Role Of Competitive Interactions Research Articles

Habitat disturbance modifies dominance, coexistence, and competitive interactions in tropical ant communities

1. Interspecific competition is a major structuring principle in ecological communities. Despite their prevalence, the outcome of competitive interactions is hard to predict, highly context‐dependent, and multiple factors can modulate such interactions.2. We tested predictions concerning how competitive interactions are modified by anthropogenic habitat disturbance in ground‐foraging ant assemblages inhabiting fragmented Inter‐Andean tropical dry forests in southwestern Colombia, and investigated ant assemblages recruiting to baits in 10 forest fragments exposed to varying level of human disturbance.3. Specifically, we evaluated how different components of competitive interactions (patterns of species co‐occurrence, resource partitioning, numerical dominance, and interspecific trade‐offs between discovery and dominance competition) varied with level of habitat disturbance in a human‐dominated ecosystem.4. Multiple lines of evidence suggest that the role of competitive interactions in structuring ground‐foraging ant communities at baits varied with respect to habitat disturbance. As disturbance increased, community structure was more likely to exhibit random co‐occurrence patterns, higher levels of monopolization of food resources by dominant ants, and disproportionate dominance of a single species, the little fire ant (Wasmannia auropunctata). At a regional scale, we found evidence for a trade‐off between dominance and discovery abilities of the 15 most common species at baits.5. Together, these results suggest that human disturbance modifies the outcome of competitive interactions in ground‐foraging ant assemblages and may promote dominant species that reduce diversity and coexistence in tropical ecosystems.

Perceived dominance status affects chemical signalling in the neriid fly Telostylinus angusticollis

Chemical communication mediates many social interactions in insects but is still less well understood than other forms of communication. In particular, chemical signalling of social dominance is believed to play an important role in competitive interactions in both sexes, but much of the evidence is correlational. Here we manipulated social dominance and examined its effect on CHC profiles in Telostylinus angusticollis, a fly with a resource defence polygyny mating system. Focal individuals' perception of their own dominance status was manipulated by placing them in an arena with larger or smaller competitors to render them ‘subordinate’ or ‘dominant.’ We found that social dominance treatment affected males' and females' social status (quantified as proximity to the larval medium/oviposition dish), as well as their CHC profiles. Dominant individuals tended to have CHC profiles less similar to those of the opposite sex. Moreover, dominant females exhibited an overall elevation of all CHC expression, relative to subordinate females, whereas males that perceived themselves as subordinate exhibited a near-significant downregulation of male-limited CHCs. Our findings suggest that T. angusticollis males and females alter their CHC profiles in response to their self-perceived social dominance status. These chemical signals could play a role in social interactions both within and between the sexes.

Fast-growing willows significantly reduce invasive knotweed spread

Competitive interactions seem to play a major role in invasive plant success. However, they have mostly been addressed through the invader impacts on other species of the plant community and rarely through the way plant communities can contain alien species. Understanding such mechanisms would help in designing restoration projects using plant community competitive properties to control invasive populations.In this study, we looked at the role of competitive interactions in the success of Fallopia japonica (Houtt.) Ronse Decraene using a native willow frequently used in bioengineering techniques: Salix viminalis L. S. viminalis has a high growth rate and is, as such, a potential candidate to compete with F. japonica in restoration projects of invaded areas.Both species were grown in semi-controlled conditions in mesocosms (truck dumpsters), alone or in competition. Morphological traits (plant height, specific leaf area) as well as biomass (aboveground and underground) were measured on each species during two growing seasons. We also quantified spatial expansion of F. japonica.Even under a dense canopy of S. viminalis, F. japonica was able to survive and grow. However, its performance was significantly reduced compared to monocultures and its spatial colonization was less extended. Although S. viminalis biomass was affected by F. japonica, F. japonica expressed competitive stress through a modification of ramet density and height. There was no significant effect of F. japonica on S. viminalis height, enabling this species to dominate.Synthesis and applications: We conclude that S. viminalis succeeded in reducing F. japonica growth by developing a competitive canopy. Bioengineering techniques aiming at restoring a competitive neighborhood can control F. japonica. However, F. japonica's broad underground extension should be taken into account in any management strategy in order to successfully limit its development and spatial spread.

