Non-random Structure Research Articles

Punishment Leads to Cooperative Behavior in Structured Societies

Altruistic punishment occurs when an agent incurs a cost to punish another but receives no material benefit for doing so. Despite the seeming irrationality of such behavior, humans in laboratory settings routinely pay to punish others even in anonymous, one-shot settings. Costly punishment is ubiquitous among social organisms in general and is increasingly accepted as a mechanism for the evolution of cooperation. Yet if it is true that punishment explains cooperation, the evolution of altruistic punishment remains a mystery. In a series of computer simulations I give agents the ability to punish one another while playing a continuous prisoner's dilemma. In simulations without social structure, expected behavior evolves-agents do not punish and consequently no cooperation evolves. Likewise, in simulations with social structure but no ability to punish, no cooperation evolves. However, in simulations where agents are both embedded in a social structure and have the option to inflict costly punishment, cooperation evolves quite readily. This suggests a simple and broadly applicable explanation of cooperation for social organisms that have nonrandom social structure and a predisposition to punish one another. Results with scale-free networks further suggest that nodal degree distribution plays an important role in determining whether cooperation will evolve in a structured population.

The role of spatial processes and environmental determinants in microgeographic shell variation of the freshwater snail Chilina dombeyana (Bruguière, 1789)

Wildlife data often show spatial organization, demonstrating positive correlations either as a result of processes occurring over the landscape or due to the influence of spatially structured environmental variables. It is, thus, essential to consider non-random spatial structure when evaluating the underlying causes of biological variation. In this study, we analyzed the population structure of Chilina dombeyana shell morphology of 14 populations that are close geographically and belong to the same hydrographic basin. We utilized a variation partitioning approach to evaluate the importance of spatial processes, such as migration, acting over the landscape, and environmental characteristics, including habitat and hydrologic characteristics, and the occurrence of aquatic predators in promoting between population variation. Our results demonstrate spatially structured variation in C. dombeyana shell morphology, with populations living near each other having more similar shell sizes than populations living farther apart. The shell size variation partition indicated that both spatially structured environmental factors and genetic relationships resulting from migration or shared common ancestry may explain this pattern. Shell shape variation, in contrast, was found to be essentially under the influence of non-spatially structured environmental factors, with habitat and water characteristics accounting for about half of the total variation among populations. The large proportion of the variation in shell size that is spatially structured demonstrates that spatial structure on morphological traits might be strong and highlights the need to consider such phenomenon in intraspecific studies of phenotypic evolution.

Quantitative Analysis of the Chromatin Proteome in Disease Reveals Remodeling Principles and Identifies High Mobility Group Protein B2 as a Regulator of Hypertrophic Growth

A fundamental question in biology is how genome-wide changes in gene expression are enacted in response to a finite stimulus. Recent studies have mapped changes in nucleosome localization, determined the binding preferences for individual transcription factors, and shown that the genome adopts a nonrandom structure in vivo. What remains unclear is how global changes in the proteins bound to DNA alter chromatin structure and gene expression. We have addressed this question in the mouse heart, a system in which global gene expression and massive phenotypic changes occur without cardiac cell division, making the mechanisms of chromatin remodeling centrally important. To determine factors controlling genomic plasticity, we used mass spectrometry to measure chromatin-associated proteins. We have characterized the abundance of 305 chromatin-associated proteins in normal cells and measured changes in 108 proteins that accompany the progression of heart disease. These studies were conducted on a high mass accuracy instrument and confirmed in multiple biological replicates, facilitating statistical analysis and allowing us to interrogate the data bioinformatically for modules of proteins involved in similar processes. Our studies reveal general principles for global shifts in chromatin accessibility: altered linker to core histone ratio; differing abundance of chromatin structural proteins; and reprogrammed histone post-translational modifications. Using small interfering RNA-mediated loss-of-function in isolated cells, we demonstrate that the non-histone chromatin structural protein HMGB2 (but not HMGB1) suppresses pathologic cell growth in vivo and controls a gene expression program responsible for hypertrophic cell growth. Our findings reveal the basis for alterations in chromatin structure necessary for genome-wide changes in gene expression. These studies have fundamental implications for understanding how global chromatin remodeling occurs with specificity and accuracy, demonstrating that isoform-specific alterations in chromatin structural proteins can impart these features.

