Degree Of Competitive Asymmetry Research Articles

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

AbstractWe 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.

Effects of Nitrogen and Plant Density on Competition between Two Maize Hybrids Released in Different Eras

Core Ideas Nitrogen application rate and plant density affect competition between maize hybrids. Competitive outcomes are invariant under different N levels and plant densities. Nitrogen deficiency increased the competitive advantage of the superior competitor. Higher plant density reduced the competitive advantage of the superior competitor. Competition is a common phenomenon in plant communities, but it remains unclear whether it is influenced by soil productivity. To investigate whether N fertilization and plant density affect competition between maize (Zea mays L.) hybrids, field experiments were conducted during the 2013 and 2014 growing seasons using a de Wit replacement series design. Two maize hybrids, YD13 and ZD909, which were released in different eras were tested. In each replacement series, dry matter accumulation (DMA), grain yield, and harvest index per plant of the component hybrids decreased as the proportion of YD13 increased under contrasting N and plant density combination. Despite different N and plant density combinations, competitive outcomes was invariant (i.e.,YD13 was always the superior competitor), the yield performance of ZD909 was always higher than YD13. When the plant density was held constant, N deficiency increased both inter‐cultivar competition and the degree of competitive asymmetry between YD13 and ZD909. When the N application rate was held constant, competition intensity intensified with increasing plant density; however, the degree of competitive asymmetry was reduced, mainly because the competitive advantage of the superior competitor (YD13) diminished at higher population densities. Our results suggest that N deficiency increases the competitive advantage of a superior competitor over a weaker competitor, but this advantage decreases in response to increased plant density. This probably implies that maize hybrids with weak competitiveness should be selected in breeding programs, especially under the condition of higher soil productivity and plant density, which is designated for dense‐planting and high‐yielding production.

Size asymmetry of resource competition and the structure of plant communities: Commentary on DeMalach et al. 2016

Size asymmetry of resource competition and the structure of plant communities: Commentary on DeMalach <i>et al</i>. 2016

Local community size mediates ecological drift and competition in metacommunities

The outcome of competitive interactions is likely to be influenced by both competitive dominance (i.e. niche-based dynamics) and ecological drift (i.e. neutral dynamics governed by demographic stochasticity). However, spatial models of competition rarely consider the joint operation of these two processes. We develop a model based on the original competition-colonization trade-off model that incorporates niche and neutral processes and several realistic facets of ecological dynamics: it allows local competition (i.e. competition within a patch) to occur within communities of a finite size, it allows competitors to vary in the degree of competitive asymmetry, and it includes the role of local migration (i.e. propagule pressure). The model highlights the role of community size, i.e. the number of competitors in the local community, in mediating the relative importance of stochastic and deterministic forces. In metacommunities where local communities are small, ecological drift is substantial enough that strong competitors become effectively neutral, creating abrupt changes in the outcome of competition not predicted by the standard competition-colonization trade-off. Importantly, the model illustrates that, even when other aspects of species interactions (e.g. migration ability, competitive ability) are unchanged, local community size can alter the dynamics of metacommunity persistence. Our work demonstrates that activities which reduce the size of local communities, such as habitat destruction and degradation, effectively compound the extinction debt.

Asymmetric competition as a natural outcome of neighbour interactions among plants: results from the field-of-neighbourhood modelling approach

Numerous attempts have been made to infer the mode of competition from size or biomass distributions of plant cohorts. However, since the relationship between mode of competition and size distributions may be obscured by a variety of factors such as spatial configuration, density or resource level, empirical investigations often produce ambiguous results. Likewise, the findings of theoretical analyses of asymmetric competition are equivocal. In this paper, we analyse the mode of competition in an individual-based model which is based on the new field-of-neighbourhood approach. In this approach, plants have a zone of influence that determines the distance up to which neighbours are influenced. Additionally, a superimposed field within the zone of influence defines phenomenologically the strength of influence of an individual on neighbouring plants. We investigated competition at both individual and population level and characterised the influence of density and of the shape of the field-of-neighbourhood on occurrence and degree of competitive asymmetry. After finding asymmetric competition emerging in all scenarios, we argue that asymmetric competition is a natural consequence of local competition among neighbouring plants.