The sil Locus in Streptococcus Anginosus Group: Interspecies Competition and a Hotspot of Genetic Diversity

The Streptococcus Invasion Locus (Sil) was first described in Streptococcus pyogenes and Streptococcus pneumoniae, where it has been implicated in virulence. The two-component peptide signaling system consists of the SilA response regulator and SilB histidine kinase along with the SilCR signaling peptide and SilD/E export/processing proteins. The presence of an associated bacteriocin region suggests this system may play a role in competitive interactions with other microbes. Comparative analysis of 42 Streptococcus Anginosus/Milleri Group (SAG) genomes reveals this to be a hot spot for genomic variability. A cluster of bacteriocin/immunity genes is found adjacent to the sil system in most SAG isolates (typically 6–10 per strain). In addition, there were two distinct SilCR peptides identified in this group, denoted here as SilCRSAG-A and SilCRSAG-B, with corresponding alleles in silB. Our analysis of the 42 sil loci showed that SilCRSAG-A is only found in Streptococcus intermedius while all three species can carry SilCRSAG-B. In S. intermedius B196, a putative SilA operator is located upstream of bacteriocin gene clusters, implicating the sil system in regulation of microbe–microbe interactions at mucosal surfaces where the group resides. We demonstrate that S. intermedius B196 responds to its cognate SilCRSAG-A, and, less effectively, to SilCRSAG-B released by other Anginosus group members, to produce putative bacteriocins and inhibit the growth of a sensitive strain of S. constellatus.

Do settlement dynamics influence competitive interactions between an alien tunicate and its native congener?

Variation in density of early stages, that is, larvae and juveniles, is a major determinant of the distribution and abundance of the adult population of most marine invertebrates. These early stages thus play a key role in competitive interactions, and, more specifically, in invasion dynamics when biologically similar native and non‐native species (NNS) come into contact in the same habitat. We examined the settlement dynamics and settlement rate of two important members of the fouling community that are common on human‐made infrastructures around the world: Ciona robusta (formerly known as Ciona intestinalis type A) and C. intestinalis (formerly known as C. intestinalis type B). In the western English Channel, the two species live in close syntopy following the recent introduction of C. robusta in the native European range of C. intestinalis. Using settlement panels replaced monthly over 2 years in four marinas (including one studied over 4 years) and species‐diagnostic molecular markers to distinguish between juveniles of both species (N = 1,650), we documented similar settlement dynamics of both species, with two settlement periods within a calendar year. With one exception, settlement times were highly similar in the congeners. Although the NNS showed lower settlement density than that of the native congener, its juvenile recruitment was high during the second settlement period that occurs after the warm season, a pattern also observed in adult populations. Altogether, our results suggest that species’ settlement dynamics do not lead to the dominance of one species over the other through space monopolization. In addition, we showed that changes over time are more pronounced in the NNS than in the native species. This is possibly due to a higher sensitivity of the NNS to changes of environmental factors such as temperature and salinity. Environmental changes may thus eventually modify the strength of competitive interactions between the two species as well as species dominance.

Multi-clone infections and the impact of intraspecific competition on trematode colonies with a division of labour

A division of labour occurs in colonies of the trematode Philophthalmus sp. within their first intermediate hosts. Two castes exist: one which reproduces and one which does not reproduce. It has been hypothesized that the benefit of the non-reproductive caste is in competitive interactions. Evidence for this from past experiments with Philophthalmus sp. colonies has been contradictory: the non-reproductive caste appears to benefit the colony in some way but not necessarily by combating interspecific competitors. The aims of this study were to consider intraspecific competition as a possible cause of the division of labour in Philophthalmus sp. colonies. Results show that mixed genotype infections occur in Philophthalmus sp. infected hosts and thus intraspecific competition is likely. Furthermore, the total number of individuals per colony is reduced in mixed genotype infections, indicating that intraspecific competition reduces colony fitness. However, the results do not indicate that the division of labour in Philophthalmus sp. plays a role in competitive interactions as the ratio of small, non-reproductive to large, reproductive individuals is unaffected by the presence of intraspecific competition. This is the first study to identify and quantify intraspecific competition in Philophthalmus sp., and to assess its selective role in this species' division of labour.