A model of co-occurrence: segregation and aggregation patterns in the mycoflora of the crayfish Procambarus clarkii in Lake Trasimeno (central Italy)

Although attention on crayfish diseases has recently proliferated, the focus is mainly on a single host-parasite relationship rather than analyzing the entire mycoflora, probably due to the fact that (1) some diseases are occasional (pathogens of which are of difficult collection) and (2) economic impact is more relevant in aquaculture (i.e., a controlled environment where only few parasites occur) than in natural conditions. Contrary to this viewpoint, fungal ectoparasites assume a great importance since they are integral components in shaping community, ecosystem structure, and energy flow. Here we described the fungal species co-occurrence patterns (species segregation/ aggregation) using null models comparing the occurrence frequencies of ectosymbiots through host individuals with those expected by chance. Non-indigenous crayfish species like Procambarus clarkii allow to observe phenomena of competitive exclusion (segregation) or mutualism (aggregation) amongst fungi. We analyzed the fungal species occurrence on 86 host crayfish monthly collected from June 2007 to June 2008 in the Lake Trasimeno (Central Italy). A total of 29 fungal taxa were detected. Taxa showed a particular fungal assemblage with a great species variation depending on sampling month and crayfish sex, size, and body region. In particular, fungal species showed different occurrence patterns in both richness and segregation/ aggregation. The fungal community shows a non-random structure not in all cases, with the majority of tests indicating segregation, not aggregation of ectosymbiontic species on hosts. The non-random patterns of fungal co-occurrence in some cases suggest that some temporally or spatially variable factors are responsible for the establishment of the mycoflora community assemblage. We also detected temporal differences in fungal co-occurrences patterns. Regarding seasonal samples, the same analyses showed a competitive structure only in the sub-communities found on pleopodal coxae. In addition, analyses considering crayfish dimension evidenced non-random co-occurrence patterns only in crayfish hosts with a cephalothorax length larger than 45 mm. Our results demonstrated that, depending on sampling month and crayfish sex, size, and body region, well structured fungal assemblages on crayfish hosts can alternate with random fungal assemblages. Whenever non-randomness of fungal co-occurrence was detected, it hinted segregation, suggesting the lack of co-evolutive phenomena of mutualism, and favoring a competitive exclusion among the mycoflora species.

Dietary relationships among coexisting anuran amphibians: a worldwide quantitative review

Anuran amphibians have traditionally been considered suitable model organisms for community ecology studies. In this paper, we review and quantitatively re-analyze the original studies published on dietary relationships among anuran communities worldwide. We re-analyzed 33 independent communities from all continents, including data on prey numbers and/or prey volumes. All datasets underwent null model re-analysis using two randomization algorithms. In addition, logistic regression models were used to identify specific assemblage patterns. We discovered three main patterns: (1) one particular algorithm (RA2, which replaces every cell in the matrix with one randomly chosen, but retains the zero structure of the matrix) performed much better at uncovering community structure; (2) nonrandom structure was more likely to occur in tropical than in nontropical regions; (3) prey volume was a better descriptor of dietary relationships than prey number when attempting to detect nonrandom structure. We showed that: pattern (1) depended on both the overall generalist nature of the anurans in general and on the mathematical properties of RA2; pattern (2) was consistent with theoretical attributes of tropicality; and pattern (3) was due to the intrinsic ecological properties of generalist organisms, which forage more or less opportunistically on prey of very different sizes; hence, when prey number is used as a diet descriptor, there is a risk of approximating different prey taxa that make different relative volume contributions to the overall diet as being of similar relevance .