Tree competition and species coexistence in a warm-temperate old-growth evergreen broad-leaved forest in Japan

The growth dynamics and mode of competition between adult trees ≥ 5.0cm in diameter at breast height (DBH) of nine abundant treespeciesoccupying ca. 85% of the total basal area were investigated in a 4ha study plot (200 m × 200 m) of awarm-temperate old-growth evergreen broad-leaved forest in the Tatera ForestReserve of Tsushima Island, southwestern Japan. In the plot, adult trees ≥5.0 cm DBH co-occurred with 35 woody plant species (except forwoody vine species). The most dominant and largest species,Castanopsis cuspidata var. sieboldiiexhibited a bimodal DBH distribution; it was found in both the upper and lowervertical layers. Other tree species had unimodal DBH distributionscorrespondingmostly to the lower vertical layer. We developed a model for individual growthincorporating both intra- and interspecific competition and degree ofcompetitive asymmetry. One-sided interspecific competition was detected in 17cases out of the 66 possible combinations on the scale of the 4 hastudy plot. The direction of interspecific competition was generally one-sidedfrom layer-I species to layer-II and III ones. The effects of two-sidedcompetition were detected only in layer-II and III species. OnlyDistylium racemosum exhibited one-sided intraspecificcompetition. We also found 11 cases of positive interspecific relationships.Generally, competitive relationships prevailed over positive relationshipsbetween adult trees in this warm-temperate evergreen broad-leaved forest.Competition between adult trees ≥ 5.0 cm in DBH did not occurinthe same vertical layer, but occurred only between trees in different verticallayers. This suggests that competition between adult trees ≥ 5.0cm in DBH plays a key role in the variation in species coexistencebetween different vertical layers on the 4 ha scale of thewarm-temperate evergreen broad-leaved forests. Moreover, it was found bycomparing with three different forest types that interspecific competition ismore intense in warm-temperate forests than in cool-temperate or sub-borealforests. We conclude that, compared to cool-temperate or sub-boreal forests(which have little interspecific competition), warm-temperate forests supportmore complex interspecific relationships and species-specific habitatpreferences that result in higher species diversity.

Ecological bistability and evolutionary reversals under asymmetrical competition.

How does the process of life-history evolution interplay with population dynamics? Almost all models that have addressed this question assume that any combination of phenotypic traits uniquely determine the ecological population state. Here we show that if multiple ecological equilibria can exist, the evolution of a trait that relates to competitive performance can undergo adaptive reversals that drive cyclic alternation between population equilibria. The occurrence of evolutionary reversals requires neither environmentally driven changes in selective forces nor the coevolution of interactions with other species. The mechanism inducing evolutionary reversals is twofold. First, there exist phenotypes near which mutants can invade and yet fail to become fixed; although these mutants are eventually eliminated, their transitory growth causes the resident population to switch to an alternative ecological equilibrium. Second, asymmetrical competition causes the direction of selection to revert between high and low density. When ecological conditions for evolutionary reversals are not satisfied, the population evolves toward a steady state of either low or high abundance, depending on the degree of competitive asymmetry and environmental parameters. A sharp evolutionary transition between evolutionary stasis and evolutionary reversals and cycling can occur in response to a smooth change in ecological parameters, and this may have implications for our understanding of size-abundance patterns.

