Effects of interference and exploitative competition from large-bodied cladocerans on rotifer community structure

I examined the effects of a graded density of the large-bodied cladoceran Daphnia pulicaria upon a natural rotifer assemblage to assess the relative magnitudes of interference and exploitative competition. An in situ, 5-day, bag-enclosure experiment was conducted to test the hypothesis that populations of the rotifers Anuraeopsis spp., Keratella cochlearis typica and Polyarthra spp. are differentially suppressed by interference competition. Rotifer density decreases and observed vs. expected mortality rates for all three rotifer taxa were consistent with the hypothesis of suppression via interference competition. Evidence of exploitative effects were also apparent, though interference effects appeared more important at higher cladoceran densities.     

Experimental studies of exploitative competition in a grazing stream insect

Summary Field and laboratory experiments were conducted to determine whether intraspecific competition for food occurs during the larval stage of the periphyton-grazing caddisfly Glossosoma nigrior (Trichoptera: Glossosomatidae). Larvae were placed in field enclosures at densities less than, equal to, or greater than their natural densities. Most of these individuals began to pupate after 3 weeks, whereupon the mass of each individual was determined. Final mass declined significantly as larval densities increased, whereas neither developmental rate nor mortality/emigration rate was significantly affected by density manipulations. a supplemental experiment comparing the final mass of individuals grown at reduced densities in a laboratory stream with individuals from a natural stream bottom confirmed the results of the more extensive field experiment: reductions in density resulted in significant increases in final mass. Periphyton availability in field enclosures declined according to a negative exponential function as larval densities increased. Over the 25-fold range of larval densities used in these experiments, the final mass of individuals increased linearly with periphyton standing crops. This result suggests that Glossosoma larvae may compete for food even at densities below those employed in this study. Path analysis was used to explore the importance of indirect (i.e., exploitative) and direct (i.e, interference) mechanisms for the observed competitive effects. The analysis indicates that a model based solely on exploitation explains nearly as much of the variance in mass as a model incorporating both interference and exploitation.     

Intraspecific competition in Tridacna crocea,a burrowing bivalve

Summary Intraspecific competition for space and light occurred when Tridacna crocea burrowed into coralline substratum of boulders on leeward coral reefs in the central Great Barrier Reef near Townsville, Australia. Intensity of competition was linearly related to clam density. Above about 200 clams/m², all clams physically contacted one another and all shells sustained damage. Mortality in isolated populations due to intraspecific competition was estimated at 40%. Principles of intraspecific competition in plants were tested for applicability to T. crocea populations. Juvenile mortality due to competitive stress was density dependent. Aggregated distributions of one year old clams changed to random or regular distribution of adults. Normal size-frequency distribution for juveniles became skewed for older groups. A bimodal size-frequency distribution of the population was related to selective mortality in 1–3 year old clams. Adult mortality due to crowding was less severe but significant. Growth rates were inhibited by competition. Deformations in morphology resulted from crowding. Intraspecific competition for space and light by adults inhibited recruitment of young. Animal adaptations to reduce mortality under crowded conditions were also important. Larvae aggregated on settling and oriented with posterior ends pointed away from nearest neighbors. Positional alignment within the substratum was selectively advantageous. Burrowing posteriorly was preferential, but anterior and sideways burrowing as well as twisting within the burrow were also observed. Movement within substratum served to reduce local damage to the shell. Proteinaceous deposits secreted through perforations in the shell reduced subsequent damage. T. crocea populations exhibited many animal adaptations that reduced mortality during the first years of life, but as cohorts matured, plant-like patterns of competitive interaction became more significant.     

Linked exploitation and interference competition drives the variable behavior of a classic predator–prey system

The potential connection between exploitation and interference competition was recognized long ago but has not been evaluated. We measured the levels of both forms of competition for the protist Didinium preying upon Paramecium. Across populations, exploitation intensity was tightly linked to interference intensity, and the form of this relationship follows from a simple model of interaction speeds. The variation in interference competition was as large across populations of Didinium as has been observed previously across species from a variety of taxa including birds, mammals, insects, crustaceans, flatworms and protists. The link between exploitation and interference competition alters our understanding of how interference competition influences population dynamics. Instead of simply stabilizing systems, variation in interference levels can shift population dynamics through qualitatively different regimes because of its association with exploitation competition. Strong interference competition pushes a system to a regime of deterministic extinction, but intermediate interference generates a system that is stable with a high competitive ability. This may help to explain why the distribution of interference values is unimodal and mostly intermediate in intensity. Synthesis Exploitation and interference competition are typically viewed as separate processes. Exploitation is described with a functional response in which the inclusion of interference competition – the effect of predator density on foraging rates – is optional. Although recent work indicates that interference competition is widespread, there is little work linking the two forms of competition. In this article we present evidence that exploitation and interference competition are linked mechanistically through movement patterns that simultaneously generate beneficial interactions of consumers with resources and detrimental interactions with other consumers. This connection alters our view of the role that interference plays in ecological dynamics.     