Temporal niche overlap among insectivorous small mammals

Being active in the same environment at different times exposes animals to the effects of very different environmental factors, both biotic and abiotic. In the present study, we used live traps equipped with timing devices to evaluate the potential role of biotic factors (competition and food abundance) on overall overlap in the temporal niche axis of 4 insectivorous small mammals in high-elevation grassland fields ('campos de altitude') of southern Brazil. Based on resources availability (invertebrates), data on animal captures were pooled in 2 seasons: 'scarcity' (June 2001-September 2001) and 'abundance' (November 2001-May 2002) seasons. We tested for non-random structure in temporal niche overlap among the species in each season. These species were the rodents Oxymycterus nasutus (Waterhouse, 1837), Deltamys sp., Akodon azarae (Fischer, 1829), and the marsupial Monodelphis brevicaudis Olfers, 1818. The studied community was mainly diurnal with crepuscular peaks. Simulations using the Pianka index of niche overlap indicated that the empirical assemblage-wide overlap was not significantly different from randomly generated patterns in the abundance season but significantly greater than expected by chance alone in the scarcity season. All the species showed an increase in temporal niche breadth during the abundance season, which appears to be related to longer daylength and high nocturnal temperatures. Patterns on both temporal niche overlap and temporal niche breadth were the opposite to those that we were expecting in the case of diel activity patterns determined by competition for dietary resources. Therefore, we conclude that competition did not seem to be preponderant for determining patterns of temporal niche overlap by the studied community.

Grouping and fission–fusion dynamics in Australian snubfin and Indo-Pacific humpback dolphins

Dolphins live in complex social systems with a wide variety of grouping and association patterns. Understanding the spatiotemporal variation of these associations (fission–fusion dynamics) is necessary to investigate the underlying factors and mechanisms shaping mammalian social systems in aquatic environments. We used boat-based surveys, photoidentification, focal observations, association analyses and social network techniques to quantify variation in the grouping patterns and fission–fusion dynamics of small, sympatric populations of Australian snubfin dolphins, Orcaella heinsohni, and Indo-Pacific humpback dolphins, Sousa chinensis, off the northeast coast of Queensland. Schools of snubfin dolphins were larger and more stable, irrespective of behavioural activity, than those of humpback dolphins. While associations of both species showed nonrandom patterns and structure, the social network of snubfin dolphins was characterized by numerous strong associations, whereas the strength of the humpback dolphin’s social network did not differ from random. Modelling of temporal patterns of association indicated long-lasting associations were an important feature of snubfin dolphins’ fission–fusion dynamics. In contrast, associations among humpback dolphins over time were best described by short-term relationships. The contrasting grouping and fission–fusion dynamics of snubfin and humpback dolphins appear to be a response to different feeding habits and prey availability. Future studies involving molecular techniques and direct quantification of food availability and predation risk will help elucidate the suite of interacting ecological, social and evolutionary factors shaping their social structures.

Site fidelity and association patterns of Indo-Pacific humpback dolphins off the east coast of Zhanjiang, China

This study examined the population of Indo-Pacific humpback dolphins, Sousa chinensis, inhabiting the waters off the east coast of Zhanjiang, China. A total of 116 dolphins were identified during 147 boat-based surveys, completed between June 2005 and June 2007. Abundance estimates indicated that a small population of 268 dolphins (95% CI = 189–413) inhabited this coastal area. The sighting frequencies of identified dolphins varied between one and five when the 2006 and 2007 data were organized into six occasions. Twenty-three percent (27 individuals) of the cataloged dolphins were identified in more than one calendar year. The two animals (ZJ001, ZJ011) with the highest degree of site fidelity were present in the study area for a period of 23 months. The most frequently (15 times) sighted dolphin (ZJ046) was recorded in nine (56%) of the 16 months surveyed. This evidence indicates the interannual site fidelity of S. chinensis to the area off the eastern coast of Zhanjiang City. Range sizes of eight identified individuals calculated by the minimum convex polygon varied from 2.07 to 331.20 km2. Associations between 34 individuals sighted at least three times and more than three times were measured by the half-weight index and SOCPROG program. The majority of these dolphins (80.57%) were not seen together during surveys and the mean association rate between dyads was low at 0.05. Temporal analyses for all the individuals were conducted using a lagged association rate. The results indicated a non-random social structure made by constant companions.

