A general model of local competition for space

Local competition for space across a wide array of taxa typically involves three mechanisms that we denote here as expansion (spreading into unoccupied habitat), lottery (replacing dead competitors), and overgrowth (encroaching on competitors along zones of contact). By formulating and analysing a simple, general model incorporating these features, we identify ecological conditions and life‐history features that lead to stable coexistence or competitive exclusion (with or without initial‐condition dependence) and gain insight by linking these to case studies in the literature. We demonstrate the importance of contact inhibition, a little‐studied feature of overgrowth, and we show how life‐history tradeoffs may influence and be influenced by local competition for space. The general model we present can help indicate whether local interactions are sufficient to explain patterns of coexistence or exclusion and can serve as the foundation for more specific, realistic models of spatial competition.     

Fitness consequences of allorecognition-mediated agonistic interactions in the colonial hydroid Hydractinia [GM

In sessile and sedentary organisms, competition for space may have fitness consequences that depend strongly on ecological context. Colonial hydroids in the genus Hydractinia use an inducible defense when encountering conspecifics, and intraspecific competition is common in natural populations, often resulting in complete overgrowth of subordinate competitors. My goal in this study was to quantify the impacts of agonistic interactions in Hydractinia [GM] (an undescribed species from the Gulf of Mexico) in terms of three primary fitness components: colony survival, growth rate, and immature gonozooid production. The results demonstrate that the fitness consequences of intraspecific competition depend on the size at which competitive encounters are initiated and the growth form (an indicator of competitive ability) of the competitors. Moreover, some competing colonies consistently produced more immature gonozooids than the controls without competition, and they exhibited extremely low mortality even after 90 days of growth. These results have several ramifications. First, agonistic interactions do not always proceed to competitive elimination. Second, the increase in production of immature gonozooids--an investment in future reproduction--in response to intraspecific competition supports the hypothesis that indeterminately growing organisms increase sexual reproductive effort when growth becomes limiting. Lastly, in light of known ontogenetic variation in the ability of Hydractinia to differentiate among genetically related colonies, strongly size-dependent fitness consequences are consistent with an adaptive, kin-discriminating allorecognition system.     

Cost of resistance and tolerance under competition: the defense-stress benefit hypothesis

Defense costs provide a major explanation for why plants in nature have not evolved to be better defended against pathogens and herbivores; however, evidence for defense costs is often lacking. Plants defend by deploying resistance traits that reduce damage, and tolerance traits that reduce the fitness effects of damage. We first tested the defense-stress cost (DSC) hypothesis that costs of defenses increase and become important under competitive stress. In a greenhouse experiment, uniparental maternal families of the host plant Arabis perennans were grown in the presence and absence of the bunch grass Bouteloua gracilis and the herbivore Plutella xylostella. Costs of resistance and tolerance manifest as reduced growth in the absence of herbivory were significant when A. perennans grew alone, but not in the competitive environment, in contrast to the DSC hypothesis. We then tested the defense-stress benefit (DSB) hypothesis that plant defenses may benefit plants in competitive situations thereby reducing net costs. For example, chemical resistance agents and tolerance may also have functions in competitive interactions. To test the DSB hypothesis, we compared differentially competitive populations for defense costs, assuming that poorer competitors from less dense habitats were less likely to have evolved defenses that also function in competition. Without competitive benefits of defenses, poorer competitors were expected to have higher net costs of defenses under competition in accordance with DSB. Populations of A. perennans and A. drummondii that differed dramatically in competitiveness were compared for costs, and as the DSB hypothesis predicts, only the poor competitor population showed costs of resistance under competition. However, cost of tolerance under competition did not differ among populations, suggesting that the poor competitors might have evolved a general stress tolerance. Although the DSC hypothesis may explain cases where defense costs increase under stress, the DSB hypothesis may explain some cases where costs decrease under competitive stress.     