Constraints on body size distributions: an experimental approach using a small-scale system

Holling’s (1992) proposition that discontinuities in biotic and abiotic processes generate structure in ecological systems is examined experimentally by imposing size-specific perturbations on marine sediment assemblages. Two kinds of perturbations were applied: organic enrichment and predation, each at two levels. Perturbations significantly affected the densities and relative abundance of the main invertebrate taxa and these effects were consistent with the known effects of enrichment and predation. However, there was little evidence of significant treatment effects on the overall benthic biomass or abundance size spectrum, supporting the contention that the spectrum is conservative and is probably constrained by habitat architecture. Received: 28 June 1999 / Accepted: 24 September 1999     

Disentangling spatio‐temporal processes in a hierarchical system: a case study in fisheries discards

In the last decade, various spatial and temporal methodologies were developed to investigate the processes that drive ecological and evolutionary patterns. However, these methods frequently fail to acknowledge that the observed patterns result from the overlap of different underlying processes. In order to understand how the patterns are formed, we must have recourse to methods that allow us to disentangle these simultaneous processes. Here we develop a hierarchical spatial predictive process (PP) combined with a separable temporal PP to disentangle and describe those overlapping processes in one very frequent setting in ecology and evolution: multilevel spatio‐temporally indexed data. We present our methodology through a case study of fisheries discards and investigate for example whether the inclusion of the hierarchical structure and the temporal processes of the system alter the observed spatial patterns. Recently it is recognized that understanding the processes driving discards is essential to sustainably manage and conserve marine resources. The results show that consideration of multiple underlying processes dramatically changes the pattern and characteristics of the discards hot‐ and coldspots. In the Irish Sea, the inclusion of the hierarchical structure of the system leads to the reduction of the hot‐ and coldspots. Simultaneously, our model identifies key bi‐annual fluctuations in the temporal process which, together with the variance associated at the level of individual fishing trips in the hierarchical structure of the data explained most of the variance driving discards. Whether the hierarchical, spatial and temporal processes are considered together or not can profoundly alter our understanding of what constitutes an appropriate mitigation measure. Misidentification of hotspots can culminate in inappropriate mitigation practices which can sometimes be irreversible. As the proposed method offers a unified approach for understanding the processes that drive observed patterns, many areas in ecology such as conservation and epidemiological studies can benefit from its use, increasing the effectiveness of management plans.     

Food robbing in ants,a form of interference competition

Summary Food robbing is a special form of interference competition in ants. It has been frequently observed in Myrmecocystus mimicus, which waylay returning foragers of several Pogonomyrmex species at their nests and take insect prey, particularly termites, away from them. Myrmecocystus more successfully robs prey from P. desertorum and P. maricopa than from P. barbatus. Usually only those Pogonomyrmex nests are affected by Myrmecocystus prey robbing that are not farther away than 10 m from the Myrmecocystus nest. Some M. mimicus workers show a high specialization in prey robbing. Other cases of food robbing in ants are described.     

The effects of interference competition on stability,structure and invasion of a multi-species system

Summary An interference competition model for a many species system is presented, based on Lotka-Volterra equations in which some restrictions are imposed on the parameters. The competition coefficients of the Lotka-Volterra equations are assumed to be expressed as products of two factors: the intrinsic interference to other individuals and the defensive ability against such interference. All the equilibrium points of the model are obtained explicitly in terms of its parameters, and these equilibria are classified according to the concept of sector stability. Thus survival or extinction of species at a stable equilibrium point can be determined analytically.The result of the analysis is extended to the successional processes of a community. A criterion for invasion of a new species is obtained and it is also shown that there are some characteristic quantities which show directional changes as succession proceeds.     

