Variation In Intensity Of Competition Research Articles

New procedure for the simulation of belowground competition can improve the performance of forest simulation models

The major part of existing models of belowground competition in mixed forest stands is limited in explaining the spatial distribution of roots as a response to competitive pressure from neighbours and heterogeneity of soil properties. We are presenting a new spatially explicit and multi-layered discrete model of belowground competition, RootInt (ROOTs INTake). It describes spatial distribution of belowground biomass and allows simulation of competition between trees for soil nutrients. The tree-specific area of root zone is calculated on the basis of stem diameter, with site-specific modifiers to account for the effect of soil fertility and moisture. The shape of root zone is dependent on the amount of available nitrogen in the current cell, distance between this cell and the stem base, and the mass of roots of other plants. RootInt was incorporated into ecosystem model EFIMOD to refine the existing description of belowground competition in forest stands with multiple cohorts and tree species. The results of simulation showed that bringing more complexity into structure of stand (including initial spatial locations of trees, species composition and age structure, vertical structure of canopy) resulted in higher spatial variation in competition intensity, as well as in higher rates of resource uptake. This indicates that stands with complex canopy structure had high plasticity in their root systems and were adapted to intensive competition for soil resources.

COMPETITION, RESOURCES, AND VEGETATION DURING 10 YEARS IN NATIVE GRASSLAND

A 10-year experiment tested for variation in competition intensity over time in a natural grassland at the northern edge of the Great Plains. Growing-season precipitation varied fivefold during the study. All ecosystem-level variables varied significantly among years, and most covaried in expected ways. The covers of all common grasses possessing the C3 photosynthetic pathway varied significantly among years; in contrast, all common species with traits associated with drought tolerance (a C4 grass, a lichen, a spikemoss, and a subshrub) did not vary. Annual transplant experiments measured the competitive effects of neighbors on the growth of individuals of the native grass Bouteloua gracilis. A significant interaction between year and competition showed that competition intensity varied among years. The size of this effect, however, was small (eta2 = 0.074) relative to the size of the direct effect of competition (eta2 = 0.20) or the year in which the experiment was conducted (eta2 = 0.51). Further, competition intensity was not significantly related to any variable describing standing crop or resources, or species richness. Species richness was highest in years with high precipitation, standing crop, and individual growth, due to the recruitment of rare species that were absent from dry years. In summary, variation in competition intensity was statistically significant but had small effects relative to the direct effects of climate.

Gradient analysis of dry grassland vegetation in Denmark

Abstract. We present a gradient analysis of 620 vegetation samples covering most of the floristic and environmental variation in semi‐natural grassland vegetation on well‐drained soils in Denmark. Vegetation was sampled using frequency in subplots. Explanatory variables were surface inclination, aspect, pH, geographical co‐ordinates together with indications of soil type. Detrended Correspondence Analysis revealed four floristic gradients that could be interpreted in ecological terms by measured variables supplemented with site calibrations based on weighted averaging of Ellenberg's indicator values. All four axes were interpreted using rank correlation statistics, and linear and non‐linear multiple regression of sample scores on explanatory variables. The first gradient was from dry calcareous to humid acidic grasslands; the second reflected an underlying gradient in fertility; the third reflected regional differentiation and the fourth was associated with variation in intensity of competition as indicated by association with calibrated Grime‐CSR values for the plots. We applied subset ordination to the data as a supplement to traditional permutation and correlation statistics to assess the consistency of ordination results. DCA axes 1 and 2 were consistent in space and time.This gradient analysis is discussed in a context of plant strategy theory and species diversity models. Ecocline patterns lend support to the view that grazing not only favours the ruderal strategy but also the stress‐tolerant strategy. The low rank of competition as an explanatory variable for the floristical gradients supports the notion that competitive effects play a subordinate role for species composition compared to microclimate and soil conditions in infertile semi‐natural grasslands.

Density Dependent Growth and Size Specific Competitive Interactions in Young Fish