Communication patterns in a psychotherapy following traumatic brain injury: A quantitative case study based on symbolic dynamics

BackgroundThe role of psychotherapy in the treatment of traumatic brain injury is receiving increased attention. The evaluation of psychotherapy with these patients has been conducted largely in the absence of quantitative data concerning the therapy itself. Quantitative methods for characterizing the sequence-sensitive structure of patient-therapist communication are now being developed with the objective of improving the effectiveness of psychotherapy following traumatic brain injury.MethodsThe content of three therapy session transcripts (sessions were separated by four months) obtained from a patient with a history of several motor vehicle accidents who was receiving dialectical behavior therapy was scored and analyzed using methods derived from the mathematical theory of symbolic dynamics.ResultsThe analysis of symbol frequencies was largely uninformative. When repeated triples were examined a marked pattern of change in content was observed over the three sessions. The context free grammar complexity and the Lempel-Ziv complexity were calculated for each therapy session. For both measures, the rate of complexity generation, expressed as bits per minute, increased longitudinally during the course of therapy. The between-session increases in complexity generation rates are consistent with calculations of mutual information. Taken together these results indicate that there was a quantifiable increase in the variability of patient-therapist verbal behavior during the course of therapy. Comparison of complexity values against values obtained from equiprobable random surrogates established the presence of a nonrandom structure in patient-therapist dialog (P = .002).ConclusionsWhile recognizing that only limited conclusions can be based on a case history, it can be noted that these quantitative observations are consistent with qualitative clinical observations of increases in the flexibility of discourse during therapy. These procedures can be of particular value in the examination of therapies following traumatic brain injury because, in some presentations, these therapies are complicated by deficits that result in subtle distortions of language that produce significant post-injury social impairment. Independently of the mathematical analysis applied to the investigation of therapy-generated symbol sequences, our experience suggests that the procedures presented here are of value in training therapists.

The effect of the surrounding landscape matrix on mangrove bird community assembly in north Australia

Mangroves are highly threatened ecosystems yet their community ecology is poorly understood. We examined the ecological determinants of bird community assemblage in floristically depauperate mangroves. Birds were surveyed using line transect methods. Large mangrove patches supported fewer species than smaller patches. Patches did not comprise nested species subsets and the bird species richness of several small patches combined was greater than a single large area. The number of mangrove dependent species in a patch was area-dependent suggesting these species may be resource limited, although there was no species density compensation. There was a clear effect of the surrounding habitat, with matrix species accounting for ∼45% of bird species in a patch. Patches surrounded by tropical savanna were relatively species-poor, while regardless of size, patches including monsoon rainforest were relatively species rich. Null model analysis of non-random assemblage structure (nestedness and species co-occurrence) revealed no deterministic structure to the overall mangrove species assemblage. These analyses described a random pattern of bird distribution and with no evidence of density compensation this suggests that competition is a weak structuring force of mangrove bird assemblages. The lack of nestedness and the random co-occurrence of species are consistent with the matrix-dependence of bird community composition. Conservation plans should treat mangrove patches as part of a habitat mosaic and incorporate many smaller mangrove patches rather than just big ones. Consideration of the nature, extent and diversity of the surrounding matrices is vital in managing and conserving mangrove bird communities.

