Measure Of Competitive Ability Research Articles

An experimental test of the relationship between seed size and competitive ability in annual plants

Competition–colonization tradeoff models explain the coexistence of competing species in terms of a tradeoff between competitive ability and colonization ability. One class of such models is based on the idea that seed size determines competitive ability, seed number determines colonization ability, and the two traits are negatively correlated such that higher competitive ability of large‐seeded species compensates for their smaller seed number. According to such models, species inhabiting the same community should show a distinct ranking of competitive ability and this ranking should be correlated with seed size. We tested these predictions using a greenhouse competition experiment focusing on 25 annual species that coexist in sandy habitats of the Mediterranean region in Israel. Rankings of species based on their competitive effects on two independent phytometers were positively correlated. Corresponding rankings based on competitive responses were also correlated. Rankings based on competitive effects were correlated with those based on competitive responses. Yet, in spite of the clear hierarchy in all measures of competitive ability, none of the measures was correlated with seed size. While lack of correlation between seed size and competitive ability has been documented in some systems, our study is the first time that absence of such correlation is documented in a system where the existence of competition is well established and the component species show a clear hierarchy of competitive ability.

Pairing patterns in relation to body size, genetic similarity and multilocus heterozygosity in a tropical monogamous bird species

The relative influence of genetic and phenotypic quality on pairing status and mating patterns in socially monogamous species remains poorly documented. We studied social status and pairing patterns in relation to genetic similarity and multilocus heterozygosity (MLH) estimates from 11 microsatellite markers, and both tarsus length and wing chord (as a measure of competitive ability in territorial defence) in a socially monogamous tropical bird species where individuals defend territories year-round, alone or in pairs, the Zenaida dove, Zenaida aurita. Tarsus length and wing chord did not differ between unpaired territorial birds and paired ones in either sex, whereas paired females, but not paired males, tended to be more heterozygous than unpaired ones. Among 84 pairs, we found no evidence for assortative mating for tarsus length, wing chord, MLH or genetic similarity. However, within pairs, male wing chord was positively related to female MLH and female tarsus length was positively related to male MLH, with no evidence for local effects, suggesting assortative mating by individual quality. Although the observed pattern of mating in Zenaida doves may be the product of mutual mate choice, further assessment of this hypothesis requires direct investigation of both mating preference in each sex and lifetime reproductive success in relation to body size and MLH.

Human Capital and Regional Growth Perspective

Accumulation of human capital is one of the major determinants of economic growth. In the last decades, theoretical and empirical literature has analysed this issue at regional level, providing interesting results. The analysis focuses on 35 regions of the Visegrad Group (NUTS 2 level) in 2002–2009 and is based on Eurostat Regional Statistics. The objective was to compare competitiveness and human capital intensity in the Visegrad Group regions, verify the existence of correlation and thus potential human capital growth effects. The analysis comprised two groups of indicators: measures of competitive position and human capital education indicators, as measures of competitive ability. The results showed that there have been and continue to be substantial differences among the regions as regards competitiveness and human capital. According to the correlation results, one can suppose — following the research assumptions — that human capital could have positive impact on regional competitiveness in the Visegrad countries in 2002–2009. However, one cannot forget that the relation between wealth and human capital can be reverse. Level of wealth and structure of the regional/local economy could affect the level and quality of human capital. Regardless of this, in the case of Visegrad Group regions, further econometrical research is still needed.

THE MEASUREMENT THEORY OF FITNESS

In this article, an approach to measure fitness is proposed that considers fitness as a measure of competitive ability among phenotypes or genotypes. This approach is based on pairwise competition tests and is related to measures of "utility" in mathematical economics. Extending the results from utility theory it is possible to recover the classical Wrightian fitness measure without reference to models of population growth. A condition, quasi-BTL, similar to the Bradley-Terry-Luce condition of classical utility theory is shown to be necessary for the existence of frequency and context-independent fitness measures. Testing for violations of this quasi-BTL condition can be used to the detect genotype-by-genotype interactions and frequency-dependent fitness. A method for the detection of genotype by environment interactions is proposed that avoids potential scaling artifacts. Furthermore the measurement theoretical approach allows one to derive Wright's selection equation. This shows that classical selection equations are entirely general and exact. It is concluded that measurement theory is able to give definite answers to a number theoretical and practical questions. For instance, this theory identifies the correct scale for measuring gene interaction with respect to fitness and shows that different scales may lead to wrong conclusions.

