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Competition Theory (competition + theory)
Kinds of Competition Theory Selected AbstractsThe ultrastructure of the spermatozoa of Boa constrictor occidentalis, with considerations on its mating system and sperm competition theoriesACTA ZOOLOGICA, Issue 1 2006M. Tourmente Abstract Sperm ultrastructure has been described for several families of Squamata in which it has been considered a valuable character source for phylogenetic studies. However, sperm competition and mating systems have been demonstrated to influence variations in the sperm morphology and dynamics. The mating system of Boa constrictor occidentalis is likely to have a high degree of sperm competition. We investigated, for the first time, the ultrastructure of the spermatozoa of B. c. occidentalis and, thus, of the family Boidae. Active mating groups were captured from the field, and the spermatozoa of the males was collected by ejaculation and processed to obtain transmission electron micrographs and fluorescence micrographs. The spermatozoa are filiform and their morphology fits the general model described for snakes, and several synapomorphies belonging to the squamates can be identified in these cells. Nevertheless, the head is 25% longer and the midpiece presents a lower frequency of mitochondrial transformations than that of other snakes. We propose that this last trait, along with the extraordinary elongation of the midpiece and the system of multilaminar membranes covering this section (both synapomorphies of the snake spermatozoa), are adaptive responses to processes of sperm competition and sperm storage. [source] Do biotic interactions shape both sides of the humped-back model of species richness in plant communities?ECOLOGY LETTERS, Issue 7 2006Richard Michalet Abstract A humped-back relationship between species richness and community biomass has frequently been observed in plant communities, at both local and regional scales, although often improperly called a productivity,diversity relationship. Explanations for this relationship have emphasized the role of competitive exclusion, probably because at the time when the relationship was first examined, competition was considered to be the significant biotic filter structuring plant communities. However, over the last 15 years there has been a renewed interest in facilitation and this research has shown a clear link between the role of facilitation in structuring communities and both community biomass and the severity of the environment. Although facilitation may enlarge the realized niche of species and increase community richness in stressful environments, there has only been one previous attempt to revisit the humped-back model of species richness and to include facilitative processes. However, to date, no model has explored whether biotic interactions can potentially shape both sides of the humped-back model for species richness commonly detected in plant communities. Here, we propose a revision of Grime's original model that incorporates a new understanding of the role of facilitative interactions in plant communities. In this revised model, facilitation promotes diversity at medium to high environmental severity levels, by expanding the realized niche of stress-intolerant competitive species into harsh physical conditions. However, when environmental conditions become extremely severe the positive effects of the benefactors wane (as supported by recent research on facilitative interactions in extremely severe environments) and diversity is reduced. Conversely, with decreasing stress along the biomass gradient, facilitation decreases because stress-intolerant species become able to exist away from the canopy of the stress-tolerant species (as proposed by facilitation theory). At the same time competition increases for stress-tolerant species, reducing diversity in the most benign conditions (as proposed by models of competition theory). In this way our inclusion of facilitation into the classic model of plant species diversity and community biomass generates a more powerful and richer predictive framework for understanding the role of plant interactions in changing diversity. We then use our revised model to explain both the observed discrepancies between natural patterns of species richness and community biomass and the results of experimental studies of the impact of biodiversity on the productivity of herbaceous communities. It is clear that explicit consideration of concurrent changes in stress-tolerant and competitive species enhances our capacity to explain and interpret patterns in plant community diversity with respect to environmental severity. [source] Coexistence of cryptic speciesECOLOGY LETTERS, Issue 3 2004Da-Yong Zhang Abstract Recent discovery of cryptic species in fig-pollinating wasps creates a puzzle for the ecological competition theory: how do two or more apparently identical species coexist? Conventional theory predicts that they should not. Chesson (Trends Ecol. Evol., 1991, 6, 26,28) identified one exception which he considered unlikely to occur in reality: coexistence might be possible if appropriate social behaviour was discriminately directed towards conspecifics and heterospecifics. Here we present an example of the exception by showing that two identical species with local mate competition and population size-dependent sex ratio adjustment may coexist. The new findings about fig-pollinating wasps provide a putative example of unexpected coexistence of identical competitors via this mechanism. [source] Competitive coexistence in spatially structured environments: a synthesisECOLOGY LETTERS, Issue 12 2003Priyanga Amarasekare Abstract Theoretical developments in spatial competitive coexistence are far in advance of empirical investigations. A framework that makes comparative predictions for alternative hypotheses is