Modeling competition between Laurencia obtusa (Ceramiales,Rhodophyta) and Hypnea spinella (Gigartinales,Rhodophyta) at Cabo Frio Island,Rio de Janeiro,Brazil

The Lotka-Volterra competition model was used to represent the interaction between Laurencia obtusa and Hypnea spinella. A new model that considers effects of competition on algal carrying capacity is suggested. To test the models, data from field experiments conducted in an intertidal region at Cabo Frio Island, Rio de Janeiro, Brazil, were used. Both models showed that Hypnea was a stronger competitor than Laurencia. The model of interaction through the carrying capacity showed a stable coexistence between the algal populations and better represented the experimental data.     

Genetics and the environment in interspecific competition: a study using the sibling species Drosophila melanogaster and Drosophila simulans

The outcome of interspecific competition of two closely related species may depend upon genetic variation in the two species and the environment in which the experiment is carried out. Interspecific competition in the two sibling species, Drosophila melanogaster and D. simulans, is usually investigated using longterm laboratory stocks that often have mutant markers that distinguish them. To examine competition in flies that genetically more closely resemble flies in nature, we utilized freshly caught wildtype isofemale lines of the two species collected at the same site in San Carlos, Mexico. Under ordinary laboratory conditions, D. melanogaster always won in competition. However, in hotter and drier conditions, D. simulans competed much more effectively. In these environmental conditions, there were genetic differences in competitive ability among lines with the outcome of competition primarily dependent upon the line of D. melanogaster used but in some cases also influenced by the line of D. simulans used. Differences in the measures of productivity and developmental time did not explain the differences in competitive ability among lines. This suggests that the outcome of competition was not due to differences in major fitness components among the isofemale lines but to some other attribute(s) that influenced competitive ability. When lines of flies were combined, the outcome of competition was generally consistent with competitive outcomes between pairs of lines. In several cases, the combination of lines performed better than the best of the constituent lines, suggesting that competitive ability was combined heterotically and that the total amount of genetic variation was important in the outcome of interspecific competition.     

Inter-and intraspecific effects of density manipulations upon females of two orb-weaving spiders (araneae: araneidae)

Summary A field experiment was conducted to establish whether or not inter-and intraspecific competition occurs between two syntopic species of orb-weaving spiders. Replicated single-species and mixed-species adult populations of the basilica spider, Mecynogea lemniscata, and the labyrinth spider, Metepeira labyrinthea, were established at a range of densities on open experimental units in the species' natural habitat. Each experimental unit was a 4mx1.6mx1m wood frame supporting branches upon which introduced spiders built webs. Survival and reproduction on the units were monitored from 1 August through 1 November 1978,There were no significant negative interspecific effects of density upon either survival or reproduction, which indicates that interspecific competition was not occurring during the experiment. There was statistically significant evidence of intraspecific competition between females for both species, but the negative density effects were small. They explained 5% of the variance in Mecynogea web height, 2% of the variance in Metepeira survival, and 1% of the variation in number of eggs per sac for Mecynogea.Hence inter-and intraspecific competition was either absent or weak in 1978, despite the fact that a field experiment conducted the previous year (Wise 1979) demonstrated that prey abundance was limiting the egg production of both species. Evidence of food limitation in 1977 followed by only weak competitive interactions the following year suggests that the significance of resource limitation and competition may vary temporally for the basilica and labyrinth spiders.     

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.     

Studies on interspecies competition between the azuki bean weevil,Callosobruchus chinensis,and the southern cowpea weevil,C. maculatus II. Competition under different environmental conditions

The experimental studies was done on the interspecies competition between the azuki bean weevil, Callosobruchus chinensis, and the southern cowpea weevil, C. maculatus, under the different environmental conditions.

1. At 30°C, 70% R. H., the change of food replenishment interval had no influence upon the competition results, and the competition always ended in the extinction of C. maculatus. But there was a tendency for the period of co-existence to be elongated as the interval of replenishment lengthened.
2. At 30°C, 70% R. H., there were no differences in the competition processes and results between the competition using the dry bean (water content about 11%) or the normal one (about 15%).
3. At 32°C, 64% R. H., the competition result was reversed from that at 30°C, 70% R. H. This is thought to be due to the differences of the developmental rates and the numbers of eggs oviposited between the two species at these conditions.
4. In some cases the competition result could be understood from the point of the phenomena of single species populations of both species, but in other cases, the results were in conflict with the expectation from those phenomena. To analyze the competition mechanism, it is necessary to investigate the interaction between species.

