Components Of Competitive Ability Research Articles

Competitive dominance of the invasive plant Impatiens glandulifera: using competitive effect and response with a vigorous neighbour

Plant invasiveness was commonly attributed to the invader’s competitive superiority over the native community, but a general pattern supporting this prediction is still lacking. This is particularly enhanced by the fact that competitive dominance and its role in plant invasiveness require the use of scarcely-practiced experimental elements. Here, we used a comprehensive experimental approach to evaluate the competitive superiority of the highly invasive annual Impatiens glandulifera. We used two competition-setting treatments, which independently examine competitive effect versus response of both native and invasive genotypes. As a neighbour species we used Urtica dioica, a vigorous perennial co-occurring with I. glandulifera at both its native and introduced ranges. By examining both components of competitive ability we were able to show that although invasive genotypes exert a weaker competitive pressure compared to their native conspecifics, they are still competitively superior to U. dioica. Our results also suggest that the high competitive ability of I. glandulifera could be attributed it to allelopathic effects on co-occurring native species. We suggest that an appropriate experimental setup, which examines competitive effect and response independently, can provide a more factual evaluation of the competitive ability of invasive plants. Furthermore, the use of a dominant species as a target plant rather than a random species or one with poor competitive ability, renders our results more general, implying that I. glandulifera might exert greater competitive effect on the less robust co-occurring species. We conclude that our approach is highly useful and advocate its application in future tests of invasion success.

Disentangling the relative importance of climate, size and competition on tree growth in Iberian forests: implications for forest management under global change

Most large-scale multispecies studies of tree growth have been conducted in tropical and cool temperate forests, whereas Mediterranean water-limited ecosystems have received much less attention. This limits our understanding of how growth of coexisting tree species varies along environmental gradients in these forests, and the implications for species interactions and community assembly under current and future climatic conditions. Here, we quantify the absolute effect and relative importance of climate, tree size and competition as determinants of tree growth patterns in Iberian forests, and explore interspecific differences in the two components of competitive ability (competitive response and effect) along climatic and size gradients. Spatially explicit neighborhood models were developed to predict tree growth for the 15 most abundant Iberian tree species using permanent-plot data from the Spanish Second and Third National Forest Inventory (IFN). Our neighborhood analyses showed a climatic and size effect on tree growth, but also revealed that competition from neighbors has a comparatively much larger impact on growth in Iberian forests. Moreover, the sensitivity to competition (i.e. competitive response) of target trees varied markedly along climatic gradients causing significant rank reversals in species performance, particularly under xeric conditions. We also found compelling evidence for strong species-specific competitive effects in these forests. Altogether, these results constitute critical new information which not only furthers our understanding of important theoretical questions about the assembly of Mediterranean forests, but will also be of help in developing new guidelines for adapting forests in this climatic boundary to global change. If we consider the climatic gradients of this study as a surrogate for future climatic conditions, then we should expect absolute growth rates to decrease and sensitivity to competition to increase in most forests of the Iberian Peninsula (in all but the northern Atlantic forests), making these management considerations even more important in the future.

Components of ‘competitive ability’ in the LHS model: Implication on coexistence for twelve co-occurring Mediterranean grasses

The leaf–height–seed (LHS) model has been proposed as a simple trait-based functional classification. We investigated whether the two components of competitive ability, i.e. competitive response (CR) and competitive effect (CE), are captured by the LHS model and whether these two components are independent for 12 coexisting Mediterranean grasses. Two greenhouse experiments were conducted to estimate competitive effect and response of 12 coexisting grass species from Mediterranean habitats in Jordan. We applied a phytometer design and calculated CR and CE using the relative interaction index (RII). Mature plant height, seed mass and leaf dry matter content (LDMC, used as the leaf trait) were measured for each species. Correlations and trade-offs between the three traits and the components of competitive ability, CR and CE, were analyzed with principal components analysis (PCA). The LHS model was a good predictor of competitive ability but CR and CE were independent and related to different traits. CR was positively correlated with seed mass and CE with plant height. LDMC was neither correlated to CR nor to CE. Based on these findings, we suggest that there are three primary strategies allowing coexistence in Mediterranean communities, which are related to competition: (1) large CE, i.e. large negative impact on other species associated with large stature, (2) large CR, i.e. resistance to competition associated with large seeds, and (3) competition avoidance associated with small seeds.