a crucial element in narrowing this gap. This review attempts to synthesize spatial competition theory into such a framework, with the goal of motivating empirical investigations that adopt the comparative approach. The synthesis presented is based on a major axis, coexistence in spatially homogeneous vs. heterogeneous competitive environments, along which the theory can be organized. The resulting framework integrates such key concepts as niche theory, spatial heterogeneity and spatial scale(s) of coexistence. It yields comparative predictions that can guide empirical investigations. [source] Factors affecting the reproductive biology of Melittobia digitata and failure to meet the sex ratio predictions of Hamilton's local mate competition theoryENTOMOLOGIA EXPERIMENTALIS ET APPLICATA, Issue 1 2003M.F. Cooperband Abstract Melittobia digitata Dahms (Eulophidae, Tetrastichini), a species of parasitic wasp satisfying all of Hamilton's local mate competition requisites, does not exhibit the predicted change in sex ratio with increased foundress number. A multifactorial design was used to test how age, oviposition experience, feeding experience, mating, and foundress number affect host-acceptance, number of offspring, and sex ratio of this species developing on honey bee pupae, Apis mellifera (L.) (Apidae, Apini). All factors significantly affected the time it took for oviposition to commence. Females oviposited soonest when they were 2 days old, mated, had previous feeding and oviposition experience, and were placed on hosts with multiple foundresses. Although the age difference between 2- and 5-day-old females is small, it significantly affected reproductive behavior. Age, mating, and foundress number were found to have an effect on sex ratio, however, the foundress effect was found to be a mathematical artifact of the limited host size. After correcting for this variable, females were found to have a constant sex ratio of approximately 0.05. Several 2-way interactions between factors were revealed: age and experience, age and foundress number, age and mating, foundress number and experience, and foundress number and mating. One 3-way interaction was found between age, mating, and foundress number. This study demonstrates that the sex ratio of M. digitata is not altered with increased foundresses, as predicted by Hamilton, and that slight changes in preconditioning may modify reproductive behavior. [source] Immigration sceptics, xenophobes or racists?EUROPEAN JOURNAL OF POLITICAL RESEARCH, Issue 6 2008Radical right-wing voting in six West European countries Given how central the immigration issue has been for the new radical right-wing parties in Western Europe, many have turned to immigration-related factors in trying to explain their emergence and electoral mobilisation. This research has convincingly shown that immigration scepticism (i.e., wanting to reduce immigration) is among the principal factors for predicting who will vote for a radical right-wing party. However, earlier studies have often uncritically equated immigration scepticism with xenophobia or even racism. By using data from the first round of the European Social Survey (2003) involving six West European countries (Austria, Belgium, Denmark, France, the Netherlands and Norway), this article differentiates between immigration scepticism and xenophobic attitudes. The analyses strongly indicate that xenophobic attitudes are a far less significant factor than immigration scepticism for predicting who will vote for the new radical right. Moreover, this article analyses the extent to which anti-immigration frames employed by radical right-wing parties resonate with attitudes held by supporting voters, and to what extent they make a difference for people's decision to vote for the radical right. The analyses indicate that frames linking immigration to criminality and social unrest are particularly effective for mobilising voter support for the radical right. Finally, the article criticises earlier research that explained radical right-wing voting with reference to ethnic competition theory. In contrast to much of the earlier research that used macro-level measures and comparisons, this study uses (self-reported) individual-level data on the degree of ethnic heterogeneity of people's area of residence. Hypotheses derived from ethnic competition theory receive less support than expected, which indicates that earlier research may have overestimated the significance of these factors. [source] EVOLUTIONARY REDUCTION IN TESTES SIZE AND COMPETITIVE FERTILIZATION SUCCESS IN RESPONSE TO THE EXPERIMENTAL REMOVAL OF SEXUAL SELECTION IN DUNG BEETLESEVOLUTION, Issue 10 2008Leigh W. Simmons Sexual selection is thought to favor the evolution of secondary sexual traits in males that contribute to mating success. In species where females mate with more than one male, sexual selection also continues after copulation in the form of sperm competition and cryptic female choice. Theory suggests that sperm competition should favor traits such as testes size and sperm production that increase a male's competitive fertilization success. Studies of experimental evolution offer a powerful approach for assessing evolutionary responses to variation in sexual selection pressures. Here we removed sexual selection by enforcing monogamy on replicate lines of a naturally polygamous horned beetle, Onthophagus taurus, and monitoring male investment in their testes for 21 generations. Testes size decreased in monogamous lines relative to lines in which sexual selection was allowed to continue. Differences in testes size were dependent on selection history and not breeding regime. Males from polygamous lines also had a competitive fertilization advantage when in sperm competition with males from monogamous lines. Females from polygamous lines produced sons in better condition, and those from monogamous