ECOLOGICAL BISTABILITY AND EVOLUTIONARY REVERSALS UNDER ASYMMETRICAL COMPETITION

How does the process of life-history evolution interplay with population dynamics? Almost all models that have addressed this question assume that any combination of phenotypic traits uniquely determine the ecological population state. Here we show that if multiple ecological equilibria can exist, the evolution of a trait that relates to competitive performance can undergo adaptive reversals that drive cyclic alternation between population equilibria. The occurrence of evolutionary reversals requires neither environmentally driven changes in selective forces nor the coevolution of interactions with other species. The mechanism inducing evolutionary reversals is twofold. First, there exist phenotypes near which mutants can invade and yet fail to become fixed; although these mutants are eventually eliminated, their transitory growth causes the resident population to switch to an alternative ecological equilibrium. Second, asymmetrical competition causes the direction of selection to revert between high and low density. When ecological conditions for evolutionary reversals are not satisfied, the population evolves toward a steady state of either low or high abundance, depending on the degree of competitive asymmetry and environmental parameters. A sharp evolutionary transition between evolutionary stasis and evolutionary reversals and cycling can occur in response to a smooth change in ecological parameters, and this may have implications for our understanding of size-abundance patterns.

Effects of competition mode on the spatial pattern dynamics of wave regeneration in subalpine tree stands

The effects of competition between individuals (the degree of competitive asymmetry and the intensity of competition; an endogenous biological factor) and the intensity of disturbance (unidirectional prevailing winds in the present paper; an exogenous environmental factor) on the spatial pattern dynamics of wave regeneration in subalpine regions were studied by using a coupled map lattice model (a lattice model with discrete time, discrete space and continuous state) of plant populations. It was shown by simulation that the wave-shaped spatial pattern of regeneration is more likely to emerge under symmetric competition than under asymmetric competition and that the likelihood of emergence increases with the intensity of disturbance. It was already shown both theoretically and by field data analysis that Abies undergoes symmetric competition and that Betula undergoes asymmetric competition. Therefore, a theoretical explanation can be given as to why the wave-shaped spatial pattern occurs in Abies forests but not in Betula forests although both the species occur in subalpine regions. Our theoretical study showed that the interaction between endogenous and exogenous factors determine the spatial pattern dynamics in relation to the regeneration of plant communities.

Experimental Evidence That Interspecific Competitive Asymmetry Increases with Soil Productivity

This study tests the assertion that the degree of competitive asymmetry between pairs of plant species increases with increasing soil productivity. We measured the average degree of competitive asymmetry, in an outdoor experiment involving 18 species of herbaceous wetland plants in each of three levels of soil productivity (1/10 NPK fertilizers + drained soil, full NPK fertilizers + drained soil and full NPK fertilizers + saturated soil). Plants, two per pot, were grown from seed over one growing season. The degree of competitive asymmetry increased 1.31 times in level 2 and 1.45 times in level three of the soil productivity treatment, relative to the least productive treatment.

Tree Competition and Species Coexistence in a Sub-boreal Forest, Northern Japan

The growth and mortality patterns and the mode of competition of six tree species forming a sub-boreal climax forest in Hokkaido, northern Japan, were investigated based on the diffusion model at the level of the individual tree ⩾ 2 m height in a 2·3-ha study site. Picea jezoensis, Picea glehnii, Betula ermanii and Abies sachalinensis were dominant species, occupying approx. 94% of the total basal area. Sorbus commixta and Acer ukurunduense were subordinate species occupying approx. 6% of the total basal area. A model for individual growth was developed, considering both intra- and inter-specific competition and the degree of competitive asymmetry. Asymmetry was found in intraspecific competition of Sorbus commixta and Acer ukurunduense. Picea jezoensis, Betula ermanii and Abies sachalinensis showed symmetric intraspecific competition. There was little interspecific competition amongst Picea jezoensis, Picea glehnii and Betula ermanii. Abies sachalinensis competed symmetrically with Picea jezoensis (only very weakly, P < 0·1) and Betula ermanii ( P < 0·01). Picea glehnii gave no indication of inter- or intra-specific competition. The growth of the four dominant species was never affected by the two subordinate species; the growth of the two subordinate species was governed by the abundances of the four dominant species, the sum of which almost amounted to stand crowdedness (i.e. symmetric competitive effect and one-sided competitive direction). On the scale of 2·3 ha of the sub-boreal forest, symmetric competition prevailed over one-sided or asymmetric competition although statistical evidence for any competitive effects was rather weak. This was probably due to the relatively low tree density and stand crowdedness of this climax forest. Little competition between the dominant species suggested by relatively low proportions of r 2-values attributable to competitive effects indicates weak organization amongst the component species (i.e. species were more or less independent of each other) at the level of the individual tree ⩾ 2 m height on the 2·3-ha scale.