The demography of range boundaries versus range cores in eastern US tree species

Regional species–climate correlations are well documented, but little is known about the ecological processes responsible for generating these patterns. Using the data from over 690 000 individual trees I estimated five demographic rates—canopy growth, understorey growth, canopy lifespan, understorey lifespan and per capita reproduction—for 19 common eastern US tree species, within the core and the northern and southern boundaries, of the species range. Most species showed statistically significant boundary versus core differences in most rates at both boundary types. Differences in canopy and understorey growth were relatively small in magnitude but consistent among species, being lower at the northern (average −17%) and higher at the southern (average +12%) boundaries. Differences in lifespan were larger in magnitude but highly variable among species, except for a marked trend for reduced canopy lifespan at the northern boundary (average −49%). Differences in per capita reproduction were large and statistically significant for some species, but highly variable among species. The rate estimates were combined to calculate two performance indices: R0 (a measure of lifetime fitness in the absence of competition) was consistently lower at the northern boundary (average −86%) whereas Z* (a measure of competitive ability in closed forest) showed no sign of a consistent boundary–core difference at either boundary.

Species’ traits predict the effects of disturbance and productivity on diversity

Disturbance is an important factor influencing diversity patterns. Ecological theory predicts that diversity peaks at intermediate levels of disturbance, but this pattern is not present in a majority of empirical tests and can be influenced by the level of ecosystem productivity. We experimentally tested the effects of disturbance on diversity and show that species' autecological traits and community relations predicted species loss. We found that - alone or in concert - increasing disturbance intensity or frequency, or decreasing productivity, reduced diversity. Our species did not exhibit a clear competition-colonization trade-off, and intrinsic growth rate was a more important predictor of response to disturbance and productivity than measures of competitive ability. Furthermore, competitive ability was more important in predicting responses when, in addition to killing individuals, disturbance returned nutrients to the ecosystem. Our results demonstrate that species' traits can help resolve conflicting patterns in the response of diversity to disturbance and productivity.

Wheat-Lolium multiflorum competence: effect of nitrogen application onto Argentinean varieties aggressivity

The objective of this study was to establish the competitive ability of Argentinean wheat (Triticum aestivum L) varieties in competence with Lolium multiflorum, and to determine the nitrogen level influence in the competitive process. Fields experiments were conducted at the La Plata National University Experimental Station (34°S, 58°W) La Plata (Argentina) during 1997 and 1998. Two wheat varieties (Klein Cacique and Pro Inta Super) and a local ecotype of L. multiflorum were used. The aggressivity index for above dry matter and grain yield and relative yield total were used as a measure of competitive ability and complementary resources, respectively. The aggressivity index variation due varieties was significant (p<0.05) for above-dry matter at first node detectable of wheat in 1997. No effect of nitrogen fertilization was observed. Neither varieties nor nitrogen fertilization effects were registered at anthesis and maturity. At these crop stages, L.multiflorum had a higher aggressivity than K. Cacique and P. I. Super. Weed registered a higher aggressivity in 0 N2 and 100N2 for grain yield. A higher aggressivity of wheat varieties at first node detectable up to 100 pl.m−2 of weed was observed in 1998 for above-dry matter. Higher weed aggressivity than wheat varieties, irrespective of nitrogen application, were obtained at anthesis. There were no resources complementarity (RYT ∼ 1) and a complete competence amongst crop-weed association was detected. A higher weed relative growth rate from first node detectable stage could explain this reversion onto weed aggressivity index. The application of nitrogen would result in losses of wheat aggressivity and that these losses would increase with increasing L.multiflorum density.