The effects of competition on fitness depend on the sex of both competitors

In intraspecific competition, the sex of competing individuals is likely to be important in determining the outcome of competitive interactions and the way exposure to conspecifics during development influences adult fitness traits. Previous studies have explored differences between males and females in their response to intraspecific competition. However, few have tested how the sex of the competitors, or any interactions between focal and competitor sex, influences the nature and intensity of competition. We set up larval seed beetles Callosobruchus maculatus to develop either alone or in the presence of a male or female competitor and measured a suite of traits: development time, emergence weight; male ejaculate mass, copulation duration, and lifespan; and female lifetime fecundity, offspring egg–adult survival, and lifespan. We found effects of competition and competitor sex on the development time and emergence weight of both males and females, and also of an interaction between focal and competitor sex: Females emerged lighter when competing with another female, while males did not. There was little effect of larval competition on male and female adult fitness traits, with the exception of the effect of a female competitor on a focal female's offspring survival rate. Our results highlight the importance of directly measuring the effects of competition on fitness traits, rather than distant proxies for fitness, and suggest that competition with the sex with the greater resource requirements (here females) might play a role in driving trait evolution. We also found that male–male competition during development resulted in shorter copulation times than male–female competition, a result that remained when controlling for the weight of competitors. Although it is difficult to definitively tease apart the effects of social environment and access to resources, this result suggests that something about the sex of competitors other than their size is driving this pattern.     

Sneaking up on ‘enemies’: alleviating inherent disadvantages in competitive outcomes in a nearly 3-million-year-old palaeocommunity from Florida,USA

Bryozoans offer one of the few systems in which competitive interactions for living space can be studied in the fossil record. Here, we describe the outcome of competitive overgrowths in a 3-million-year-old bryozoan palaeocommunity encrusting shells of the bivalve Anomia simplex from the lower Tamiami Formation in Florida (upper Pliocene, Piacenzian). We found that win–lose overgrowths are more common than stand-offs in interspecific encounters, while stand-offs are more common than win–lose overgrowths in intraspecific encounters. We observed more intraspecific encounters and fewer interspecific interactions than expected under a null hypothesis, suggesting that bryozoans of the same species are likely to be clustered. For some species, intraspecific encounters are more likely to result in the apparent fusion of the two colonies, with the development of rows of kenozooids along the contact edge, probably reflecting relatively low dispersal. We also identified some clear winners and some clear losers. A negative correlation was found between the number of colonies observed and the probability of winning for a species, resulting in a dominance of loser species in the assemblage, a pattern previously described as typical for early colonizers of hard substrates. Our results also confirm the finding of earlier studies that having large zooids and, subordinately, multilayered growth are key traits for success in overgrowth competition, with angle of encounter also having an effect for both poor and good competitors that take advantage of ‘attacking’ colonies of other species from the rear and the flank.     

Bryozoan assemblages on hard substrata: species abundance distribution and competition for space

Bryozoans are colonial invertebrates that often dominate epibenthic assemblages on the lower surfaces of hard substrata. Competition among neighbouring organisms is usually a critical process regulating biodiversity, and hard substrata have proved to be a suitable model habitat to analyse spatial interactions. We explored the relationships among abundance, species richness, diversity, competitive ability, coverage, available surface, depth and substratum size in an assemblage of bryozoans encrusting pebbles and cobbles in a bank off the eastern mouth of the Strait of Magellan. We also tested whether overgrowth competition can be regarded as hierarchical, and whether the species abundance distribution shows a mode of rare species and a decreasing frequency of increasingly abundant species. Abundance, species richness, diversity and overgrowth competition were highest on the largest substrata. Smaller pebbles tended to be encrusted by the commonest bryozoans, while the rarest species were mainly found on relatively larger clasts. A high proportion of the lower surfaces of most substrata was available for growth. Diversity values of relatively shallow stations were lower than expected under Caswell’s neutral model. Interspecific competition was hierarchical, but the abundance of colonies was not related to the competitive ability of each species. The species abundance distribution was bimodal, with a main mode of rare species and a second one partly composed of relatively abundant bryozoans with poor competitive abilities. This study shows that even in an encrusting assemblage where competition is hierarchical, the weakest competitors persist and the dominant species are far from being capable of monopolizing space.     