Rethinking Community Assembly through the Lens of Coexistence Theory

Although research on the role of competitive interactions during community assembly began decades ago, a recent revival of interest has led to new discoveries and research opportunities. Using contemporary coexistence theory that emphasizes stabilizing niche differences and relative fitness differences, we evaluate three empirical approaches for studying community assembly. We show that experimental manipulations of the abiotic or biotic environment, assessments of trait-phylogeny-environment relationships, and investigations of frequency-dependent population growth all suggest strong influences of stabilizing niche differences and fitness differences on the outcome of plant community assembly. Nonetheless, due to the limitations of these approaches applied in isolation, we still have a poor understanding of which niche axes and which traits determine the outcome of competition and community structure. Combining current approaches represents our best chance of achieving this goal, which is fundamental to conceptual ecology and to the management of plant communities under global change.

Conserved Mutations in the Pneumococcal Bacteriocin Transporter Gene, blpA , Result in a Complex Population Consisting of Producers and Cheaters

ABSTRACTAll fully sequenced strains of Streptococcus pneumoniae possess a version of the blp locus, which is responsible for bacteriocin production and immunity. Activation of the blp locus is stimulated by accumulation of the peptide pheromone, BlpC, following its secretion by the ABC transporter, BlpA. The blp locus is characterized by significant diversity in blpC type and in the region of the locus containing putative bacteriocin and immunity genes. In addition, the blpA gene can represent a single large open reading frame or be divided into several smaller fragments due to the presence of frameshift mutations. In this study, we use a collection of strains with blp-dependent inhibition and immunity to define the genetic changes that bring about phenotypic differences in bacteriocin production or immunity. We demonstrate that alterations in blpA, blpC, and bacteriocin/immunity content likely play an important role in competitive interactions between pneumococcal strains. Importantly, strains with a highly conserved frameshift mutation in blpA are unable to secrete bacteriocins or BlpC, but retain the ability to respond to exogenous peptide pheromone produced by cocolonizing strains, stimulating blp-mediated immunity. These “cheater” strains can only coexist with bacteriocin-producing strains that secrete their cognate BlpC and share the same immunity proteins. The variable outcome of these interactions helps to explain the heterogeneity of the blp pheromone, bacteriocin, and immunity protein content.

Natural functions of lipopeptides fromBacillusandPseudomonas: more than surfactants and antibiotics

Lipopeptides constitute a structurally diverse group of metabolites produced by various bacterial and fungal genera. In the past decades, research on lipopeptides has been fueled by their antimicrobial, antitumour, immunosuppressant and surfactant activities. However, the natural functions of lipopeptides in the lifestyles of the producing microorganisms have received considerably less attention. The substantial structural diversity of lipopeptides suggests that these metabolites have different natural roles, some of which may be unique to the biology of the producing organism. This review gives a detailed overview of the versatile functions of lipopeptides in the biology of Pseudomonas and Bacillus species, and highlights their role in competitive interactions with coexisting organisms, including bacteria, fungi, oomycetes, protozoa, nematodes and plants. Their functions in cell motility, leading to colonization of novel habitats, and in the formation and development of highly structured biofilms are discussed in detail. Finally, this review provides an update on lipopeptide detection and discovery as well as on novel regulatory mechanisms and genes involved in lipopeptide biosynthesis in these two bacterial genera.