     

Competition theory,evolution, and the concept of an ecological niche

This article examines some of the main tenets of competition theory in light of the theory of evolution and the concept of an ecological niche. The principle of competitive exclusion and the related assumption that communities exist at competitive equilibrium - fundamental parts of many competition theories and models - may be violated if non-equilibrium conditions exist in natural communities or are incorporated into competition models. Furthermore, these two basic tenets of competition theory are not compatible with the theory of evolution. Variation in ecologically significant environmental factors and non-equilibrium in population numbers should occur in most natural communities, and such changes have important effects on community relations, niche overlap, and the evolution of ecosystems. Ecologists should view competition as a process occurring within a complexdynamic system, and should be wary of theoretical positions built upon simple laboratory experiments or simplistic mathematical models.In considering the relationship between niche overlap and competition, niche overlap should not be taken as a sufficient condition for competition; many factors may prevent or diminish competition between populations with similar resource utilization patterns. The typically opposing forces of intraspecific and interspecific competition need to be simultaneously considered, for it is the balance between them that in large part determines niche boundaries.     

Competitive exclusion through reproductive interference

A simple differential equation model was developed to describe the competitive interaction that may occur between species through reproductive interference. The model has the form comparable to Volterra's competition equations, and the graphical analysis of the outcome of the two-species interaction based on its zero-growth isoclines proved that: (1) The possible outcome in this model, as in usual models of resource competition, is either stable coexistence of both species or gradual exclusion of one species by the other, depending critically upon the values of the activity overlapping coefficient c_ij; (2) but, for the same c_ij-values, competitive exclusion is much more ready to occur here than in resource competition; (3) and moreover, the final result of the competition is always dependent on the initial-condition due to its non-linear isoclines, i.e., even under the parameter condition that generally allows both species to coexist, an extreme bias in intial density to one species can readily cause subsequent complete exclusion of its counterparts. Thus, it may follow that the reproductive interference is likely to be working in nature as an efficient mechanism to bring about habitat partitioning in either time or space between some closely related species in insect communities, even though they inhabit heterogeneous habitats where resource competition rarely occurs so that they could otherwise attain steady coexistence.     

Moose and snowshoe hare competition and a mechanistic explanation from foraging theory

Summary Moose (Alces alces) and snowshoe hare (Lepus americanus) appear to compete with each other. This was determined using the natural experiments of populations found in sympatry and allopatry on islands at Isle Royale National Park, Michigan, and manipulated exclosures. The population densities from these areas are fit to a series of competition models based upon different competitive mechanisms (Schoener 1974a), using non-linear regression techniques. A model of competition for food where the food can be separated into exclusively used and shared categories is found to predict observed densities of moose and hare best. Finally, the competition model's parameters (fraction of food shared and competition coefficients) are shown to agree with values predicted independently from a foraging model.     

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.     

Differential introgression of a female competitive trait in a hybrid zone between sex‐role reversed species

Mating behavior between recently diverged species in secondary contact can impede or promote reproductive isolation. Traditionally, researchers focus on the importance of female mate choice and male–male competition in maintaining or eroding species barriers. Although female–female competition is widespread, little is known about its role in the speciation process. Here, we investigate a case of interspecific female competition and its influence on patterns of phenotypic and genetic introgression between species. We examine a hybrid zone between sex‐role reversed, Neotropical shorebird species, the northern jacana (Jacana spinosa) and wattled jacana (J. jacana), in which female–female competition is a major determinant of reproductive success. Previous work found that females of the more aggressive and larger species, J. spinosa, disproportionately mother hybrid offspring, potentially by monopolizing breeding territories in sympatry with J. jacana. We find a cline shift of female body mass relative to the genetic center of the hybrid zone, consistent with asymmetric introgression of this competitive trait. We suggest that divergence in sexual characteristics between sex‐role reversed females can influence patterns of gene flow upon secondary contact, similar to males in systems with more typical sex roles.     

A genomic island in a plant beneficial rhizobacterium encodes novel antimicrobial fatty acids and a self-protection shield to enhance its competition

Rhizobacteria devote a relatively large percentage of their genomes to encode bioactive natural products that are important for competition in the rhizosphere. In this study, a plant beneficial rhizobacterium Bacillus velezensis SQR9 was discovered to produce novel antibacterial fatty acids, Bacillunoic acids, which are encoded on a genomic island (GI). This GI contains a hybrid type I fatty acid synthase (FAS)-polyketide synthase (PKS) system and an ABC transporter. The FAS was predicted to synthesize a primer that was transferred to the PKS to synthesize Bacillunoic acids. The synthesized Bacillunoic acids inhibit the growth of diverse bacteria, with the strongest activity against closely related Bacillus strains, the ABC transporter exported the toxic Bacillunoic acids upon their induction for protecting the producing strain. The inhibition of other Bacillus strains by Bacillunoic acids extended the antimicrobial spectrum of SQR9 and enhanced its competition with closely related root-associated bacteria. So, through the obtaining of this GI by horizontal gene transfer, strain SQR9 not only acquired a competitive weapon but also acquired a self-protecting shield, which increased its competition with other rhizobacteria.     