Digest: Structuring interactions in Streptomyces*

For bacteria growing in colonies, spatial structure can allow maintenance of costly traits such as the production of antibiotics. Using spatially structured environments, Westhoff et al. examined the benefits of streptomycin production for the bacterium Streptomyces griseus in competition with a streptomycin-susceptible strain. Streptomyces griseus outcompeted susceptible competitors, but the benefit of its antibiotic decreased as competitor resistance to streptomycin increased. Spatial structure also increased the ability of S. griseus to invade susceptible competitor populations from low starting densities. These results demonstrate that spatially structured environments can both provide and amplify benefits of antibiotics to antibiotic-producing bacteria on a microbial scale.     

Competitive relationships in natural and artificial algal communities

Modern data on competitive relationships and their role in the succession of natural and artificial algal communities are reviewed. The mechanisms of macroalgae competition and the factors that affect the competitive outcomes are considered. The conception of competitive interactions between seaweeds in the field and culture is suggested. (1) Competitive relationships are possible only between seaweeds which live together and are able to exchange signals. (2) Success in the competition for light is the basis for wins in the competition for space. (3) The competition for nutrients never results directly in the exclusion of the competitor from the community. It inhibits the competitor and allows the winner to overgrow, shade, act allelopathically, and to displace the inferior competitor in the community. (4) People, creating an artificial monodominant community, either increase the competitive potential of cultivated species by selection of growth conditions or exclude the competitors.     

Simultaneous evolution of competitiveness and defense: induced switching in Arabis drummondii

Optimality theory for plant defense against herbivores predicts an evolutionary tradeoff between the abilities to compete and defend. We tested this hypothesis by studying the effects of genetic variation in competitiveness on defense expression. Two closely related and differentially competitive congeners were compared for levels of resistance, tolerance, and secondary metabolite production. In a growth room experiment, plants of Arabis drummondii and A. holboellii were grown in the presence and absence of the common bunch grass Boutelloua gracilis, the specialist herbivore Plutella xylostella, and generalist herbivore Trichoplusia ni. Tolerance to competition, measured as growth next to the grass relative to controls in the absence of grass, was greatest for A. drummondii, the species that occurred in communities with higher densities of inter-specific neighbors. Measures of defense (resistance to herbivores, tolerance to damage, and concentrations of glucosinolates) varied inconsistently between the Arabis, species, depending on type of herbivore, competition level, and type of defense. The better competitor A. drummondii was more resistant to specialist herbivores, as in the field, and exhibited greater herbivore- and competition-induced changes in glucosinolate profiles. Further, when plants of A. drummondii were fed upon in competitive environments, the induced glucosinolate response was reduced while tolerance levels increased in an apparent switching of induced strategies. We suggest that competitiveness and defense responses are sometimes positively correlated because some defensive traits also function as competitive traits. A competitive function for defenses may also explain why defenses were affected by competition. Alternatively, since the induced response did not increase estimates of total glucosinolate content significantly, minimal defense costs might also allow the simultaneous evolution of competitiveness and defense. Finally, when faced with both herbivory and competition, some competitive species, such as A. drummondii, may switch to growth-based rather than toxin-based strategies as recent theoretical models predict.     