lines increased their sons condition by mating polygamously. Rather than being costly for females, multiple mating appears to provide females with direct and/or indirect benefits. Neither body size nor horn size diverged between our monogamous and polygamous lines. Our data show that sperm competition does drive the evolution of testes size in onthophagine beetles, and provide general support for sperm competition theory. [source] Co-evolution of male and female reproductive traits across the Bruchidae (Coleoptera)FUNCTIONAL ECOLOGY, Issue 5 2008P. F. Rugman-Jones Summary 1Despite the obvious importance of spermatozoa to individual reproductive success a general explanation of variation in spermatozoan form and function is still lacking. In species with internal fertilization, sperm not only have to interact with the physical and biochemical environment of the female reproductive tract, but frequently face competition from the sperm of rival males. Both sperm competition theory and adaptation to the selective environment of the female reproductive tract have been implicated in the evolution of spermatozoan morphological diversity. 2Using the comparative method, we examine variation in sperm length in relation to (i) sperm competition intensity (as measured by relative testis size) and (ii) female reproductive characters, across 15 species of beetle belonging to the family Bruchidae. 3Stepwise multiple regression within a phylogenetic framework revealed sperm length to be positively correlated with female spermathecal duct length and negatively related to spermathecal volume, but not testes size, indicating that the female reproductive environment rather than sperm competition per se exerts selection on sperm length in this taxonomic group. 4A positive association between testes volume and the volume of the female spermatheca was also evident suggesting correlated evolution of these traits. 5A number of models of sexual selection could lead to the correlated evolution of male and female reproductive characters, although the underlying mechanisms of cause and effect remain elusive. Divergence between species (and populations) in primary reproductive traits is likely to present a significant barrier to hetero-specific fertilization, and thus contribute to reproductive isolation. [source] Park's Tribolium competition experiments: a non-equilibrium species coexistence hypothesisJOURNAL OF ANIMAL ECOLOGY, Issue 5 2003Jeffrey Edmunds Summary 1In this journal 35 years ago, P. H. Leslie, T. Park and D. B. Mertz reported competitive exclusion data for two Tribolium species. It is less well-known that they also reported ,difficult to interpret' coexistence data. We suggest that the species exclusion and the species coexistence are consequences of a stable coexistence two-cycle in the presence of two stable competitive exclusion equilibria. 2A stage-structured insect population model for two interacting species forecasts that as interspecific interaction is increased there occurs a sequence of dynamic changes (bifurcations) in which the classic Lotka,Volterra-type scenario with two stable competitive exclusion equilibria is altered abruptly to a novel scenario with three locally stable entities; namely, two competitive exclusion equilibria and a stable coexistence cycle. This scenario is novel in that it predicts the competitive coexistence of two nearly identical species on a single limiting resource and does so under circumstances of increased interspecific competition. This prediction is in contradiction to classical tenets of competition theory. [source] Resource competition and plant traits: a response to Craine et al.JOURNAL OF ECOLOGY, Issue 2 2007Summary 1Resource competition theory incorporates the mechanisms that underlie consumer,resource interactions and the trade-offs that constrain these mechanisms. Contrary to assertions by Craine, the concept of R* as the measure of resource reduction and the predictor of resource competition has not changed since it was proposed more than two decades ago. 2Resource reduction, as summarized in R*, is readily observed. Soil concentrations of nitrate and water are decreased by plant uptake, and are lowered to different levels by different species. Tests have shown R* theory to correctly predict competitive outcomes for a variety of organisms and ecosystems. 3Consumer-resource mechanisms are a building block for theories that incorporate other trade-offs faced by plants, such as those between competitive ability and dispersal. 4Numerous plant traits interactively determine R* in a manner predictable from trait-based resource competition theory. The same traits shown by comparative research to be associated with plant dominance in low-nutrient habitats give lower R* values, greater predicted competitive ability and greater predicted abundances in nutrient-limited habitats. 5Plant ecology needs closer links between analytical theory, observations and experiments. Simple verbal theories can generate novel ideas but the logical implications of such scenarios are best explored using the rigorous logic of mathematics. Predictions of theory can then be tested via experiments and comparative studies. [source] Measuring the components of competition along productivity gradientsJOURNAL OF ECOLOGY, Issue 2 2007MARK V. WILSON Summary 1Controversy surrounds the measurement of competition intensity. Moreover, when biomass varies systematically along productivity and other environmental gradients, common indices of competitive outcome mask important ecological interactions. 