Influence of size and spatial competition on the bioactivity of coral reef sponges

Sponges biosynthesize a wealth of secondary metabolites, many with novel structures and strong biological activity. Such compounds may serve multiple ecological roles including anti-predation, anti-fouling functionalities and are implicated in border defense or attack during spatial competition. Relative size of benthic organisms may also play an important role in competitive interactions. To determine if a relationship exists between individual size and bioactive metabolite production in the context of spatial competition, we examined three sponge species with different morphologies: the massive Coscinoderma matthewsi, the club-shaped branching Hyrtios erecta, and the fan-shaped Ianthella basta. Extracts from sponges of various sizes and competitive environments were examined using a cell based bioassay as a proxy of bioactivity. For I. basta, sponge size was correlated with bioactivity; the largest individuals generally being the most bioactive. In contrast, there was no correlation between size and bioactivity for either C. matthewsi or H. erecta. Bioactivity of sponges in this study were however highly variable among individuals, regardless of levels of competition. The prevalence of encroaching organisms was not correlated with sponge size for any of the three sponge species, suggesting that potential bioactivity is not influenced by surrounding competition.

Numerical assessment affects aggression and competitive ability: a team-fighting strategy for the ant Formica xerophila

The relationship between numerical advantage and competitive ability is a fundamental component in contests between groups of social animals. An individual's ability to correctly assess the numerical state of its group is of vital importance. In addition to numerical dominance, the group's fighting ability also plays an important role in competitive interactions. By staging experimental fights between two Formica ant species, I show that Formica xerophila are able to assess their own group's strength prior to any competitive encounter. Ants that perceive themselves as part of a large group act more aggressively toward a competitor than ants that perceive themselves as isolated individuals. This increase in aggression improves F. xerophila's competitive ability. Furthermore, the number of individuals in a contest was found to affect competitive ability. In contests with equal number of competitors, groups of F. xerophila were more successful than individual F. xerophila. Contrary to previous predictions using Lanchester's laws of fighting, F. xerophila's ability to kill competitors increased nonlinearly with group size. This nonlinearity was due to the collective fighting strategy of an F. xerophila group isolating and engaging a single Formica integroides competitors.

Aggregative Root Placement: A Feature During Interspecific Competition in Inland Sand-Dune Habitats

Segregation of roots is frequently observed in competing root systems. However, recently, intensified root growth in response to a neighbouring plant has been described in pot experiments [Gersani M, Brown J S, O'Brien E E, Maina G M and Abramsky Z 2001. J. Ecol. 89, 660-669]. This paper examines whether intense root growth towards a neighbour (aggregation) plays a role in competitive interactions between plant species from open nutrient-poor mid-European sand ecosystems. In a controlled field-competition experiment, root distribution patterns of intra- and interspecific pairs as well as single control plants of Corynephorus canescens, Festuca psammophila, Hieracium pilosella, Hypochoeris radicata and Conyza canadensis were investigated after one growing season. Under intraspecific competition plants tended to segregate their root systems, while under interspecific competition most species tended to aggregate roots towards their neighbours even at the expense of root development at the opposite competition-free side of the target. Preference of a root aggregation strategy over the occupation of competition-free soil in interspecific competition emphasizes the importance of contesting between individuals in relation to mere resource acquisition. It is suggested that in the presence of a competitor the plants might use root aggregation as a defensive reaction to maintain a strong competitive response and exclusive access to the resources of already occupied soil volumes.

Role of competitive interactions in growth rate trends of subtilisin s88 crystals

An orthorhombic crystal form of subtilisin BPN’ variant s88 exhibits a systematic variation in growth rates of its three unique faces, resulting in pronounced variations in crystal morphology as a function of the ionic strength (Gallagher et al., J. Crystal Growth 193 (1998) 665). We have sought to explain these observations by performing energetics calculations using the full structural details of the protein. The approach of correlating growth rates with the strength of interactions of molecular pairs in the crystal proved unsatisfactory. The attachment of protein molecules to growing crystal faces is a stochastic process: a protein molecule samples numerous configurational (orientational and translational) states before attaching to the crystal in a configuration consistent with the crystal symmetry. This search-and-bind process determines the rate of crystal growth. We present a simplified approach that mimics this dynamical process: we perform docking of a protein molecule with another protein molecule or a cluster of molecules. An analysis of the bound configurations reveals the presence of orientations that are incommensurate with the crystal symmetry but in which a protein molecule can also bind strongly to the crystal face. Such improperly oriented molecules can be thought of as competitive inhibitors, and their presence is shown to affect the growth rate. Thus procedures aimed at decreasing the effect of such competitive inhibition, or conversely increasing the specificity of protein–protein interactions forming the crystal, are expected to offer better control of crystal growth rates.