Pattern of species occurrence in detritus-based communities with variable connectivity

Abstract. Patterns and mechanisms of species occurrence in space and time are outstanding questions in community and conservation ecology. Much of the current debate focuses on randomness or non-randomness in the structuring process of ecological communities, and the extent to which local/deterministic or regional/stochastic processes may drive their composition. However, a categorical subdivision could be misleading, as community composition may be driven by a continuum between neutral- and niche-based processes. For instance, in spatially structured systems local processes may surpass regional processes over time or across space to derive non-random metacommunity structure, suggesting the filtering role of the environment in mediating the pattern of species occurrence. In this work we study the temporal composition of detritus-based communities, which are an essential component in many aquatic and terrestrial ecosystems. We used data about macroinvertebrate colonization of leaf detritus in different sites of a patchy-connected system, to measure the co-occurrence of species on the detrital resource and evaluate the role of the spatial configuration and the environmental variability in determining the community’s composition. Our results show the importance of considering the joint role of regional and local processes in mediating stochastic and deterministic mechanisms in species assemblage, with important outcomes from a conservation point of view.

Characteristics of Human Lysozyme and Its Disease‐Related Mutants in their Unfolded States

Characteristics of Human Lysozyme and Its Disease‐Related Mutants in their Unfolded States

Self-Organization, Layered Structure, and Aggregation Enhance Persistence of a Synthetic Biofilm Consortium

Microbial consortia constitute a majority of the earth's biomass, but little is known about how these cooperating communities persist despite competition among community members. Theory suggests that non-random spatial structures contribute to the persistence of mixed communities; when particular structures form, they may provide associated community members with a growth advantage over unassociated members. If true, this has implications for the rise and persistence of multi-cellular organisms. However, this theory is difficult to study because we rarely observe initial instances of non-random physical structure in natural populations. Using two engineered strains of Escherichia coli that constitute a synthetic symbiotic microbial consortium, we fortuitously observed such spatial self-organization. This consortium forms a biofilm and, after several days, adopts a defined layered structure that is associated with two unexpected, measurable growth advantages. First, the consortium cannot successfully colonize a new, downstream environment until it self-organizes in the initial environment; in other words, the structure enhances the ability of the consortium to survive environmental disruptions. Second, when the layered structure forms in downstream environments the consortium accumulates significantly more biomass than it did in the initial environment; in other words, the structure enhances the global productivity of the consortium. We also observed that the layered structure only assembles in downstream environments that are colonized by aggregates from a previous, structured community. These results demonstrate roles for self-organization and aggregation in persistence of multi-cellular communities, and also illustrate a role for the techniques of synthetic biology in elucidating fundamental biological principles.

The Non-Random Brain: Efficiency, Economy, and Complex Dynamics

Modern anatomical tracing and imaging techniques are beginning to reveal the structural anatomy of neural circuits at small and large scales in unprecedented detail. When examined with analytic tools from graph theory and network science, neural connectivity exhibits highly non-random features, including high clustering and short path length, as well as modules and highly central hub nodes. These characteristic topological features of neural connections shape non-random dynamic interactions that occur during spontaneous activity or in response to external stimulation. Disturbances of connectivity and thus of neural dynamics are thought to underlie a number of disease states of the brain, and some evidence suggests that degraded functional performance of brain networks may be the outcome of a process of randomization affecting their nodes and edges. This article provides a survey of the non-random structure of neural connectivity, primarily at the large scale of regions and pathways in the mammalian cerebral cortex. In addition, we will discuss how non-random connections can give rise to differentiated and complex patterns of dynamics and information flow. Finally, we will explore the idea that at least some disorders of the nervous system are associated with increased randomness of neural connections.