Fruit,flies and filamentous fungi – experimental analysis of animal–microbe competition using Drosophila melanogaster and Aspergillus mould as a model system

In addition to their fundamental role in nutrient recycling, saprobiotic microorganisms may be considered as typical consumers of food‐limited ephemeral resource patches. As such, they may be engaged in inter‐specific competition with saprophagous animals feeding from the same resource. Bacteria and filamentous fungi are known to synthesise secondary metabolites, some of which are toxic and have been proposed to deter or harm animals. The microorganisms may, however, also be negatively affected if saprophagous animals do not avoid microbe‐laden resources but feed in the presence of microbial competitors. We hypothesised that filamentous fungi compete with saprophagous insects, whereby secondary metabolites provide a chemical shield against the insect competitors. For testing this, we developed a new ecological model system representing a case of animal–microbe competition between saprobiotic organisms, comprising Drosophila melanogaster and species of the fungus Aspergillus (A. nidulans, A. fumigatus, A. flavus). Infestation of Drosophila breeding substrate with proliferating fungal colonies caused graduated larval mortality that strongly depended on mould species and colony age. Confrontation with conidiospores only, did not result in significant changes in larval survival, suggesting that insect death may not be ascribed to pathogenic effects. When confronted with colonies of transgenic fungi that lack the ability to express the global secondary metabolite regulator LaeA (ΔlaeA), larval mortality was significantly reduced compared to the impact of the wild type strains. Yet, also in the ΔlaeA strains, inter‐specific variation in the influence on insect growth occurred. Competition with Drosophila larvae impaired fungal growth, however, wild type colonies of A. nidulans and A. flavus recovered more rapidly from insect competition than the corresponding ΔlaeA mutants (not in A. fumigatus). Our findings provide genetic evidence that toxic secondary metabolites synthesised by saprotrophic fungi may serve as a means to combat insect competitors. Variation in the ability of LaeA to control expression of various secondary metabolite gene clusters might explain the observed species‐specific variation in Drosophila–Aspergillus competition.     

Disentangling the genetic origins of a plant pathogen during disease spread using an original molecular epidemiology approach

The advent of molecular epidemiology has greatly improved our ability to identify the population sources and track the pathogen movement. Yet the wide spatial and temporal scales usually considered are useful only to infer historical migration pathways. In this study, Bayesian genetic assignments and a landscape epidemiology approach were combined to unravel genetic origin and annual spread during a single epidemic of a plant pathogen: the poplar rust fungus Melampsora larici-populina. The study focused on a particular area-the Durance River valley-which enabled inoculum sources to be identified and channelled spread of the epidemic along a one-dimensional corridor. Spatio-temporal monitoring of disease showed that the epidemic began in the upstream part of the valley and spread out downstream. Using genetic assignment tests, individuals collected at the end of the epidemic were sorted into two genetic groups; very few hybrids were detected, although individuals from both groups coexisted locally downstream in the valley. The epidemic originated from two genetically distinct inoculum sources. Individuals of each group then dispersed southwards along the Durance River and became mixed in poplar riparian stands. These two genetic groups were found previously at a wider spatial scale and proved to result from distinct evolutionary histories on either wild or cultivated poplars. This study showed that the two groups can mix during an epidemic but do not hybridize because they then reproduce asexually. In general, the methods employed here could be useful for elucidating the genetic origin and retracing the colonization history and migration pathways of recent epidemics.     

Cirripede phylogeny using a novel approach: molecular morphometrics

We present a new method using nucleic acid secondary structure to assess phylogenetic relationships among species. In this method, which we term "molecular morphometrics," the measurable structural parameters of the molecules (geometrical features, bond energies, base composition, etc.) are used as specific characters to construct a phylogenetic tree. This method relies both on traditional morphological comparison and on molecular sequence comparison. Applied to the phylogenetic analysis of Cirripedia, molecular morphometrics supports the most recent morphological analyses arguing for the monophyly of Cirripedia sensu stricto (Thoracica + Rhizocephala + Acrothoracica). As a proof, a classical multiple alignment was also performed, either using or not using the structural information to realign the sequence segments considered in the molecular morphometrics analysis. These methods yielded the same tree topology as the direct use of structural characters as a phylogenetic signal. By taking into account the secondary structure of nucleic acids, the new method allows investigators to use the regions in which multiple alignments are barely reliable because of a large number of insertions and deletions. It thus appears to be complementary to classical primary sequence analysis in phylogenetic studies.     