Allelopathy as a competitive strategy in persistent thickets of Lantana camara L. in three Australian forest communities

Field experiments were established to assess possible allelopathic suppression by Lantana camara L. of two indigenous tree species. The design allowed comparison of allelopathic effects with density-dependent resource competition effects. Fire and its role in competitive interactions was included as an experimental treatment. Allelopathic responses were measured in L. camara thickets by germinating and growing Alectryon subcinereus (A. Gray) Radlk. in dry rainforest ecotones (Macleay River) or Cryptocarya rigida (Meissner) in warm temperate rainforest and wet sclerophyll forest (Lake Macquarie) at 10, 20 and 30 seedlings m-2, where L. camara was either physically removed (LR), burnt (LB), or cut and left in place (LT). Germination for both species increased significantly by completely removing L. camara (LR) whereas burning (LB) was significant only for C. rigida. Seedling growth for both species was negatively related to increasing density when all L. camara was removed (LR) but was positively related in the other two treatments (LB and LT). C. rigida seedling biomass increased 47.4% (1.75%2.58 g) and 68.6% (1.98%2.95 g) with increasing seedling density for LT and LB respectively and decreased 23.2% (2.93–2.25 g) for LR. A. subcinereus seedling biomass increased 29.7% (1.95–2.53 g) and 34.7% (2.25–3.03 g) with increasing seedling density for LT and LB respectively and decreased 27.9% (3.30–2.38 g) for LR. Phytotoxin dilution effects were inferred in LT and LB rather than density-dependent intraspecific competition, whereas the reverse was true for LR. Seedling biomass for C. rigida resulting from potential phytotoxin dilution at high seedling density was not significantly different from the response of LR at low seedling density but, for A. subcinereus, the phytotoxin dilution response was significantly less than LR at low seedling density. Moderately intense fire (LB) was not significantly different from the LT treatment at both locations, emphasising that moderate to low intensity fires should not be used to control existing invasions of L. camara. Competitive strategies for invasive populations are identified that may modify succession following disturbance, thereby allowing thicket formation and long-term persistence to affect community dynamics. Such strategies need to be recognised in managing natural communities, particularly for biodiversity conservation. Nomenclature: Harden (1990).

Bryozoan stolonal outgrowths: a role in competitive interactions?

The use of stolons in competitive interactions between several species of bryozoans growing in Townsville Harbour, North Queensland is described. Stolonal growth resulted in overgrowth reversals and redirected growth in interactions between Schizoporella unicornis and Watersipora subtorquata. These results suggest that stolons may play an important role in bryozoan interactions

Induced resistance in mountain birch: defence against leaf-chewing insect guild and herbivore competition

The effect of leaf damage simulating the feeding of early season insect herbivore species, e.g. Epirrita autumnata, to mountain birch, Betula pubescens ssp. tortuosa, on the performance of insect larvae was studied with eleven leaf-chewing sawfly species. I found variation in the results that was due to short- and long-term inducible responses and to the phenology of herbivore species. In general, early and mid-season species were more strongly affected by induced reactions than late-season species. This finding is in accordance with earlier results but I could show that the persistance of induced reactions rather than the influence of timing of damage is responsible for the result. The growth of the larvae of mid-season sawfly species was affected by both short- and long-term induced reactions. This result shows that early season species may escape short-term induced reactions of mountain birch in current year but may not avoid long-term effects. It is supposed that seasonal deterioration of leaf quality either masks the effects of induced defences or late-season species are better adapted to low-quality leaves. Some species show variation in their response to induced defence in different years. This may be due to yearly differences in induced reactions as well as to species-specific responses. Induced defence reactions may play a role in competitive interactions between herbivore species in leaf-chewing guild of mountain birch.