Spatial genetic structure and dispersal of giant pandas on a mountain-range scale

Understanding the spatial genetic structure of populations can provide insight into the ecological or evolutionary processes of the species, and enable wise conservation decisions. We examined the spatial genetic structure of a giant panda (Ailuropoda melanoleuca) population in a heterogeneous mountainous landscape using noninvasive genetic sampling and 12 microsatellite loci. Nonrandom genetic structure was detected through spatial autocorrelation analysis, demonstrating a significantly positive autocorrelation over closer distances. Additional spatial analyses showed significantly positive genetic correlation among spatially-proximate males, and no correlation among females and among male–female pairs. These findings suggest a female-biased dispersal pattern and cryptic family grouping among giant pandas on a large mountain-range scale. The spatial extent of genetic structure occurred within 12.5 km, measured by a least-cost path distance model integrating information of habitat quality and habitat preferences of this species. Using the bearing analysis of PASSAGE, we found that directional genetic autocorrelations were in agreement with habitat structure, and habitat heterogeneity may affect the direction of giant panda dispersal. The characterization of spatial genetic structure can provide potentially valuable information for the conservation and management of giant pandas and their habitat.

Mapping sources of correlation in resting state FMRI, with artifact detection and removal

Many components of resting-state (RS) FMRI show non-random structure that has little to do with neural connectivity but can covary over multiple brain structures. Some of these signals originate in physiology and others are hardware-related. One artifact discussed herein may be caused by defects in the receive coil array or the RF amplifiers powering it. During a scan, this artifact results in small image intensity shifts in parts of the brain imaged by the affected array components. These shifts introduce artifactual correlations in RS time series on the spatial scale of the coil's sensitivity profile, and can markedly bias RS connectivity results. We show that such a transient artifact can be substantially removed from RS time series by using locally formed regressors from white matter tissue. This is particularly important in arrays with larger numbers of coils, which may generate smaller artifact zones. In such a case, brain-wide average noise estimates would fail to capture the artifact. We also examine the anatomical structure of artifactual variance in RS FMRI time series, by identifying sources that contribute to these signals and where in the brain are they manifested. We consider current methods for reducing confounding sources (or noises) and their effects on connectivity maps, and offer an improved approach (ANATICOR) that can also reduce hardware artifacts. The methods described herein are currently available with AFNI, in addition to tools for rapid, interactive generation of seed-based correlation maps at single-subject and group levels.

Residual structure in a peptide fragment of the outer membrane protein X under denaturing conditions: a molecular dynamics study

The Escherichia coli outer membrane protein X (OmpX) contains two polypeptide segments that present nonrandom residual structure in 8 M aqueous urea, whereas the remainder of the protein is in a flexibly disordered conformation (Tafer et al. in Biochemistry 43:860-869, 2004). In the present study, the results of two long-timescale (0.4 micros) unrestrained explicit-solvent molecular dynamics (MD) simulations of a tetradecapeptide representative of one of these two segments in 8 M aqueous urea are reported and analyzed. The two simulations were initiated either from the conformation of the corresponding segment in an NMR model structure of the unfolded protein or from an entirely extended configuration. The sampled conformational ensembles agree qualitatively with the experimentally observed NOEs, but not quantitatively, suggesting that a number of relevant configurations were not visited on the 2 x 0.4 micros timescale. Major conformational transitions occur on the 0.1 micros timescale, and the ensembles corresponding to the two independent simulations overlap only to a limited extent. However, both simulations show in multiple events the reversible formation and disruption of alpha-helical secondary structure (characteristic of the urea-denatured state) and beta-turn secondary structure (characteristic of the native state). Events of helix formation are correlated with the appearance of hydrogen bonds between two side chains (Asp75-Ser78) and of a persistent hydrophobic contact (Trp76-Tyr80). They also evidence a peculiar helix stabilization and N-terminal capping role for a negatively charged residue (Asp75). These features are in good qualitative agreement with the NMR model for the structured state of the corresponding segment in the urea-denatured protein. The analysis of the simulations provides a detailed picture of the structural and dynamic features of the considered peptide at atomic resolution that is of high relevance in the understanding of the OmpX folding process.