Relative competitive ability does not change over time in blackbirds

Summary Individual variation in feeding rate in the absence of competitors (absolute feeding rate) and decline in feeding rate in the presence of competitors (susceptibility to interference) are frequently used as measures of competitive ability or fitness in short‐term foraging studies. They are also crucial parameters of individual‐based population models that predict the distribution of animals across patches. There are, however, few data on whether absolute feeding rate and susceptibility to interference competition remain constant for individuals between years. The hypothesis that absolute feeding rate and susceptibility to interference were similar between years was tested by observing 25 European blackbirdsTurdus merulafeeding in seminatural experimental patches, during January to March over a 2–4‐year period 1995–98. Absolute feeding rate was measured as the feeding rates of solitary blackbirds. Susceptibility to interference was measured as the change in feeding rate of a focal bird when it fed in the presence of different numbers of competitors. Individuals changed significantly in their feeding rate between years, but most individuals did not change in their feeding rate relative to others in the population. The absolute feeding rate of an individual was significantly positively correlated with its feeding rate in the subsequent year. There was no significant variation in susceptibility to interference for individuals between years. Only two of 27 blackbirds showed a significant change in susceptibility to interference between years. Relative absolute foraging rate and susceptibility to interference competition were reasonably similar between years in blackbirds. Relative fitness measures derived from short‐term measures of foraging ability may therefore be valid over long periods. In ideal free models that incorporate individual competitive ability, rather than population averages, temporal changes in competitive ability can possibly be ignored.

Absolute foraging rate and susceptibility to interference competition in blackbirds varies with patch conditions

Summary Individual variation in feeding rate in the absence of competitors (absolute feeding rate) and change in feeding rate in the presence of competitors (susceptibility to interference) are crucial parameters used in individual‐based population models that predict the distribution of animals across patches. There are, however, few data on how these parameters are dependent on local foraging conditions, which will vary widely in natural systems. We tested the hypotheses that absolute feeding rate and change in feeding rate with group size were independent of patch conditions. We observed 27 European blackbirdsTurdus merulafeeding in seminatural experimental patches that were either open (where prey was easily visible) or covered with leaves (where prey was invisible until the leaves had been moved aside by the bird). Absolute feeding rate and susceptibility to interference were measured by recording feeding rates of blackbirds alone and in the presence of increasing numbers of competitors, respectively. There was significant individual variation in absolute feeding rate dependent on patch type, with some birds showing relatively large increases in open patches. There was no significant correlation between absolute feeding rate in open and leaf patches. There was significant variation in individual susceptibility to interference dependent on patch type. There was no correlation between an individual’s susceptibility to interference in open and leaf patches. Mathematical models that seek to make predictions of the distribution of animals across heterogeneous patches should incorporate measures of competitive ability over a wide range of patch conditions, or use more complex parameters that incorporate individual changes in density dependent pay‐offs dependent on patch conditions.

Traits Related to Competitive Ability of Wheat (Triticum aestivum) Varieties against Italian Ryegrass (Lolium multiflorum)

ABSTRACT The objective of this study was to determine which specific morphological characters are related with competitive ability and to establish their potential to suppress Lolium multiflorum growth while maintaining wheat grain yields. Field experiments were conducted at the La Plata National University Experimental Station (34°S 58°W), La Plata (Argentina) during 1997 and 1998. Wheat varieties used were Buck Charrúa, Klein Cacique and Pro Inta Super. A local ecotype of L. multiflorum was used. 280 plants m−2 of wheat and 50, 100 and 150 plants m−2 of Italian ryegrass were tested. The aggressivity index for aboveground dry matter and grain yield were used as a measure of competitive ability. A higher aggressivity of wheat varieties at first node, detectable up to 100 plants m−2 of weed was observed. For the 150 plants m−2 of weed, L. multiflorum had higher aggressivity than wheat varieties. At crop anthesis, weed aggressivity was higher than wheat varieties for the evaluated densities. No differences between varieties at 50 and 100 plant m−2 of L. multiflorum were observed. At first node detectable, aboveground dry matter and height of last expanded leaf of wheat, and photosynthetically active radiation interception were related with aggressivity. At anthesis, flag leaf length, flag leaf area, penultimate expanded leaf length and relative crop grain yield were positively correlated with aggressivity. This study demonstrates that variety differences in competitive ability of Argentinean wheat genotypes are significant at early crop stages up to 100 plants m−2 of L. multiflorum. Conversely there are no characters associated with wheat aggressivity at anthesis (for aboveground dry matter) and for grain yield.