A novel approach to estimate trabecular bone anisotropy using a database approach

Continuum finite element (FE) models of bones have become a standard pre-clinical tool to estimate bone strength. These models are usually based on clinical CT scans and material properties assigned are chosen as isotropic based only on the density distribution. It has been shown, however, that trabecular bone elastic behavior is best described as orthotropic. Unfortunately, the use of orthotropic models in FE analysis derived from CT scans is hampered by the fact that the measurement of a trabecular orientation (fabric) is not possible from clinical CT images due to the low resolution of such images. In this study, we explore the concept of using a database (DB) of high-resolution bone models to derive the fabric information that is missing in clinical images. The goal of this study was to investigate if models with fabric derived from a relatively small database can already produce more accurate results than isotropic models.     

Fast screening target sites for RNA interference using a cell-free system

A fast and simple procedure to screen target sites for RNA interference by using RNA in a cell-free system of Hela cells, and then evaluating the efficiency by Northern blotting, is described. This procedure produces results with an identical reliability compared to those described previously but which are more time-consuming than this present method.     

Arsenic removal from the aqueous system using plant biomass: a bioremedial approach

Metal species released into the environment by technological activities tend to persist indefinitely, circulating and eventually accumulating throughout the food chain, thus becoming a serious threat to the environment. Environment pollution by toxic metals occurs globally through military, industrial, and agricultural processes and waste disposal. Bioremediation processes are the target of recent research and are considered low-cost, ecofriendly methods to alleviate the current problems of water decontamination, particularly for remote and rural areas. The present piece of work reports the unexploited sorption properties of the powdered seed of the plant Moringa oleifera (SMOS) for the removal of Arsenic [As(III) and As(V)] from aqueous solutions. Sorption studies, using standard practices, result in the standardization of optimum conditions such as biomass dosages (2.0 g), metal concentrations (25 ppm), contact time (60 min) and volume of the test solutions (200 ml) at pH 7.5, for As(III) and pH 2.5 for As(V). Maximum sorption for As(III) and As(V) species is 60.21 and 85.6%, respectively. Protein/Amino acid-Arsenic interactions are found to play an important role in the biosorption process using plant biomass SMOS.     

Removing biofilms from microstructured titanium ex vivo: a novel approach using atmospheric plasma technology

The removal of biofilms from microstructured titanium used for dental implants is a still unresolved challenge. This experimental study investigated disinfection and removal of in situ formed biofilms from microstructured titanium using cold atmospheric plasma in combination with air/water spray. Titanium discs (roughness (Ra): 1.96 µm) were exposed to human oral cavities for 24 and 72 hours (n = 149 each) to produce biofilms. Biofilm thickness was determined using confocal laser scanning microscopy (n = 5 each). Plasma treatment of biofilms was carried out ex vivo using a microwave-driven pulsed plasma source working at temperatures from 39 to 43°C. Following plasma treatment, one group was air/water spray treated before re-treatment by second plasma pulses. Vital microorganisms on the titanium surfaces were identified by contact culture (Rodac agar plates). Biofilm presence and bacterial viability were quantified by fluorescence microscopy. Morphology of titanium surfaces and attached biofilms was visualized by scanning electron microscopy (SEM). Total protein amounts of biofilms were colorimetrically quantified. Untreated and air/water treated biofilms served as controls. Cold plasma treatment of native biofilms with a mean thickness of 19 µm (24 h) to 91 µm (72 h) covering the microstructure of the titanium surface caused inactivation of biofilm bacteria and significant reduction of protein amounts. Total removal of biofilms, however, required additional application of air/water spray, and a second series of plasma treatment. Importantly, the microstructure of the titanium discs was not altered by plasma treatment. The combination of atmospheric plasma and non-abrasive air/water spray is applicable for complete elimination of oral biofilms from microstructured titanium used for dental implants and may enable new routes for the therapy of periimplant disease.     

Demonstration of tubulin-glycolytic enzyme interactions using a novel electrophoretic approach

A gel electrophoretic technique was used to demonstrate an interaction with the soluble enzymes aldolase, glyceraldehydephosphate dehydrogenase, pyruvate kinase and muscle type lactate dehydrogenase to the cytoskeletal protein tubulin. It is suggested that tubulin, like actin, is a key cytoskeletal structure with which soluble proteins may associate.