An unlocking/relocking barrier in conformational fluctuations of villin headpiece subdomain

A reversible structural unlocking reaction, in which the close-packed van der Waals interactions break cooperatively, has been found for the villin headpiece subdomain (HP35) using triplet-triplet-energy transfer to monitor conformational fluctuations from equilibrium. Unlocking is associated with an unfavorable enthalpy change (DeltaH(0) = 35 +/- 4 kJ/mol) which is nearly compensated in free energy by the entropy change (DeltaS(0) = 112 +/- 20 Jxmol(-1)xK(-1)). The unlocking reaction has a time constant of about 1 mus at 5 degrees C and is enthalpy-limited with an activation energy of 32 +/- 1 kJ/mol and a large Arrhenius preexponential factor of A = 7.5 x 10(11) s(-1). In the unlocked state a fast local conformational fluctuation with a time constant of 170 ns and a low activation barrier of 17 +/- 1 kJ/mol leads to unfolding of the C-terminal helix and to its undocking from the core. On a much slower time scale, global unfolding occurs from the unlocked state. These results suggest that native protein structures are locked into conformations with low amplitude motions. Large scale motions and global unfolding require an initial structural unlocking step leading to a state with properties of a dry molten globule. The experiments additionally yielded information on the dynamics of loop formation between different positions in unfolded HP35. Comparison of the results with dynamics in unstructured model peptides indicates slightly decelerated kinetics of local loop formation in the C-terminal region which points at residual, nonrandom structure. Dynamics of long-range loop formation, in contrast, are not influenced by residual structure, which argues against unfolded state properties as molecular origin for ultrafast folding of HP35.

The inverse niche model for food webs with parasites.

Although parasites represent an important component of ecosystems, few field and theoretical studies have addressed the structure of parasites in food webs. We evaluate the structure of parasitic links in an extensive salt marsh food web, with a new model distinguishing parasitic links from non-parasitic links among free-living species. The proposed model is an extension of the niche model for food web structure, motivated by the potential role of size (and related metabolic rates) in structuring food webs. The proposed extension captures several properties observed in the data, including patterns of clustering and nestedness, better than does a random model. By relaxing specific assumptions, we demonstrate that two essential elements of the proposed model are the similarity of a parasite’s hosts and the increasing degree of parasite specialization, along a one-dimensional niche axis. Thus, inverting one of the basic rules of the original model, the one determining consumers’ generality appears critical. Our results support the role of size as one of the organizing principles underlying niche space and food web topology. They also strengthen the evidence for the non-random structure of parasitic links in food webs and open the door to addressing questions concerning the consequences and origins of this structure.Electronic supplementary materialThe online version of this article (doi:10.1007/s12080-009-0069-x) contains supplementary material, which is available to authorized users.

Co-occurrence analyses show that non-random community structure is disrupted by fire in two groups of soil arthropods (Isopoda Oniscidea and Collembola)

In this paper, we tested the hypothesis that natural catastrophes may destroy non-random community structure in natural assemblages of organisms. As a study system, we selected fire as the catastrophic event, and two groups of soil arthropods (Collembola and Isopoda Oniscidea) as target organisms. By co-occurrence analyses and Monte Carlo simulations of niche overlap analysis (C-score, with fixed-equiprobable model; RA2 and RA3 algorithms) we evaluated whether the community structure of these two groups were random/non-random at three unburnt sites and at three neighbour burnt sites that were devastated by a large-scale fire in summer 2000. Both taxa experienced a remarkable reduction in the number of species sampled in burnt versus unburnt sites, but the difference among sites was not statistically significant for Oniscidea. We determined that community structure was clearly non-random at the unburnt sites for both Collembola (according to RA3 algorithm) and Isopoda Oniscidea (according to co-occurrence analysis) and that, as predicted by theory, the catastrophic event did deeply alter the community structure by removing the non-random organization of the species interactions. We also observed a shift from segregation to aggregation/randomness in soil arthropods communities affected by fire, a pattern that was similar to that observed in natural communities of organisms perturbed by the introduction of alien species, thus indicating that this pattern may be generalizable when alteration of communities may occur.