Relative intra‐species competitive ability of nodding thistle biotypes with varying resistance to the herbicide 2,4‐D

Resistance of pasture weed species to phenoxy herbicides is emerging as a potential problem in New Zealand and is especially apparent in the case of nodding thistle (Carduus nutans). We selected eight biotypes of nodding thistle known to differ in resistance to the herbicide 2,4‐D and evaluated their relative competitive abilities during vegetative growth in terms of their effects on growth of the other biotypes, growth response to the other biotypes, and growth and seedling emergence responses to pre‐established swards of perennial ryegrass (Lolium perenne). Although biotypes differed in terms of competitive ability for each of these measures, there was no detectable agreement between any pair of measures across the eight biotypes. None of the measures of competitive ability was significantly correlated with herbicide resistance across the biotypes. Our results show no clear causative relationship between competitive ability as measured in this experiment and herbicide resistance. Since resistant populations are apparently of comparable competitive ability to susceptible ones, cessation of herbicide use is unlikely to be followed by a significant loss of resistant individuals from a population.

Effects of Elevated Temperatures on Threshold Food Concentrations and Possible Competitive Abilities of Differently Sized Cladoceran Species

We tested the hypothesis that at elevated temperatures in temperate lakes small cladoceran species are competitively superior to large species, which would cause a shift to smaller species in pelagic communities. As a measure of competitive ability, threshold food concentrations for growth (C 0 ) were determined in a flow-through system at 16, 20, 24 and 28°C. Four lake species were tested (in the order of decreasing size): Daphnia pulicaria, D. galeata, D. ambigua and Ceriodaphnia reticulata. C 0 increased at temperatures above 20°C, but the rank order of species was retained, i.e., the larger species had lower C 0 at all temperatures. The two smallest species showed a dramatic increase of C 0 at 28°C, hence their competitive ability was severely reduced. The hypothesis was rejected. In the temperature range tested, small cladoceran species were not competitively superior to large ones. The observed size shift from larger to smaller zooplankton in lakes with increasing temperature may be due to selective factors other than direct competition (e.g., predation, food quality).

How Species with Different Regeneration Niches Coexist in Patchy Habitats with Local Disturbances

We used a two-species simulation model to study mechanisms of coexistence of annual plants in patchy habitats with local disturbances. In habitats with nested scales of patchiness, short dispersal is advantageous because favorable habitat tends to be aggregated. The invasion of a resident population with short dispersal distance by a species with longer-range dispersal was simulated for combinations of habitat pattern, disturbance frequency and germination strategies. A germination strategy was defined by the type of response to disturbance (disturbance-broken when disturbances trigger germination, risk-spreading when germination is insensitive to disturbance) and the dormancy fraction at dispersal. Simulations estimated the long-term low-density growth rate of the invader, the mean local crowding (number of competing seeds per invader seed at each site) and the effective fecundity of each species (the mean number of seeds successfully dispersed per adult plant). Crowding increased with habitat suitability and decreased with increasing dormancy fractions for the resident. Effective fecundity in a landscape can be taken as a measure of competitive ability. The short-dispersing resident invariably had higher effective fecundity, but this difference decreased with increasing suitability, i.e. competitive differences decreased. Coexistence depended on both habitat suitability and disturbance frequency. Maximum coexistence was obtained for habitats of intermediate suitability with moderately frequent disturbances. General linear modelling of the long-term low-density growth rate showed that coexistence results from a reduction in local crowding. This growth rate also increased for increasing habitat suitability and connectivity, and for a higher dormancy fraction of the resident species. The effects of disturbance frequency and of invader's dormancy fraction depended on the type of dormancy of the resident species. The analysis showed that 2 different mechanisms are involved in the coexistence of species with different niches. Differences in regeneration niches permit coexistence through competitive equivalency with trade-offs between dispersal and germination traits, but for a limited range of habitat pattern and disturbance conditions. On the other hand, coexistence through density fluctuations of a disturbance-broken species and storage effects can be achieved for a broad range of environmental conditions and species germination strategies. Species coexistence thus results from the combination of two mechanisms. Evidence from natural communities is discussed. Our results also demonstrate the importance of detailed attention to spatial patterns and dispersal because of the complexity of spatial effects. Further, spatial pattern and disturbance frequencies need to be considered jointly to understand the dynamics of diversity.

