Differences In Intensity Of Competition Research Articles

Relationships between tree survival, stand structure and age in trembling aspen dominated stands

Canopy openings resulting from tree death provide opportunities for shade intolerant species such as aspen to establish and grow in the understory leading to development of aspen stands with multiple canopy layers and multiple cohorts. Stand structure is likely to influence survival probability since it will lead to differences in competition intensity and in resource utilization. We examined relationships among stand age (years after stand establishment), age structure (age diversity), and tree size diversity (i.e., diversity of diameter classes), using cores collected from 531 aspen trees in 18 stands distributed across the province of Alberta, Canada. Results show positive relationships between stand age, age diversity, and tree size diversity. Logistic mixed effects survival probability models were developed using data from 199 remeasured permanent sample plots. Results show an inverted U-shaped relationship between survival probability and tree diameter. Survival probability trends varied between young (≤60), mature (>60 and ≤ 100), and old (>100) age groups in relation to differences in competition intensity and stand structure. Intraspecific competition (deciduous basal area larger) was negatively related to tree survival and the magnitude of the negative impact was stronger in the young age group. Negative effects of structural diversity (tree size diversity) on survival probability were strongest in the mature age group. We suggest that effects of tree size diversity relate to environmental stresses and competition at the stand level while the influence of deciduous basal area larger serves as a measure of competition experienced by individual trees. The significance of mean annual temperature in our models indicates that, under warmer conditions, competition variables (e.g., deciduous basal area larger and tree size diversity) had stronger effects on tree mortality with the magnitude of effects varying between age groups.

Does experience with competition matter? Effects of source competitive environment on mean and plastic trait expression in Erodium cicutarium

Abstract Interspecific competition is likely to act as an agent for selection on local scales, although evidence in plants is sparse so far. We hypothesize that in annual shade-avoiding grassland species, heterogeneity in the intensity of aboveground competition for light may shape patterns of genetic variation and induce phenotypic plasticity in traits affecting competitive ability. We collected maternal seed families of Erodium cicutarium from replicated high and low competition environments and exposed them to different levels of aboveground competition in a glasshouse. We examined effects of seed source and competition treatment on expression of plant traits related to competitive ability and fitness. Source environments with high levels of competition were significantly more heterogeneous in competition intensity at both intermediate (approx. 10 m) and small (approx. 0.1 m) spatial scales. Seed source and competition treatment both had highly significant effects on trait expression. Greater intensity of competition experienced by maternal plants was coupled with lower vegetative biomass production and slower growth rates, and at the same time lower abortion rates in the offspring. We interpret these findings as an indication of greater reproductive efficiency in the next generation, in response to competition experienced by parents. There was higher total phenotypic variability in the plants from high competition source sites, but equivalent levels of phenotypic plasticity across source-site competition levels; no costs of phenotypic plasticity were detected. We concluded that differences in competition intensity can lead to trait differentiation in the next generation. For E. cicutarium , experience with competition matters: it leads to substantial phenotypic differences and more total variability in the offspring generation.

Dichromatic and Trichromatic Callithrix geoffroyi Differ in Relative Foraging Ability for Red-Green Color-Camouflaged and Non-camouflaged food

An advantage for trichromatic color vision in primates is shown by its presence in many lineages, but little attention has been paid to the potential disadvantages of trichromacy. Most New World monkey species are polymorphic for color vision, with both dichromats and trichromats present within a single population. We tested the foraging ability of trichromatic and dichromatic Geoffroy's marmosets (Callithrix geoffroyi) for colored cereal balls (Kix®) under conditions of red-green color camouflage (orange/green Kix® against an orange/green background) or lack of camouflage (Kix® same color as background) in a naturalized captive setting. In separate experiments designed to test foraging ability at long distances (<6 m) and short distances (<0.5 m), trichromats found significantly fewer Kix® under the camouflage condition than in the non-camouflage condition. In contrast, there is no difference in the ability of dichromats to detect color-camouflaged versus non-camouflaged Kix®. There is no significant difference between dichromats and trichromats for either camouflaged or non-camouflaged Kix®, though the power in the tests is low because of high individual variation. The results have clear implications for the foraging strategies of trichromatic marmosets. Differences in intensity of competition between trichromats and dichromats for items of food of different colors in relation to background may also have consequences for the foraging behavior of dichromats.

Food Prices and Market Structure in Sweden

This paper examines retail grocery price levels across a large panel of stores in Sweden. We explain price variation across stores by market structure variables to capture differences in competition intensity and a number of store– and region–specific factors. Most of the explained variation in prices can be attributed to store–specific factors such as size and chain affiliation. Overall, the relation between market structure variables and food prices is weak, and effects are small in percentage terms. Nevertheless, higher local concentration of stores, higher regional wholesaler concentration and a lower market share of large stores are all correlated with higher prices. JEL classification: D43; L13; L81

Effects of herbivory on competition intensity in two arctic‐alpine tundra communities with different productivity

The effects of long‐term (11 yr) exclusion of vertebrate herbivores on competition intensity and plant community structure were studied using manipulative field experiments in two arctic‐alpine plant communities with contrasting productivity: an unproductive snowbed and a considerably more productive tall herb meadow. In the snowbed, the exclusion of herbivorous mammals resulted in a significant increase in the biomasses of vascular plants and cryptogams, whereas no corresponding response was observed on the tall herb meadow. The intensity of competition, measured with a neighbour removal experiment, did not differ significantly between three of the four habitat×treatment combinations – snowbed exclosures, meadow exclosures and open meadow plots – but was significantly lower on open snowbed plots. Our results thus suggest that the low competition intensity in the unproductive snowbed is caused by herbivorous mammals, which tend to depress plant biomass in relatively unproductive habitats. When herbivorous mammals have been excluded for a sufficiently long time to allow the build‐up of plant biomass even in unproductive habitats, between‐habitat differences in competition intensity disappear.