Shoot Growth and Mortality Patterns of Urtica dioica, a Clonal Forb

The growth and mortality patterns of the clonal forb Urtica dioica were investigated at the level of the individual shoot in two growing seasons, 1991 and 1992, in a natural stand. Shoot height and diameter at ground level of each shoot tagged in spring were measured repeatedly five times during the growing season. Dry weights of these repeatedly measured shoots were estimated using an allometric relationship between dry weight, height and diameter of harvested shoots. A large decrease in shoot density occurred with stand development from the beginning of the growing season in both the years: (1) shoot survival rate was about 30% at the end of the growing season; (2) shoot mortality rate per 10 × 10 cm subplot between censuses was positively dependent on shoot density per subplot; (3) the mortality rate of individual shoots was negatively dependent on shoot size (height, diameter and weight) at each growing stage, suggesting one-sided competition between living and dying shoots; (4) shoot size (height, diameter and weight) variability in terms of the coefficient of variation and skewness decreased in accordance with shoot mortality. Symmetric competition between living shoots was detected by regression analysis based on a model for individual shoot growth considering the degree of competitive asymmetry. However, the competitive effect on individual shoot growth was very small (nearly absent). The mortality pattern of Urtica dioica indicates that shoot self-thinning occurred from the early growing stage as in non-clonal crowded monospecific stands, and contrasts with many clonal plants where shoot self-thinning rarely occurs or, if any, is confined only to a short period of the later growing stage. The pattern of growth and competition between living shoots of Urtica dioica contrasts with non-clonal crowded plants undergoing intense competition (usually asymmetric) between individuals, but is a common feature of many clonal plants where shoot competition is supposed to be reduced by 'physiological integration' between shoots. These form a new pattern not reported yet for clonal plants. It is pointed out that clonal plants show a wider spectrum of the growth, competition and mortality patterns of shoots than non-clonals. Some possible mechanisms for the pattern of Urtica dioica are discussed.

Tree competition and species coexistence in a cool‐temperate old‐growth forest in southwestern Japan

Abstract. The growth dynamics and mode of competition between adult trees ≥ 4 cm in DBH (stem diameter at breast height 1.3 m) of eight abundant species occupying ca. 90 % of the total basal area were investigated in a 4‐ha study plot (200 m × 200 m) of a cool‐temperate, old‐growth forest on Mount Daisen, southwestern Japan. In the study plot, 30 tree species with individuals ≥ 4.0 cm DBH co‐occurred. A bimodal DBH distribution showing upper and lower‐canopy layers was found for the most dominant and largest species, Fagus crenata (ca. 78 % of the total basal area), whilst other tree species showed unimodal DBH distributions corresponding mostly to the lower‐canopy layer. We developed a model for individual growth incorporating both intra and interspecific competition and the degree of competitive asymmetry. Onesided interspecific competition was detected only from Fagus crenata (upper‐canopy species) to Acer japonicum and Acanthopanax sciadophylloides (lower‐canopy species) on the scale of the 4‐ha study plot. Only Acanthopanax sciadophylloides showed symmetric intraspecific competition. However, a positive (non‐competitive) interspecific relationship between adult trees prevailed over a competitive relationship; for example, individual DBH growth rate of Fagus crenata (especially lower‐canopy trees) was correlated with the abundance of Acer mono. The positive relationship represented a group of species with similar habitat preference [soil type (mature or immature) caused by landslide disturbance and the presence/absence of Sasa dwarf bamboos in the understorey], where tree densities were not so high as to bring about competition. Competitive interactions between adult trees ≥ 4 cm in DBH occurred only locally between a few specific species and were suggested to be almost irrelevant to the variation in species coexistence on the 4‐ha scale of cool‐temperate forest. Rather, the coexistence of 30 tree species (species diversity) on this large scale was suggested to be governed by the regeneration pattern of each component species (habitat preference, seedling establishment, sapling competition) with respect to landslide disturbance.