Spatial heterogeneity, source-sink dynamics, and the local coexistence of competing species

Patch occupancy theory predicts that a trade-off between competition and dispersal should lead to regional coexistence of competing species. Empirical investigations, however, find local coexistence of superior and inferior competitors, an outcome that cannot be explained within the patch occupancy framework because of the decoupling of local and spatial dynamics. We develop two-patch metapopulation models that explicitly consider the interaction between competition and dispersal. We show that a dispersal-competition trade-off can lead to local coexistence provided the inferior competitor is superior at colonizing empty patches as well as immigrating among occupied patches. Immigration from patches that the superior competitor cannot colonize rescues the inferior competitor from extinction in patches that both species colonize. Too much immigration, however, can be detrimental to coexistence. When competitive asymmetry between species is high, local coexistence is possible only if the dispersal rate of the inferior competitor occurs below a critical threshold. If competing species have comparable colonization abilities and the environment is otherwise spatially homogeneous, a superior ability to immigrate among occupied patches cannot prevent exclusion of the inferior competitor. If, however, biotic or abiotic factors create spatial heterogeneity in competitive rankings across the landscape, local coexistence can occur even in the absence of a dispersal-competition trade-off. In fact, coexistence requires that the dispersal rate of the overall inferior competitor not exceed a critical threshold. Explicit consideration of how dispersal modifies local competitive interactions shifts the focus from the patch occupancy approach with its emphasis on extinction-colonization dynamics to the realm of source-sink dynamics. The key to coexistence in this framework is spatial variance in fitness. Unlike in the patch occupancy framework, high rates of dispersal can undermine coexistence, and hence diversity, by reducing spatial variance in fitness.     

Oxidative stress induces cytotoxicity during rejection reaction in the compound ascidian Botryllus schlosseri

When genetically incompatible colonies of the compound ascidian Botryllus schlosseri contact each other, a rejection reaction occurs, characterised by the appearance of cytotoxic foci along the touching borders. In the course of this reaction, morula cells, a common haemocyte-type in ascidians, release their vacuolar content, mainly phenoloxidase and its polyphenol substrata, upon the recognition of soluble factors diffusing from the alien colony through the partially fused tunic. In a previous paper, we demonstrated the relationship between phenoloxidase and cytotoxicity. Here, we investigated the effects of superoxide dismutase, catalase and sorbitol (scavengers of superoxide anions, peroxides and hydroxyl radicals, respectively) on the cytotoxicity observed in haemocyte cultures incubated with heterologous blood plasma. Although the above compounds have no effects on morula cell degranulation and phenoloxidase activity, they suppress cell death, suggesting that oxidative stress plays a key role in in vitro cytotoxicity. In addition, sorbitol reduces the extent of the cytotoxicity occurring in the rejection reaction between incompatible colonies, which stresses the important role of hydroxyl radicals in this process. The observation of a decrease in total and non-protein thiols in haemocytes previously incubated with heterologous blood plasma fits the hypothesis of oxidative stress as the main cause of phenoloxidase-related cytotoxicity.     

Sex increases the probability of evolutionary rescue in the presence of a competitor

The explanation for the continued existence of sex, despite its many costs, remains one of the major challenges of evolutionary biology. Previous experimental studies have demonstrated that sex increases the rate of adaptation in novel environments relative to asexual reproduction. Whereas these studies have investigated the impact of sex on adaptation to stressful abiotic environments, the potential for biotic interactions to influence this advantage of sex has been largely ignored. Species rarely exist in isolation in natural conditions, so the impact of sex on adaptation to a stressful abiotic environment may be altered by the interactions between coexisting species. To investigate the interplay of sex and competition on adaptation to deteriorating conditions, we allowed populations of the unicellular alga (Chlamydomonas reinhardtii) to evolve in an environment to which they were initially poorly adapted. We manipulated both their mode of reproduction and the presence of a competitor, and monitored population size and proportion of evolutionary rescue events for each mode of reproduction. The results indicate that sex may be the beneficial strategy in the presence of the competitor. Sexual populations had highest probability of evolutionary rescue irrespective of the presence of the competitor. The overall advantage of sex was also manifested through higher level of adaptedness of survived sexual populations relative to asexual populations. Since competitive interactions are commonplace in nature, one of the explanations for the maintenance of sex by natural selection may be the increased rate of adaptation of sexual populations both in the presence and absence of competitors.     