A Model of Simultaneous Evolution of Competitive Ability and Herbivore Resistance in a Perennial Plant

Plant populations often experience the joint effects of intraspecific competition and herbivory, yet the impact of the interaction of these two factors on the outcome of evolution is largely unknown. Here, we develop a spatially explicit simulation model to examine interactions between the evolution of herbivore resistance and competitive ability in the goldenrod Solidago altissima. We define competitive ability as either competitive effect, the ability of a plant to deplete resources and make them unavailable to competitors, or competitive response, the ability to grow, survive, and reproduce despite depletion of resources by neighboring competitors. We considered symmetric and asymmetric modes of competition and explored the following questions: (1) Does the selective effect of competition differ for the two components of competitive ability? (2) What are the effects of the evolution of competitive ability and resistance on each other? (3) Can trade-offs between competitive ability and resistance emerge, given no relationship between these two traits prior to selection? Our results showed that competitive response evolved quickly regardless of the mode of competition, but self-suppression hindered the evolution of competitive effect. The evolution of resistance appeared to be independent of the evolution of competitive ability. Intraspecific competition was the major selective force in our model. At natural levels of herbivory, selection for resistance played a secondary role in structuring the population. Resistant genotypes were only favored at very low resistance costs. At high cost levels, the costs of maintaining resistance far outweighed the benefits. The selective forces of competition and herbivory resulted in trade-offs between competitive response and herbivore resistance, but only at low costs of resistance. Vigorous growth associated with a high competitive response might translate into trade-offs between herbivore tolerance and resistance. The strong selective effects of competitors, coupled with the weaker selection from herbivores, suggest that plant traits directly associated with growth that confer tolerance to both competitors and consumers may be the targets of selection.

A MODEL OF SIMULTANEOUS EVOLUTION OF COMPETITIVE ABILITY AND HERBIVORE RESISTANCE IN A PERENNIAL PLANT

Plant populations often experience the joint effects of intraspecific compe- tition and herbivory, yet the impact of the interaction of these two factors on the outcome of evolution is largely unknown. Here, we develop a spatially explicit simulation model to examine interactions between the evolution of herbivore resistance and competitive ability in the goldenrod Solidago altissima. We define competitive ability as either competitive effect, the ability of a plant to deplete resources and make them unavailable to competitors, or competitive response, the ability to grow, survive, and reproduce despite depletion of resources by neighboring competitors. We considered symmetric and asymmetric modes of competition and explored the following questions: (1) Does the selective effect of com- petition differ for the two components of competitive ability? (2) What are the effects of the evolution of competitive ability and resistance on each other? (3) Can trade-offs between competitive ability and resistance emerge, given no relationship between these two traits prior to selection? Our results showed that competitive response evolved quickly regardless of the mode of competition, but self-suppression hindered the evolution of competitive effect. The evolution of resistance appeared to be independent of the evolution of com- petitive ability. Intraspecific competition was the major selective force in our model. At natural levels of herbivory, selection for resistance played a secondary role in structuring the population. Resistant genotypes were only favored at very low resistance costs. At high cost levels, the costs of maintaining resistance far outweighed the benefits. The selective forces of competition and herbivory resulted in trade-offs between competitive response and herbivore resistance, but only at low costs of resistance. Vigorous growth associated with a high competitive response might translate into trade-offs between herbivore tolerance and resistance. The strong selective effects of competitors, coupled with the weaker selection from herbivores, suggest that plant traits directly associated with growth that confer tol- erance to both competitors and consumers may be the targets of selection.

The Balance of Terror: An Alternative Mechanism for Competitive Trade-Offs and Its Implications for Invading Species.