Growth and competition in clonal plants-persistence of shoot populations and species diversity

This paper reviews studies on growth and size-structure dynamics of shoots and clones in clonal plants in comparison with those in non-clonal plants, and discusses the characteristics of clonal plants. The mode of competition between individuals (symmetric versus asymmetric, degree of competitive asymmetry), growth dynamics of individuals, allocation pattern between organs and spatial pattern of individuals are closely correlated with each other in non-clonal plant populations. Theoretical and field studies based on the diffusion model revealed that plants of “height-growth” type (mostly early-successional tree species) and plants of “diameter-growth” type (mostly late-successional tree species) tend to exhibit asymmetric competition and symmetric competition respectively. Moreover, asymmetrically competing plants show smaller effects of variation in individual growth rate and spatial pattern on the size-structure dynamics of the population than symmetrically competing plants. Thefefore, the spatial pattern of inviduals should be considered especially for plants undergoing symmetric competition. These results for non-clonal plants should have a significant implication also for the growth dynamics and competition in clonal plants. The mean growth rate of shoots [G(t,x) function] and hence the mode of competition between shoots differs among clonal plant species as in non-clonal plants. However, a large magnitude and size-independence (or slightly negative size-dependence) of the variation in growth rate of shoots [D(t,x) function], especially at the early stage in a growing season is a common characteristic of many clonal plant species, in contrast to the positively size-dependent variation in individual growth rate in non-clonal plants. This type of variation in shoot growth rate leads to the persistence of stable shoot populations even when the mean growth rate function is changed, and also in cases where the shoot population structure would be unstable in the absence of variation in growth rate. It is suggested that competition between clones is symmetric in most clonal plant species, which brings about small-scale spatio-temporal changes in species abundance and hence species diversity.

Mode of Competition and Size‐structure Dynamics in Plant Communities

Abstract Competition between individual plants is defined as a mathematical function for the mean growth rate of individuals in a vertically mono‐layered crowded stand as compared with growth of an isolated individual of the same size. The degree of competitive asymmetry can be described mathematically by using this definition. A canopy photosynthesis model for individual growth in a mono‐layered stand shows that physiological and environmental variations have little effects on the size‐structure dynamics of plant populations undergoing strongly asymmetric competition. These variations affect greatly the size‐structure dynamics under symmetric competition. It has also been shown that one‐sided competition (the most asymmetric competition) is never realised in a mono‐layered stand. A spatial competition model shows that variation in the spatial distribution pattern of individuals greatly affects the size‐structure dynamics of populations undergoing symmetric competition, whilst it has little effect on populations undergoing strongly asymmetric competition. Therefore, it can be concluded that a population undergoing strongly asymmetric competition is a stable system little affected by variations in various factors and that a population undergoing symmetric competition is an unstable system greatly affected by these variations.Based on these theoretical results, the relationships between the mode of competition, species diversity and community stability are discussed and a hypothesis on these processes and mechanisms is proposed for grassland and forest plant communities. One‐sided competition is associated with the stability of community structure, and this type of competition works as a structuring force for plant communities. Symmetric competition is associated with species diversity, and this type of competition cannot be a structuring force of plant communities, i.e. symmetric competition is irrelevant to the deterministic community dynamics. Distinctly multi‐layered forests show “apparent” one‐sided competition between layers (i.e. little direct interactions between canopies in different layers, but higher‐layer trees suppress lower‐layer trees one‐sidedly), bringing about structural stability, but trees in each layer undergo symmetric competition, bringing about species diversity. Mono‐layered grassland shows symmetric competition, bringing about small‐scale spatio‐temporal changes in species' abundance and hence species diversity. The real world lies between the two extreme types of competition, one‐sided and completely symmetric, and each plant community has its own position in this continuous spectrum of the competition mode.