Jean Rouch's ethnographic path

This article considers the multiple influences that have shaped Jean Rouch's ethnographic gaze upon the Songhay people of Niger. Marcel Griaule, Rouch's mentor, undoubtedly influenced his student's ethnographic approach to Songhay. But Rouch was equally influenced, I am convinced, by his direct and unforgettable confrontation with the mysteries of the Songhay world. After a discussion of the methodological and theoretical orientations of the Griaule school, I analyze Rouch's major ethnographic work, La Religion et la magie Songhay, focusing upon how Griaulian epistemology and Songhay realities molded the textual presentation. I then discuss several of Rouch's major films—Les Magiciens de Wanzerbé, The Lion Hunters, Les Maîtres Fous, and Tourou et Bitti, placing them within an ethnographic and epistemological context. This analysis leads to a consideration of Rouch's unrecognized contribution to anthropology.     

Using growth analysis to interpret competition between a C3 and a C4 annual under ambient and elevated CO2

Summary Detailed growth analysis in conjunction with information on leaf display and nitrogen uptake was used to interpret competition between Abutilon theophrasti, a C₃ annual, and Amaranthus retroflexus, a C₄ annual, under ambient (350 l l^-1) and two levels of elevated (500 and 700 l l^-1) CO₂. Plants were grown both individually and in competition with each other. Competition caused a reduction in growth in both species, but for different reasons. In Abutilon, decreases in leaf area ratio (LAR) were responsible, whereas decreased unit leaf rate (ULR) was involved in the case of Amaranthus. Mean canopy height was lower in Amaranthus than Abutilon which may explain the low ULR of Amaranthus in competition. The decrease in LAR of Abutilon was associated with an increase in root/shoot ratio implying that Abutilon was limited by competition for below ground resources. The root/shoot ratio of Amaranthus actually decreased with competition, and Amaranthus had a much higher rate of nitrogen uptake per unit of root than did Abutilon. These latter results suggest that Amaranthus was better able to compete for below ground resources than Abutilon. Although the growth of both species was reduced by competition, generally speaking, the growth of Amaranthus was reduced to a greater extent than that of Abutilon. Regression analysis suggests that the success of Abutilon in competition was due to its larger starting capital (seed size) which gave it an early advantage over Amaranthus. Elevated CO₂ had a positive effect upon biomass in Amaranthus, and to a lesser extent, Abutilon. These effects were limited to the early part of the experiment in the case of the individually grown plants, however. Only Amaranthus exhibited a significant increase in relative growth rate (RGR). In spite of the transitory effect of CO₂ upon size in individually grown plants, level of CO₂ did effect final biomass of competitively grown plants. Abutilon grown in competition with Amaranthus had a greater final biomass than Amaranthus at ambient CO₂ levels, but this difference disappeared to a large extent at elevated CO₂. The high RGR of Amaranthus at elevated CO₂ levels allowed it to overcome the difference in initial size between the two species.This study was supported by a grant from the US Department of Energy     

Effect of Azotobacter Exudates on Certain Micro-Fungi Associated with the Phylloplane of Soybean

The effect of different concentrations of Azotobacter chroococcum exudates on microfungi isolated from Soybean phylloplane was studied in vitro and in vivo. It has been found that the growth of some fungi was stimulated or suppressed whereas others showed fluctuations or remained unaffected depending upon the age of the plant, growth stage of the leaf, concentration of the exudates, method used to harvest fungi, and environmental conditions. Nutrient competition or production of biologically active substances like GA may be responsible for retarding the growth of fungi.     

A biogeochemistry‐based dynamic vegetation model and its application along a moisture gradient in the continental United States

Abstract. We develop and evaluate a large‐scale dynamic vegetation model, TEM‐LPJ, which considers interactions among water, light and nitrogen in simulating ecosystem function and structure. We parameterized the model for three plant functional types (PFTs): a temperate deciduous forest, a temperate coniferous forest, and a temperate C₃ grassland. Model parameters were determined using data from forest stands at the Harvard Forest in Massachusetts. Applications of the model reasonably simulated stand development over 120 yr for Populus tremuloides in Wisconsin and for Pinus elliottii in Florida. Our evaluation of tree‐grass interactions simulated by the model indicated that competition for light led to dominance by the deciduous forest PFT in moist regions of eastern United States and that water competition led to dominance by the grass PFT in dry regions of the central United States. Along a moisture transect at 41.5° N in the eastern United States, simulations by TEM‐LPJ reproduced the composition of potential temperate deciduous forest, temperate savanna, and C3 grassland located along the transect.     