Body size is a major determinant affecting an individual's performance. In this study, four factors affecting an organism's competitive ability as related to body size: the attack rate, the maximum growth capacity, the metabolic demands and the size spectrum of available resources, were investigated for small stages of Eurasian perch (Percu fluviatilis) and roach (Rutilus rutilus). The size dependence of the attack rate on a 0.5-mm cladoceran zooplankton of larval and juvenile perch was estimated and compared with the results from similar experiments for roach. At equal body sizes, roach always had a higher attack rate than perch. In contrast the reverse was the case for maximum growth capacity. The maintenance requirements at the same size were higher for perch than for roach. Based on the above data we were able to gain a mechanistic understanding of the outcome of two field enclosure competition experiments, one between perch larvae and I-yr-old roach and one between perch larvae and roach larvae. In the first experiment, we found strong intraspecific density dependent growth in perch larvae, while the presence of I-yr-old roach had no effect on perch larvae growth. High perch densities had strong negative effects on the zooplankton resource. Due to the larger size of I-yr-old roach and consequently higher metabolic demands, I-yr-old roach showed a stronger negative response in growth to high perch larvae densities than perch despite roach's higher attack rate on zooplankton. In the second experiment, perch larvae were negatively affected by high densities of roach larvae and had a reduced growth at high intraspecific densities. In contrast, the growth of roach larvae was not affected by perch larvae or high intraspecific densities. This difference between species could be related to the simultaneous lower attack rate and higher growth capacity of perch, leading to a higher sensitivity in growth of perch than of roach to decreasing resource levels. Temporal variation in competition intensity was present in both experiments. This variation could be related to the foraging efficiencies and different growth capacities of the competitors and the species composition and size structure of the zooplankton resource. Our study points to the potential for both intra- and interspecific competitive interactions in fish larvae in freshwater systems. Our results also suggest that species specific differences in how foraging, growth and food processing capacities relate to body size are of vital importance for interactions in ecological communities.

COMPETING PREDATORS AND PREY: JUVENILE BOTTLENECKS IN WHOLE-LAKE EXPERIMENTS

In size-structured populations, prey have the potential to restrict the recruit- ment of their predator by decreasing the growth rate of young predators through interspecific competition, a situation referred to as a ''competitive juvenile bottleneck.'' Two mechanisms have been advanced by which a decrease in growth may increase the mortality of young predators: (1) increased time during which juvenile predators are susceptible to gape-limited predation and (2) increased susceptibility to starvation. To study the effects of competition from a prey fish (roach, Rutilus rutilus) on the recruitment of a piscivorous fish (perch, Perca fluviatilis), roach were introduced in two of four small adjacent unproductive lakes inhabited by natural populations of perch. We thereafter studied the diet, growth, and survival of the new-born cohorts of perch during a 13-mo period. Growth and survival of larval perch were not affected by roach whereas during the latter part of the growing season, growth and condition of young-of-the-year (YOY) perch were negatively affected by roach. The growth retardation of YOY perch in roach-treatment lakes coincided with a shift in diet from free-swimming zooplankton to benthic cladocerans and chironomids, which in turn was related to a stronger decline in pelagic zooplankton resources in these lakes. The small size and poor condition of YOY perch in the roach- treatment lakes resulted in almost all these perch dying from starvation during the winter and subsequent spring. There was a tendency for there to be higher densities of YOY perch in the roach-treatment lakes, possibly due to a strong die-off of adult perch in these lakes that in turn may have reduced cannibalism in these lakes. The results from an enclosure experiment on intra-cohort competition among YOY perch, however, suggested that the discrepancy in growth of YOY perch between the lakes was mainly due to direct competition from roach for the zooplankton resource. Temporal variation in competition intensity and in duration of periods for growth and energy gain (e.g., summer) vs. periods of only energy loss (e.g., winter) are suggested to have major effects on juvenile predator growth and subsequent recruitment to the adult stage through competitive juvenile bottlenecks, and thus to affect overall community dy- namics. The relative importance of size-dependent winter starvation mortality compared to mortality caused by gape-limited predators as a mortality agent on juveniles is hypoth- esized to increase with latitude. Whole-lake experiments lend themselves both to testing the importance in natural systems of mechanisms previously identified at small spatial and short temporal scales, and also to identifying important mechanisms that cannot be studied at smaller scales.

Competition between tree seedlings and herbaceous vegetation: support for a theory of resource supply and demand

1 We measured competition intensity (CI) between herbaceous vegetation and tree seedlings (Quercus macrocarpa and Q. ellipsoidalis) along an experimental moisture–light gradient. Contrasting theories were tested by comparing variation in competition intensity to changes in neighbour biomass and resource supply and demand. 2 CI based on survival was inversely correlated with net soil water supply (gross supply minus demand by herbaceous vegetation). CI was not positively correlated with either gross resource supply or neighbour biomass, contrary to predictions of Grime's triangular model for plant strategies. 3 Many of the inconsistencies and conflicting results that have characterized the recent literature on plant competition could be eliminated if changes in competition intensity along a resource gradient are compared with changes in net resource supply rather than changes in productivity or neighbour biomass. 4 Tree seedling success in savannas and grasslands may be strongly influenced by the intensity of competition from herbaceous vegetation. Factors that reduce soil water content are likely to increase competition intensity (and reduce seedling success) in these environments, while factors that increase soil water content will favour seedling success through decreased competition for water with herbaceous vegetation.