This article uses models to propose an explanation for three observations in community ecology: the apparent overreaction of prey to attack by specialist predators, the existence of a common trade-off among components of competitive ability in communities of unrelated competitors, and the ability of invading species to break the native trade-off. Strategies that increase resource collection ability are assumed to increase vulnerability to attack by specialist consumers according to a vulnerability function. If competitors compete for a common resource and share the same form of the vulnerability function, then they are favored to converge on the same evolutionarily stable level of competitiveness or trade-off curve even if the parameters describing their specialized consumers differ. The position of the common strategy or trade-off curve depends on the whole guild, with more speciose guilds tending to favor higher levels of competitiveness. Invaders can break the native trade-off if they come from a guild with a higher trade-off curve, an effect possibly enhanced evolutionarily by escape from specialist consumers.

The Balance of Terror: An Alternative Mechanism for Competitive Trade-Offs and Its Implications for Invading Species

This article uses models to propose an explanation for three observations in community ecology: the apparent overreaction of prey to attack by specialist predators, the existence of a common trade‐off among components of competitive ability in communities of unrelated competitors, and the ability of invading species to break the native trade‐off. Strategies that increase resource collection ability are assumed to increase vulnerability to attack by specialist consumers according to a vulnerability function. If competitors compete for a common resource and share the same form of the vulnerability function, then they are favored to converge on the same evolutionarily stable level of competitiveness or trade‐off curve even if the parameters describing their specialized consumers differ. The position of the common strategy or trade‐off curve depends on the whole guild, with more speciose guilds tending to favor higher levels of competitiveness. Invaders can break the native trade‐off if they come from a guild with a higher trade‐off curve, an effect possibly enhanced evolutionarily by escape from specialist consumers.

Fecundity Allocation in Herbaceous Plants

The relationship between fecundity and plant size (fecundity allocation) was studied in twenty-one herbaceous species collected from natural populations. Within species, size/fecundity regressions were predominantly linear but the slopes often varied significantly among sites. Species varied widely in r2 and slope. Monocarpic species generally had greater r2 values and steeper slopes than polycarpic species, especially clonal perennials. The implications of variation in fecundity allocation among species are explored in terms of its role as a component of competitive ability (i.e. fitness under competition). For linear relationships, the role of fecundity allocation as a component of competitive ability is defined by both the slope and the X-intercept of the fecundity versus size regression. Their relative roles depend on the intensity of competition (i.e. the resource supply/demand ratio). We predict that a greater slope beconres more important in defining relative competitive abilities when plants are

SELECTION FOR COMPETITIVE ABILITY: NEGATIVE RESULTS IN DROSOPHILA.

The occurrence of competition between populations may result in selection pressures favoring increases in competitive ability. Competition is used in the following sense: competition occurs when two or more species or genotypes experience depressed fitness (growth rate or carrying capacity) attributable to their mutual presence in an area. This is a modification of a definition given by Emlen (1973, p. 307). Competitive ability is used to mean the sum of all those characteristics of an individual relevant to competition. It is likely in most species that the components of competitive ability are polygenic traits, and as such, affected by epistasis, geneenvironment interactions, and correlations with other traits. Consequently, the evolutionary effects of the occurrence of competition are not straightforward. In particular, the question, Does competitive ability change rapidly and predictably in response to selection pressures arising from the occurrence of competition? has no obvious answer. This is a matter of some interest, as failure to improve rapidly could result in exclusion by superior competitors and, if this were the case, preadaptation would be important to the structure of natural communities and the success of invasions by new species. In efforts to examine the evolution of competitive ability, laboratory investigations of the effect of intraspecific competition have been done for some short-lived insect species. The results of these studies were dissimilar. In some populations significant improvements were found within a small number of generations (Seaton and Antonovics, 1967; Bryant and Turner, 1972; Sulzbach and Emlen, 1979), while in others no change was detected (Sokal et al., 1970; Dawson, 1972; Sulzbach and Emlen, 1979). Cases of rapid evolution may be dependent upon giving the larvae or ovipositing females of one of the competing populations a temporal headstart in mixtures (Bryant and Turner, 1972). However, such initial asymmetries have been found to be insufficient in themselves to cause rapid increases in competitive ability, and it was proposed that rapid changes may be exceptional and highly dependent on the particular populations used (Sulzbach and Emlen, 1979). Most studies of the evolution of intraspecific competitive ability have employed samples from single laboratory populations, and the period of selection under constant conditions was short, generally less than ten generations. The present experiments employ hybrid populations and a selection protocol maintained for a longer period.