Effects of clonal variation of the host plant,interspecific competition,and climate on the population size of a folivorous thrips

Summary The effects of clonal variation, interspecific competition, and climate upon the population size of Apterothrips secticornis was assessed by a series of observations and experimental manipulations. Three clones of the host plant, Erigeron glaucus, consistently supported different numbers of thrips during monthly censuses. When rosettes of the three clones were transplanted to a common garden, relative numbers of thrips on the clones remained the same as those observed where the clones grew in situ. The presence or absence of other hervivores had no effect on thrips numbers in the common garden. Plume moth caterpillars and thrips were observed to co-occur less often than expected in the field but this was caused by differences in habitat selection by these two species rather than being the result of interspecific competition. Populations of thrips were affected by climate, but analyses suggest that the host clone was a more important factor.     

Dispersal polymorphisms from natal phenotype–environment interactions have carry‐over effects on lifetime reproductive success of a tropical parrot

We examined how interactions between an individual's phenotype and its environment affect natal dispersal at multiple scales and the effects on lifetime reproductive success using a 22‐year study of green‐rumped parrotlets (Forpus passerinus). Dispersal increased or decreased lifetime reproductive success depending upon an individual's natal environment and phenotype. Many of the phenotypic traits and environmental conditions that influenced lifetime reproductive success also influenced dispersal, such as clutch size and competition, and this differed with scale. By examining phenotype–environment interactions, we observed both positive and negative effects of rainfall, habitat quality and competition on dispersal depending upon phenotype. The dispersal behaviours of juveniles typically resulted in higher lifetime reproductive success. Thus, individuals commonly exhibit ideal free behaviour and results provide support for the occurrence and maintenance of dispersal polymorphisms. This study highlights the long‐term, carry‐over effects of natal environment, natal phenotype and dispersal tactic on lifetime reproductive success.     

Oviposition Deterrence Is Likely an Effect,Not a Mechanism,in the Leaf Beetle <Emphasis Type="Italic">Chrysophtharta bimaculata</Emphasis> (Olivier) (Coleoptera: Chrysomelidae)

Oviposition deterrence is common in many insects as an evolutionary mechanism to reduce subsequent larval competition. We investigated a suspected case of oviposition deterrence by the paropsine chrysomelid, Chrysophtharta bimaculata. In paired choice tests, gravid females were found to prefer ovipositing on host leaves without conspecific eggs, confirming the presence of an apparent oviposition deterrence mechanism. Washing egg batches in water, hexane, or ethanol did not change this preference, suggesting that a soluble marking pheromone was not involved. Furthermore, it is unlikely that a plant-derived oviposition deterring substance is produced as beetles showed no significant oviposition preference between leaves which had been oviposited upon, but then had the eggs removed, and those that had never been oviposited upon. In trials using artificial leaves and mimic egg batches, “leaves” with “egg batches” placed near the tip of the leaf (the preferred site of oviposition in this species) were significantly less likely to be laid upon than artificial leaves where mimic eggs were placed away from the tip. In combination, the results strongly infer that oviposition deterrence in C. bimaculata is due to the mechanical blocking of the oviposition site by the first laid egg-batch, rather than a specific oviposition deterring cue. The apparent oviposition deterrence in this insect may well be an outcome or evolutionary effect of oviposition-site selection, rather than a clear adaptive mechanism to decrease larval competition.     

Strain competition restricts colonization of an enteric pathogen and prevents colitis

The microbiota is a major source of protection against intestinal pathogens; however, the specific bacteria and underlying mechanisms involved are not well understood. As a model of this interaction, we sought to determine whether colonization of the murine host with symbiotic non‐toxigenic Bacteroides fragilis could limit acquisition of pathogenic enterotoxigenic B. fragilis. We observed strain‐specific competition with toxigenic B. fragilis, dependent upon type VI secretion, identifying an effector–immunity pair that confers pathogen exclusion. Resistance against host acquisition of a second non‐toxigenic strain was also uncovered, revealing a broader function of type VI secretion systems in determining microbiota composition. The competitive exclusion of enterotoxigenic B. fragilis by a non‐toxigenic strain limited toxin exposure and protected the host against intestinal inflammatory disease. Our studies demonstrate a novel role of type VI secretion systems in colonization resistance against a pathogen. This understanding of bacterial competition may be utilized to define a molecularly targeted probiotic strategy.     

Survivorship of Different Strains of the Bacteria Listeria monocytogenesis and Yersinia pseudotuberculosisin Sea and River Water

It has been shown that the survival capabilities of the bacteria Listeria monocytogenes and Yersinia pseudotuberculosis in water environments is different depending upon the strain of the bacteria. When entering sea or river water, the pathogenic bacteria must withstand keen competition against numerous saprophytic microflora. It is found that different strains of Listeria and Yersinia adapted to low positive temperatures can reproduce in non-sterile sea and river water under low positive temperatures and remain there for long periods of time.