EVOLUTION OF BROAD AND SPECIFIC COMPETITIVE ABILITY IN NOVEL VERSUS FAMILIAR ENVIRONMENTS IN DROSOPHILA SPECIES

We used nine pairs of competing Drosophila melanogaster and Drosophila simulans populations to test three hypotheses. (1) Weaker competitors undergo greater evolutionary increases in competitive ability, compared with stronger ones. (2) Increased competitive ability against a specific competitor population causes a correlated increase in competitive ability against other competitor populations. (3) In a novel environment, adaptation to the abiotic environment contributes more to competitive ability than adaptation to the competitor population. After 11 generations of competition, initially weaker competitor populations showed relatively greater increases in competitive ability. Broad and specific competitive abilities, the latter being specific to a particular competitor population, were positively correlated in both familiar and novel environments. Adaptation to the abiotic environment seemed to be a more important component of competitive ability in the novel environments. We conclude that in geographically structured species, biotic and abiotic factors affecting the evolution of competitive ability may interact to help create a mosaic of outcomes that can affect the evolutionary dynamics of the interaction over the range of the competing species.     

Newly dominant benthic invertebrates reshape competitive networks on contemporary Caribbean reefs

Competition is a fundamental process structuring ecological communities. On coral reefs, space is a highly contested resource and the outcomes of spatial competition can dictate community composition. In the Caribbean, reefs are increasingly dominated by non-scleractinian species like sponges, gorgonians, and zoanthids, yet there is a paucity of data on interactions between these increasingly common organisms and historically dominant corals. Here, we investigated interactions among these groups of sessile benthic invertebrates to better understand the role of spatial competition in shaping benthic communities on Caribbean reefs. We coupled surveys of competitive interactions on the reef with a common garden competition experiment to determine the frequency and outcome of interference competition among eight focal species. We found that competitive interactions were pervasive on Florida reefs, with 60% of sessile benthic invertebrates interacting with at least one other invertebrate. Increasingly common non-scleractinian species were some of the most abundant taxa and consistently outcompeted the contemporarily common scleractinian species Porites porites and Siderastrea siderea. The encrusting gorgonian, Erythropodium caribaeorum, was the most aggressive species, reducing the live area of its competitors on average 42% ± 7.04 (SE) over the course of 5 months. Surprisingly, the most aggressive species declined in size when competing, while some less aggressive species were able to increase or maintain area, suggesting a trade-off between aggressiveness and growth. Our findings suggest that competition among sessile invertebrates is likely to remain an important process in structuring coral reefs, but that the optimal strategies for maintaining space on the benthos may change. Importantly, many non-scleractinian species that now dominate reefs appear to be superior competitors, potentially increasing the stress on corals on contemporary reefs.

     

Morphological response to competition for light in the clonal Trifolium repens (Fabaceae)

? Premise of the study: Plant communities in temperate zones are dominated by clonal plants that can plastically modify their growth characteristics in response to competition. Given that plants compete with one another, and the implications this has for species coexistence, we conducted a study to assess how clonal species morphologically respond to competition for light depending on its intensity and heterogeneity, which are determined by the competitor species. ? Methods: We assessed the morphological response to competition for light of the clonal species Trifolium repens L. by measuring its growth performance, and vertical and horizontal growth traits. We used five competitive environments, i.e., one without competitor and four differing by their competitor species creating different conditions of competition intensity and heterogeneity. ? Key results: The morphological response of Trifolium repens to competition for light depended on the competitor identity. Competition intensity and heterogeneity, determined by competitor identity, had an interactive effect on most traits. The increase in petiole elongation and specific leaf area due to increased competition intensity was observed only at low to intermediate competition heterogeneity. Competition heterogeneity promoted the elongation of clone connections allowing space exploration. ? Conclusions: Our results demonstrated that the intensity and heterogeneity of competition, which depended on competitor identity, are of primary importance in determining the plastic response of Trifolium repens. This emphasizes that it is important to consider the fine-scale spatial distribution of individuals when studying their interactions within plant communities.     