2This study presents two indices derived from how neighbours interact with target plants. The first, relative crowding, increases directly with the abundance of neighbours present and decreases inversely with the potential size and vigour of the target plant itself. The second, interaction strength, is the integral of suppression of the target by neighbours over the range of neighbour abundance. Relative crowding and interaction strength are derived independently, but when multiplied produce the commonly used relative competitive index, showing the biological underpinnings of the relative competition index in terms of crowding and strength of interaction. Since the new indices of relative crowding and interaction strength explicitly account for the amount of neighbour biomass, they serve as a valid method to track the effects of changing habitat conditions on the components of competition. 3The new indices are applied to three published data sets. In each case, relative crowding increased with standing crop. In one case competition was reported as unchanged along a productivity gradient, whereas the new indices show that relative crowding and interaction strength both had significant patterns, but their effects were counteracting. These results do not fit current theories of competition. Further empirical studies are needed to see if competition theory needs revision. 4Separating the mechanisms of competition into relative crowding and strength of interaction reveals previously hidden patterns that help bring to light underlying processes of competition along productivity gradients. [source] Pollen and sperm heteromorphism: convergence across kingdoms?JOURNAL OF EVOLUTIONARY BIOLOGY, Issue 1 2005I. Till-Bottraud Abstract Sperm competition theory predicts that males should produce many, similar sperm. However, in some species of animals and plants, males exhibit a heteromorphism that results in the production of at least two different types of sperm or pollen grains. In animals, sperm heteromorphism typically corresponds to the production of one fertile morph and one (or more) sterile morph(s), whereas in plants two or more pollen morphs (one of which can be either sterile or fertile) are produced in all flowers but sometimes in different anthers. Heteromorphism has arisen independently several times across phyla and at different phylogenetic levels. Here, we compare and contrast sperm and pollen heteromorphism and discuss the evolutionary hypotheses suggested to explain heteromorphism in these taxa. These hypotheses include facilitation, nutritive contribution, blocking, cheap filler, sperm flushing or killing for animals; outcrossing and precise cross-pollen transfer or bet-hedging strategy for plants; cryptic female choice for both. We conclude that heteromorphism in the two phyla is most likely linked to a general evolutionary response to sexual selection, either to increase one male's sperm or pollen success in competition with other males, or mediate male/female interactions. Therefore, although sperm and pollen are not homologous, we suggest that heteromorphism represents an example of convergence across kingdoms. [source] Sperm competition selects for increased testes mass in Australian frogsJOURNAL OF EVOLUTIONARY BIOLOGY, Issue 3 2002P. G. Byrne Game theory predicts that investment in spermatogenesis will increase with the risk and intensity of sperm competition. Widespread support for this prediction has come from comparative studies of internal fertilizing species reporting positive associations between testes mass and the probability that females mate with more than one male. Data for external fertilizers have generated conflicting results. We investigated how risk of sperm competition affects testes size in two families of Australian frogs: the Myobatrachidae and the Hylidae. We also examined effects of clutch size, egg size and oviposition location as alternative factors that might influence sperm production. Species were ranked according to probability of group spawning, and hence risk of sperm competition. Controlling for body size and phylogenetic relationships, we demonstrated that within the Myobatrachidae, the risk of sperm competition explained a significant amount of variation in testes mass. Oviposition location had a weak influence, with species ovipositing into foam having smaller testes. No significant effects of clutch size or egg size were detected. In hylids, the relationship between testes mass and risk of sperm competition was positive but not significant, again with no predictable effects related to egg size or number. These data provide an important test of sperm competition theory for externally fertilizing taxa. [source] Does intergenerational social mobility affect antagonistic attitudes towards ethnic minorities?THE BRITISH JOURNAL OF SOCIOLOGY, Issue 2 2009Jochem Tolsma Abstract Up till now, no study satisfactorily addressed the effect of social mobility on antagonistic attitudes toward ethnic minorities. In this contribution, we investigate the effect of educational and class intergenerational mobility on ethnic stereotypes, ethnic threat, and opposition to ethnic intermarriage by using diagonal mobility models. We test several hypotheses derived from ethnic competition theory and socialization theory with data from the Social and Cultural Developments in The Netherlands surveys (SOCON, waves 1995, 2000, and 2005) and The Netherlands Kinship and Panel Study (NKPS, wave 2002). We find that the relative influence of social origin and social destination depends on the specific origin and destination combination. If one moves to a more tolerant social destination position, the influence of the social origin position is negligible. If on the other hand, one is socially mobile to a less tolerant social position, the impact of the origin on antagonistic attitudes is substantial and may even exceed the impact of the destination category. This confirms our hypothesis that adaptation to more tolerant norms is easier than adaptation to less tolerant norms. We find only meagre evidence for the hypothesis that downward mobility leads to frustration and consequently to more antagonistic attitudes. [source] | |||||||||||||