COMPETITIVE RESPONSE HIERARCHIES FOR GERMINATION, GROWTH, AND SURVIVAL AND THEIR INFLUENCE ON ABUNDANCE

Among plants, differences in competition intensity and in the ranking of competitive ability for traits such as germination, growth, and survival may suggest that the importance of competition for population success varies with the trait measured. If a species is a good competitor when measured by one component of fitness (e.g., seedling growth), but a poor competitor when measured by a different component of fitness (e.g., seedling survival), understanding the relative importance of each component of fitness for persistence in the community is critical to understanding how communities are structured. Using a field and a garden experiment in southeastern Michigan, we generated competitive response hierarchies among 4–8 old-field perennials for five components of fitness (germination, seedling growth, seedling survival, adult growth, and adult survival). We examined how the overall response to neighbors changes among components of fitness, the concordance of hierarchies within and among components of fitness, and the correlations between competitive hierarchies and natural abundance. We found little to no overall effect of neighbors on germination and seedling survival, indicating a high tolerance (= strong response competitive ability) of species for neighbors as measured by these components of fitness. We found a strong effect of neighbors on seedling growth, and on adult survival and growth, indicating poorer response competitive ability overall for these components. Although rankings of competitive ability were concordant among species across all components of fitness, certain demographic parameters were more consistent in their rankings than others. Specifically, competitive response hierarchies based on size were strongly concordant, while those based on survival were not. This suggests that future studies may effectively estimate response competitive ability for growth with a single estimate, but that estimates of response competitive ability for survival at both the seedling and adult stages may be required. Finally, competitive response rankings based on germination and seedling growth were most strongly correlated with abundance, suggesting that these components of fitness more strongly influence success in the community than do seedling survival and adult growth and survival.

Competitive Response Hierarchies for Germination, Growth, and Survival and Their Influence on Abundance

Among plants, differences in competition intensity and in the ranking of competitive ability for traits such as germination, growth, and survival may suggest that the importance of competition for population success varies with the trait measured. If a species is a good competitor when measured by one component of fitness (e.g., seedling growth), but a poor competitor when measured by a different component of fitness (e.g., seedling survival), understanding the relative importance of each component of fitness for persistence in the community is critical to understanding how communities are structured. Using a field and a garden experiment in southeastern Michigan, we generated competitive response hierarchies among 4–8 old-field perennials for five components of fitness (germination, seedling growth, seedling survival, adult growth, and adult survival). We examined how the overall response to neighbors changes among components of fitness, the concordance of hierarchies within and among components of fitness, and the correlations between competitive hierarchies and natural abundance. We found little to no overall effect of neighbors on germination and seedling survival, indicating a high tolerance (= strong response competitive ability) of species for neighbors as measured by these components of fitness. We found a strong effect of neighbors on seedling growth, and on adult survival and growth, indicating poorer response competitive ability overall for these components. Although rankings of competitive ability were concordant among species across all components of fitness, certain demographic parameters were more consistent in their rankings than others. Specifically, competitive response hierarchies based on size were strongly concordant, while those based on survival were not. This suggests that future studies may effectively estimate response competitive ability for growth with a single estimate, but that estimates of response competitive ability for survival at both the seedling and adult stages may be required. Finally, competitive response rankings based on germination and seedling growth were most strongly correlated with abundance, suggesting that these components of fitness more strongly influence success in the community than do seedling survival and adult growth and survival.

Clonal Growth in Grassland Perennials: I. Density and Pattern-Dependent Competition Between Plants with Different Growth Forms

(1) Bellis perennis and Prunella vulgaris are grassland perennials with contrasting growth forms. Clones of Bellis perennis have tightly aggregated rosettes (phalanx type) and vary little in size. Rosette populations grow logistically, probably regulated by change in birth rate. Branches of Prunella vulgaris are only loosely aggregated within clones (guerrilla type). The frequency distribution of clone sizes in P. vulgaris is hierarchical. Branch populations grow exponentially and exceed the equilibrium density before regulation, through increased death rate, occurs. (2) This pair of species was used to investigate competition between and within clones and between species as functions of growth form, planting pattern, and planting density. (3) No differences in intensity of competition within and between clones were found in either species. (4) Plants of P. vulgaris showed a more fugitive behaviour in mixed than in pure populations. (5) Competition was density-dependent; in mixtures of Bellis perennis and Prunella vulgaris the phalanx species was aggressive at high but suppressed at low densities and the guerrilla species was aggressive at low but suppressed at high densities. This outcome is caused by the differences in growth form of the two species. (6) Different planting patterns had little influence on the competitive relationship of the two species.