Overgrowth competition in an assemblage of encrusting bryozoans settled on artificial substrata

Overgrowth competition in an assemblage of encrusting bryozoans settled on artificial substrata

Early Interference between Cotton (Gossypium hirsutum) and Four Weed Species

Early weed and crop interference was examined in a greenhouse pot experiment combining aspects of additive, substitutive, and diallel designs. Cotton (Gossypium hirsutumL. 'Stoneville 213′) and four weeds {purple nutsedge (Cyperus rotundusL.), large crabgrass [Digitaria sanguinalis(L.) Scop.], velvetleaf (Abutilon theophrastiMedic.) and prickly sida (Sida spinosaL.)} were grown singly and in all possible combinations of pairs including reciprocals for 5 weeks in 10-cm plastic pots. After 5 weeks large crabgrass was the tallest species in all combinations except when grown with purple nutsedge. Purple nutsedge, however, had more leaf area and greater dry weight than all other species. Height, leaf area, and dry weight were lowest in prickly sida. Mean relative yields in mixtures (a measure of competitive ability) were 1.90, 1.15, 1.13, 0.86, and 0.41 for purple nutsedge, large crabgrass, cotton, velvetleaf, and prickly sida, respectively, when grown with the other four species as associates. Conversely, purple nutsedge and cotton as associates depressed mean relative yields of competing species to 0.59 and 0.94. Large crabgrass, velvetleaf, and prickly sida as associates increased mean relative yields of competing species to 1.17, 1.18, and 1.55, respectively. Analysis of these data and replacement-series diagrams indicated that purple nutsedge was the most competitive, prickly sida the least, and the other species were intermediate and approximately equivalent in competitive ability.

Competition, Predation, and the Structure of the Ambystoma‐Rana Sylvatica Community

Populations of six species of amphibians were manipulated in field enclosures to study the biological tractability of current concepts of the organization of natural communities. Experimental communities with a known composition of mature eggs were introduced into screen enclosures in a pond to assay the importance of competition and predation to the ecology of amphibian larvae in temporary ponds. The competitive ability of each population was measured by its survivorship, mean length of its larval period, and mean weight at metamorphosis. Three simultaneous experiments (requiring 70 enclosures and 137 populations) were replicated in a randomized complete—block design for variance analysis. The assumptions of the classical Lotka—Volterra model of competition were tested by raising Ambystoma laterale, Ambystoma tremblayi, and Ambystoma maculatum in all combinations of three initial densities (0, 32, and 64). All three measures of competitive ability were affected by competition with other species. Higher—order interactions decreased the variance of the outcomes of the experiments as species were added to the communities. The statistical effects of these higher—order interactions between the densities of competing species often exceeded the simple effects of competition. The increase in community stability with the addition of species to the community is not predicted by the classical models of community ecology. The second experiment tested the effects of adjacent trophic levels on the structure of the three—species community. Eggs of Ambystoma tigrinum, a predator, and Rana sylvatica, an alternate prey of Ambystoma tigrinum, were added singly and together into systems with 16 eggs of species in the Maculatum species—group. Ambystoma tigrinum was a predator if it acquired an initial size advantage by preying on Rana sylvatica tadpoles; otherwise it was principally a competitor. Rana sylvatica adversely affected the Maculatum group by competing with invertebrate prey for periphyton and photoplankton. The three species in the Maculatum group had nearly the same response to the addition of both A. tigrinum and R. sylvatica. Ambystoma texanum, which occurs sporadically in southn Michigan at the northern limit of its range but not on the study area, was introduced as a test for community saturation. Ambystoma texanum was successfully raised alone. When mixed with the Maculatum group, Ambystoma texanum had a low survivorship, a small body size, and a long larval period. The native species were affected equally by the introduction of Ambystoma texanum, demonstrating the complexity of the food web and the ecological pliability of salamander larvae. The uncertainty of the temporary pond environment precludes extreme ecological specialization among these species of salamanders. Coexistence is a consequence of the relative advantages of the species in different years and the long adult life spans. The complexity of the food web and "predator switching" are probably important elements of the density dependent interactions that contribute to the stability of communities within seasons.