Variation in competition in eucalypt forests: the importance of standardization in pattern analysis

Abstract. Contrasting views have been put forward that competition intensity should either increase with community standing crop or remain constant. Testing these hypotheses with long‐lived plants in natural communities is possible using pattern analysis, involving statistical summaries of the relationships between the total size and distance of nearest neighbors. This approach, however, may produce a spurious negative correlation between competition intensity and standing crop in cases where individual size increases with standing crop. A method for overcoming this problem using data standardization is illustrated using data from 15 stands in a variety of forest types (subalpine woodland, dry forest, warm wet forest, cool wet forest, and near‐rainforest) in southeastern Australia. Standing crop, estimated from stand volume, and water availability, estimated from evapotranspiration, varied significantly among forest types. Total size and nearest neighbor distance was measured for 50 pairs of trees in three stands of each forest type. The sum of nearest neighbor sizes increased significantly with interneighbor distance in all forest types, indicating that competition contributed to pattern at all levels of standing crop. The slopes of the regression equations describing these relationships varied significantly among forest types and with evapotranspiration, suggesting that competition intensity varied. This result, however, was shown to be the product of a significant positive correlation between individual size and standing crop. Once this correlation was removed by using standardized measures of tree size, there was no evidence for variation in competition intensity among forest types or with evapotranspiration.

Competition, Fire, and Nutients in a Mixed‐Grass Prairie

Variation in the intensity of competition has been proposed as a mechanism that may control the species diversity and composition of many kinds of communities, including North American prairie. Competition intensity is predicted to increase with soil fertility and decrease with disturbance, causing variation in the abundances of competitively subordinate species. Variation in competition intensity was examined in an experiment in which fire and fertilizer were applied to native mixed—grass prairie. Treatments consisted of annual burns, a single burn, and untreated control, and fertilization (N ≥ 15 replicates) applied to 7 x 5 m plots for two growing seasons. The experiment was repeated at a second site 15 km away. Fire reduced standing crop and litter, and increased the frequency of bare ground. Nutrient addition increased standing crop and decreased bare ground. Species composition and diversity were largely unaffected by fire. Two common species, Bouteloua gracilis and Carex obusata, were significantly more abundant in fertilized vegetation than in controls at both sites; species diversity decreased in fertilized plots. Vegetation responses to the treatments were similar at the two sites. To tests for variation in competition intensity with fire and nutrient availability, removal experiments were performed in each treatment at one site. Two tussocks each of Festuca ovina and Stipa spartea were chosen in each plot. One plant of each pair had all neighbors within 30 cm clipped and removed during two growing seasons. Each species grew larger and faster in the absence of neighbors, but the magnitude of this effect did not vary with fire, fertilizer, or standing crop. Fire and nutrient addition produced significant changes in the community structure of mixed—grass prairie but did not alter competition intensity in the manner predicted.

Patterns of Coexistence in Synaptomys Cooperi and Microtus Pennsylvanicus

The microtine rodents Synaptomys cooperi and Microtus pennsylvanicus are sympatric, have a similar life styles, and occur in the same general habitats. The object of this study was to determine the importance of competition in a Microtus—Synaptomys system in southwestern Virginia as a test of the general hypothesis that herbivorous rodents compete directly for space and indirectly for food. The following patterns were observed: (1) Undisturbed populations of Microtus and Synaptomys exhibited microhabitat partitioning when coexisting in heterogeneous habitats that were marginal for Microtus. (2) In the presence of Microtus, Synaptomys microhabitats were characterized by significantly higher densities of deciduous trees and shrubs. (3) Disappearance of Microtus from some habitats during a population decline was accompanied by a shift of Synaptomys into areas formerly occupied by Microtus. Recolonization by Microtus resulted in a return to the previous pattern of microhabitat segregation. (4) Removal of Microtus from an experimental grid was followed by immediate colonization by Synaptomys. (5) One year following removal of Synaptomys from an experimental grid, most of the former Synaptomys habitat remained empty even though Microtus numbers were increasing. (6) Fecal analysis indicated that Synaptomys diet consisted largely of vegetation that is low digestible nutrients (Andropogon in summer, moss in winter). Although Microtus ate Andropogon, this species included more dicotyledons and other monocotyledons in its diet. (7) When Synaptomys was living on a grid from which Microtus had been removed, similarity to food habits of Microtus previously living on the removal grid was much greater than on a grid where the two species coexisted in separate microhabitats (72 vs. 37%). These results suggest that in the southern Appalachians, Synaptomys is excluded from preferred habitats by Microtus and, as a result, lives where cover is sparse and food is low in nutritional value. Although these species do compete, this competition is relaxed when Microtus populations decline. When given access to preferred habitats the food habits of Synaptomys change and resemble those of Microtus, suggesting that the ultimate limiting resource is food. The combination of wider habitat tolerances by Synaptomys and temporal variation in intensity of competition allows coexistence of these species on a regional basis.