Role of competitive interactions in the postnatal development of X and Y retinogeniculate axons

The cat's retinogeniculate pathway is largely composed of X and Y axons, which represent two distinct neuronal streams organized in parallel. Our earlier data, summarized in the previous paper, suggest that the postnatal development of retinogeniculate axon arbors is characterized by competitive interactions between the X and Y axons. Thus, during development, X arbors in lamina A or A1 are initially broad or exuberant before the Y arbors begin to develop adultlike arbors; the X arbors then shrink to their adult form as the Y arbors grow and establish their mature complement of connections; monocular lid suture prevents the rapid growth of Y arbors, which in turn prevents the pruning of X arbors; and monocular enucleation at birth allows X arbors from the remaining eye to retain their exuberance although completely confined to their appropriate lamina A or A1, whereas the Y arbors develop aberrant extensions into adjacent, previously denervated laminae. We now provide additional evidence for the role of competition between retinogeniculate X and Y axons during development. The addition of visual deprivation by lid suture of the remaining eye to monocular enucleation at birth causes no apparent change in the morphology of X arbors in laminae A and A1. In contrast, the Y arbors of such cats continue to form extensive translaminar sprouts in the previously denervated laminae despite severely reduced terminations in the lamina A or A1 normally innervated by the remaining eye. We interpret these new data, in conjunction with our earlier data, as follows. If retinogeniculate X and Y arbors complete for synaptic space during postnatal development, terminations of Y axons can be affected by lid suture only in geniculate laminae where terminations of X axons are also present. Thus, Y axon arbors are severely reduced in deprived lamina A or A1 following lid suture whether or not the other eye is removed. Where X arbors are not present, such as in lamina C or the laminae inappropriate for the remaining eye after removal of the other, the lid suture has no obvious effect on development of the Y arbors.

Generalist Herbivore Foraging and Its Role in Competitive Interactions

Whether herbivores are food limited and compete interspecifically for food has been debated by population/community ecologists. To examine this proposition, a mechanistic approach based upon autecological observations is employed: 1. 45 foraging studies of 20 species of generalist herbivores indicate that the herbivores behave as energy maximizers in their diet selection, as determined by a linear programming model. 2. The constraints that determine the forager's energy maximizing diet can be used to determine whether a food item should be consumed, based upon a minimum digestibility that determines energy value and a minimum item size or abundance that determines cropping rate. 3. Population densities of a number of herbivore species depend upon the biomass of plants in the environment that satisfy the minimum characteristics sought by the herbivore. 4. Using experimental populations for several herbivore species that are known to be food limited, it can be demonstrated that these species compete with each other. The competitive isoclines are non-linear and arise from the fractions of each forager's food resources that it shares and exclusively uses, which depend upon each species' minimum food characteristics. 5. The results indicate that a mechanistic approach can provide considerable insights into herbivore community structure. The foraging constraints underlying the entire analysis may be body size dependent, providing a more general view.

Descending and ascending influences on neurons of the zona incerta

Characteristics of responses of single neurons in zona incerta to electrical stimulation of the amygdala and central gray matter of the midbrain were compared in acute experiments on rats. Analysis of the results showed that stimulation of the central gray matter evokes predominantly excitation (86%), where stimulation of the amygdala invokes inhibition (64%). Convergence of impulses from the amygdala and central gray matter was observed on 58 of 174 responding neurons. Most frequently (57%) the responses were opposite in direction. Stimulation of the amygdala invoked inhibitory effects and stimulation of the central gray matter activation effects. Possible functional relation in the amygdala-zona incerta-central gray matter system are discussed and the role of competitive interactions of flows of impulses descending from the amygdala and ascending from the central gray matter of neurons of zona incerta is examined.