate species to preferentially occupy gaps in the foraging ranges of dominant species, and we found no relationship between the abundance of dominant ants and nondominant species richness at fine spatial scales. However, we found a negative relationship between subdominant and subordinate ants, a negative relationship between dominant and subdominant ants, and a positive relationship between dominant and subordinate ants. These results suggest that dominant species actually promote species richness by neutralizing the effects of subdominant species on subordinate species. Such indirect interactions have very close parallels with three-tiered trophic cascades in food webs, and we propose a "competition cascade" where the interactions are through a competition rather than trophic hierarchy. © 2011 Springer-Verlag.
Arnan X., Rodrigo A., Retana J. (2011) What are the consequences of ant-seed interactions on the abundance of two dry-fruited shrubs in a Mediterranean scrub?. Oecologia. 167: 1027-1039.EnllaçDoi: 10.1007/s00442-011-2034-9
Strong interactions between dry-fruited shrubs and seed-harvesting ants are expected in early successional scrubs, where both groups have a major presence. We have analysed the implications of the seed characteristics of two dry-fruited shrub species (Coronilla minima and Dorycnium pentaphyllum) on seed predation and dispersal mediated by harvester ants and the consequences of these processes on spatio-temporal patterns of plant abundance in a heterogeneous environment. We found that large C. minima seeds were collected much more (39%) than small D. pentaphyllum seeds (2%). However, not all of the removed seeds of these plant species were consumed, and 12.8% of the seeds were lost along the trails, which increased dispersal distances compared with abiotic dispersal alone. Seed dropping occurred among all microhabitats of the two plant species, but especially in open microhabitats, which are the most suitable ones for plant establishment. The two plant species increased their presence in the study area during the study period: C. minima in open microhabitats and D. pentaphyllum in high vegetation. The large size of C. minima seeds probably limited the primary seed dispersal of this species, but may have allowed strong interaction with ants. Thus, seed dispersal by ants resulted in C. minima seeds reaching more suitable microhabitats by means of increasing dispersal distance and redistribution among microhabitats. In contrast, the smaller size of D. pentaphyllum seeds arguably allows abiotic seed dispersal over longer distances and colonization of all types of microhabitats, although it probably also limits their interaction with ants and, consequently, their redistribution in suitable microhabitats. We suggest that dyszoochory could contribute to the success of plant species with different seed characteristics in scrub habitats where seeds are abundantly collected by seed-harvesting ants. © 2011 Springer-Verlag.
Espelta J.M., Arnan X., Rodrigo A. (2011) Non-fire induced seed release ina weakly serotinous pine: Climatic factors, maintenance costs or both?. Oikos. 120: 1752-1760.EnllaçDoi: 10.1111/j.1600-0706.2011.19570.x
The advantages of canopy seed retention (serotiny) for plants inhabiting fire-prone ecosystems are well documented. However, very few species are completely serotinous and non-fire induced opening of serotinous fruits is commonly observed (weak serotiny). Two non-mutually exclusive causes are envisaged to contribute to this process: mechanical changes in serotinous fruits mediated by climatic conditions (e.g. drought) or the costs of maintenance for the plant of these long-lasting structures. However, their relative contribution to the spontaneous opening of serotinous fruits remains elusive as well as the consequences for the build-up of the canopy seed bank and inter-individual differences in serotiny. In this study we monitored the dynamics of cone production and cone opening in the weakly serotinous Pinus halepensis for five years (2004-2008), including two severe drought episodes (2005, 2006). Drought decreased the production of conelets, increased the abortion of immature cones, reduced the seed quality in the cohorts of cones produced during these years, and increased the opening of serotinous cones. During the first drought episode, a higher proportion of serotinous cones opened in those pines bearing a larger crop of younger cones. This suggests that not only passive changes induced by drought but also competition among cones for resources (e.g. water) might be involved in this process. The opening of serotinous cones in pines bearing more cones made inter-individual differences in the size of the canopy cone bank to narrow or even to reverse from 2004 to 2008. These results may help to understand the decrease in serotiny when pines grow and accumulate more cones and the large inter-individual variability in the degree of serotiny observed in P. halepensis forests. In addition, the negative effects of drought episodes for the size of the canopy cone bank and the seeds contained can be an unexplored cause of post-fire regeneration constraint. © 2011 The Authors.
Jenkins C.N., Sanders N.J., Andersen A.N., Arnan X., Brühl C.A., Cerda X., Ellison A.M., Fisher B.L., Fitzpatrick M.C., Gotelli N.J., Gove A.D., Guénard B., Lattke J.E., Lessard J.-P., Mcglynn T.P., Menke S.B., Parr C.L., Philpott S.M., Vasconcelos H.L., Weiser M.D., Dunn R.R. (2011) Global diversity in light of climate change: The case of ants. Diversity and Distributions. 17: 652-662.EnllaçDoi: 10.1111/j.1472-4642.2011.00770.x
Aim To use a fine-grained global model of ant diversity to identify the limits of our knowledge of diversity in the context of climate change. Location Global. Methods We applied generalized linear modelling to a global database of local ant assemblages to predict the species density of ants globally. Predictors evaluated included simple climate variables, combined temperature×precipitation variables, biogeographic region, elevation, and interactions between select variables. Areas of the planet identified as beyond the reliable prediction ability of the model were those having climatic conditions more extreme than what was represented in the ant database. Results Temperature was the most important single predictor of ant species density, and a mix of climatic variables, biogeographic region and interactions between climate and region yielded the best overall model. Broadly, geographic patterns of ant diversity match those of other taxa, with high species density in the wet tropics and in some, but not all, parts of the dry tropics. Uncertainty in model predictions appears to derive from the low amount of standardized sampling of ants in Asia, in Africa and in the most extreme (e.g. hottest) climates. Model residuals increase as a function of temperature. This suggests that our understanding of the drivers of ant diversity at high temperatures is incomplete, especially in hot and arid climates. In other words, our ignorance of how ant diversity relates to environment is greatest in those regions where most species occur - hot climates, both wet and dry. Mainconclusions Our results have two important implications. First, temperature is necessary, but not sufficient, to explain fully the patterns of ant diversity. Second, our ability to predict ant diversity is weakest exactly where we need to know the most, the warmest regions of a warming world. This includes significant parts of the tropics and some of the most biologically diverse areas in the world. © 2011 Blackwell Publishing Ltd.
Arnan X, López BC, Martínez-Vilalta J, Estorach M, Poyatos R (2011) The age of monumental olive trees (Olea europaea) in northeastern Spain. Dendrochronología doi: 10.1016/j.dendro.2011.02.002.
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