Multi-proxy evidence for competition between savanna woody species

Coexistence of trees and grasses in savannas should be possible if competition between the woody and the grassy components is less intense than the competition within each component. Although several studies have investigated competition between trees and grasses, little is known about tree–tree interactions. We used a multi-proxy approach to examine the spatial pattern of Acacia mellifera and other savanna woody species in a semi-arid savanna in South Africa. Spatial analysis of the point patterns of young and reproductively mature shrubs detected decreasing aggregation with size/age over all spatial scales. This indicated the prevalence of competition although the overall spatial shrub pattern was aggregated. In contrast to point pattern statistics that detect changes only when competition has led to the death of the inferior competitor, we also applied methods identifying the competitive effect on sizes of individual trees. Competition should lead to a negative spatial autocorrelation in size, which we observed in half of the studied cases. Quantile regressions show that nearest-neighbour distance increased steeply with combined size of the target shrub and its neighbours indicating strong competitive effects. The medians of the distributions of maximum root lengths of A. mellifera, of the scale of regular patterns, and of negative autocorrelations were not significantly different, suggesting that overlapping root systems mediate competitive interactions. A competitor removal experiment did not lead to increased shrub sizes, which may be due to the limited duration of the experiment. From the nearest neighbour and autocorrelation analyses, we conclude that competition had a strong impact on growth rates of savanna woody species. Competition-induced mortality only becomes obvious when analysing the shift towards less aggregated spatial patterns when shrubs become reproductively mature. As the overall clustered spatial pattern masks the perceptible effect of competition, a time component should always be included in spatial pattern-based inference of competition.     

Effects of spatial pattern and relatedness in an experimental plant community

Many plant species show limited dispersal resulting in spatial and genetic substructures within populations. Consequently, neighbours are often related between each other, resulting in sibling competition. Using seed families of the annuals Capsella bursa-pastoris and Stachys annua we investigated effects of spatial pattern (i.e. random versus aggregated) on total and individual performance at the level of species and seed families under field conditions. At the level of species, we expected that inferior competitors increase, while superior competitors decrease their performance within neighbourhoods of conspecifics. Thus, we expected a species by spatial pattern interaction. Sibling competition, however, might reduce the performance of competitors, when genetically related, rather than non-related individuals are competing. Therefore, aggregations at the level of seed families could decrease the performance of competitors. Alternatively, if the opposite outcome would be observed, kin selection might be hypothesized to have occurred in the past. Because heavy seeds are expected to disperse less than light seeds, we further hypothesized that kin selection might be more likely to occur in superior competitors with heavy, locally dispersed seeds (e.g. Stachys) compared to inferior competitors with light, more distantly dispersed seeds (e.g. Capsella). We found a significant species by spatial pattern interaction. Indeed, the inferior competitor, Capsella, showed increased reproductive biomass production in aggregated compared to random patterns. Whereas, the performance of the superior competitor, Stachys, was to some extent decreased by intraspecific aggregation. Although statistically not significant, effects of intrafamily aggregations tended to be rather negative in Capsella but positive in Stachys. Our results confirmed that spatial patterns affect growth and reproduction of plant species promoting coexistence in plant communities. Although, we could not provide strong evidence for sibling competition or kin selection, our results suggested that competition among relatives was more severe for Capsella (lighter seeds) compared to Stachys (heavier seeds).     

Coral mortality and interaction with algae in relation to sedimentation

The impact of sedimentation on coral–algal interactions was studied by monitoring tissue mortality and radial growth in two coral species, Colpophyllia natans and Siderastrea siderea, over a continuum of sediment input intensities. This study sets out to investigate (1) whether sedimentation can facilitate algal overgrowth of corals and (2) whether this was a significant cause of coral mortality. Over a 15-month period, 198 coral colonies were tagged and photographed at six sites along two replicate gradients of sediment input, spanning high inputs near river mouths to low inputs at exposed headlands. Photographs were taken so that they covered the interface between colonies and algae. Radial growth was measured along colony edges in contact with algae and unaffected by tissue loss from causes other than competition with algae. To establish whether algal overgrowth was a significant cause of coral mortality, tissue mortality on the colony surface area visible in the photographs was related to different causes, including sediment smothering, disease, and algal overgrowth. Radial growth became negative with increasing proximity to river mouths in C. natans and remained negative or close to zero throughout the gradients in S. siderea, overall suggesting that sedimentation can facilitate algal overgrowth on corals. However, the analysis of tissue mortality revealed that algal overgrowth was a relatively minor cause of tissue loss. In contrast, the most important cause of coral mortality in relation to sedimentation was from sediment smothering, probably during intense episodes of deposition associated with heavy rainfall. We conclude that sedimentation may lead to reef degradation by causing coral mortality through sediment smothering and burial, and then by suppressing the regrowth of surviving adult colonies through increased competition with algae.     

Intrinsic competition between primary hyperparasitoids of the solitary endoparasitoid Cotesia rubecula

1. In nature, competitive interactions occur when different species exploit similar niches. Parasitic wasps (parasitoids) often have narrow host ranges and need to cope with competitors that use the same host species for development of their offspring. When larvae of different parasitoid species develop in the same host, this leads to intrinsic and often contest competition. Thus far, most studies on intrinsic competition have focused on primary parasitoids. However, competition among primary hyperparasitoids, parasitic wasps that use primary parasitoids as a host, has been little studied. 2. This study investigated intrinsic competition between two primary hyperparasitoids, the gregarious Baryscapus galactopus and the solitary Mesochorus gemellus, which lay their eggs in primary parasitoid larvae of Cotesia rubecula, while those in turn are developing inside their herbivore host, Pieris rapae. The aims were to identify: (i) which hyperparasitoid is the superior competitor; and (ii) whether oviposition sequence affects the outcome of intrinsic competition. 3. The results show that B. galactopus won 70% of contests when the two hyperparasitoids parasitised the host at the same time, and 90% when B. galactopus oviposited first. When M. gemellus had a 48 h head start, the two hyperparasitoids had an equal chance to win the competition. This suggests that B. galactopus is an intrinsically superior competitor to M. gemellus. Moreover, the outcome of competition is affected by time lags in oviposition events. 4. In contrast to what has been reported for primary parasitoids, we found that a gregarious hyperparasitoid species had a competitive advantage over a solitary species.     

Interspecific competition in perennial sedentary organisms: An individual-based model

We investigated a mathematical model of the dynamics of the ecological system consisting of two competing perennial species, each of which leads a sedentary life. It is an individual-based model, in which the growth of each individual is described. The rate of this growth is weakened by competition from neighboring individuals. The strength of the competitors' influence depends on their size and distance to them. The conditions, in which the competitive exclusion of one of the competitors and the coexistence of both competitors take place are provided. The influence of the parameters responsible for the strength of competition, the degree of competitive asymmetry, and consideration of the importance of specific elements of the spatial structure of this ecological system on the results of the competition were analyzed. Both species co-exist when they are equal competitors. Permanent coexistence is possible only when interspecific competition is weaker than intraspecific. When interspecific competition is stronger, the coexistence of equal interspecific competitors is random. Both species have equal probability of extinction. If species are not equal competitors, the stronger one wins. This result can be modified by different strengths of intraspecific competition. The weaker interspecific competitor can permanently coexist with stronger one, when its individuals suffer stronger intraspecific competition.