Differences In Competitive Ability Research Articles

Size-specific effects of increased sediment loads on gastropod communities in Lake Tanganyika, Africa

The remarkable biodiversity of the littoral zone of Lake Tanganyika appears to be at risk through increasing sediment input caused by anthropogenic pressures. An in-situ field experiment was done to investigate the effects of increased sediment loads on the size-structure of gastropod communities on a rocky shore site in the lake. Gastropods were removed prior to the addition of sediment, and subsequent further removals were done after seven days and six months. Relative to controls, mean size of individuals of both Lavigeria grandis (Smith) and Lavigeria sp. Q decreased following the addition of sediment, while that of Lavigeria sp. P increased. A subsequent laboratory experiment found that survivorship of larger individuals of Lavigeria grandis was less than smaller individuals in the sediment-impacted treatment. It is hypothesised that decreased algal availability on sediment-impacted rock surface increased starvation of larger snails. Alterations to the size-structure of Lavigeria sp. P populations appear related to intraspecific differences in competitive ability. These results highlight the complex and unpredictable effects of increases in sediment loads on aquatic ecosystems and have implications for the conservation of littoral communities in sediment-impacted areas.

CLUTCH SIZE REDUCTION AS A RESPONSE TO INCREASED NEST PREDATION RATE IN THE COLLARED FLYCATCHER

Nest predation has been hypothesized as a major factor influencing clutch size in birds. Indirect and correlative evidence for clutch size reduction under high nest predation risk is accumulating, but such a reduction has never been shown experimentally. We experimentally tested this “nest predation hypothesis” by manipulating nest predation rate in the Collared Flycatcher, Ficedula albicollis. As predicted, clutch size decreased in the year following the experiment in “depredated” areas (in which 20–35% of the nests were totally depredated) compared to control ones (no predation). This could result from (1) phenotypic plasticity in clutch size, or (2) differential breeding habitat selection, with dominant individuals or individuals laying large clutches preferentially leaving depredated areas and settling in control ones. There was no evidence for differential dispersal from areas of the two treatments, nor differential competitive abilities of breeders in areas of the two treatments. Conversely, all ind...

Interactions between hatchery and wild salmonids in streams: differences in biology and evidence for competition

Competition between hatchery-reared and wild salmonids in streams has frequently been described as an important negative ecological interaction, but differences in behavior, physiology, and morphology that potentially affect competitive ability have been studied more than direct tests of competition. We review the differences reported, designs appropriate for testing different hypotheses about competition, and tests of competition reported in the literature. Many studies have provided circumstantial evidence for competition, but the effects of competition were confounded with other variables. Most direct experiments of competition used additive designs that compared treatments in which hatchery fish were introduced into habitats containing wild fish with controls without hatchery fish. These studies are appropriate for quantifying the effects of hatchery fish at specific combinations of fish densities and stream carrying capacity. However, they do not measure the relative competitive ability of hatchery versus wild fish because the competitive ability of hatchery fish is confounded with the increased density that they cause. We are aware of only two published studies that used substitutive experimental designs in which density was held equal among treatments, thereby testing for differences in competitive ability. Additional substitutive experiments will help managers to better understand the ecological risk of stocking hatchery fish.

Patterns of coalition formation and spatial association in a social carnivore, the African wild dog (Lycaon pictus)

In many social species, relationships within groups seem to be non‐random but related to variables such as rank, kinship or sexual attractiveness. The endangered African wild dog Lycaon pictus is a social carnivore that lives in large, stable packs, and intra‐pack associations might be expected to display similar patterns. We investigated patterns of coalition formation (support during dominance interactions, and partnership interactions) and resting associations between members of a captive pack of 19 wild dogs. The social organization of the captive pack was similar to that of free‐ranging packs in many respects. Polyadic (group) incidents of coalition support were also observed in a free‐ranging pack. Patterns of coalition formation in the captive pack were related to rank. Most aggressive interactions involved high‐ranking individuals (particularly the alpha, beta and third‐ranking males) and coalitionary support tended to reinforce the existing hierarchy. However, there was at least one example of support influencing a successful rank challenge. Support was affected by potential risks and benefits, the latter including dominance through association and revolutionary alliances. An even stronger pattern overlaid associations between pack members: coalitions and resting associations were strongest between members of the same age–sex cohort, and may have enabled the eventual dominance of younger pack members over adults. Among adults, coalitionary associations were sometimes overridden by intersexual relationships. The results from this captive pack suggest that wild dogs are sensitive to differences in competitive ability. This information, in conjunction with strong affiliative bonds between littermates, is used to manoeuvre for position in the social hierarchy. It may also be important during dispersal, in encounters with other dispersing groups of the same sex. Although most features of the social structure of the captive pack were comparable to those of free‐ranging packs, aspects such as the influence of relatedness on coalition formation still need to be explored.

Reproduction and survival in Melanoplus sanguinipes (Orthoptera: Acrididae) in response to resource availability and population density: the role of exploitative competition

AbstractThe relative importance of exploitative competition for resources on grasshopper reproductive allocation has not been fully examined. Given the large fluctuations in grasshopper densities that periodically occur in western North America, an increased understanding of how grasshopper survival and reproduction vary in response to intraspecific densities and per capita resource availability is important. I examined if exploitative resource competition could explain variation in reproductive allocation in Melanoplus sanguinipes (Fabricius) in response to resource availability and grasshopper population density. I also examined whether individual differences in competitive ability resulted in increased variance in egg production with low per capita resource availability. As expected with exploitative resource competition, per capita resource availability explained a significant amount of the variation in all reproductive characteristics examined. There was no effect of per capita resource availability on survival. Residuals of the regressions of egg production and vitellogenesis versus per capita resource availability did not differ for resource or density treatments, indicating that exploitative competition for resources played a more important role than interference competition in determining reproductive allocation in M. sanguinipes. Individual differences were evident, as variation around the mean of egg production increased with resource limitation. Exploitative competition for resources was important in determining both individual and population-level reproductive responses of grasshoppers to resource availability.

Contrasting Responses of Intermountain West Grasses to Soil Nitrogen

The mechanisms responsible for soil-N-mediated species replacement of native perennial grasses by the invasive annual grasses cheatgrass (Bromus tectorum L.) and medusahead (Taeniatherum caput-medusae [L.] Nevski) on rangelands are not completely understood. In addition, the contributions of distinct forms of inorganic N (i.e., NH 4 + and NO 3 -) to these shifts in species composition are currently unclear. Consequently, we conducted a greenhouse experiment to test 2 hypotheses: 1) that low N availability reduces growth (root and shoot) and N allocation of invasive annual seedlings more than native perennial species, and 2) that seedling growth and N allocation of invasive annual grasses is more responsive than native perennial grasses when supplied with NO 3 - relative to NH 4 +. We grew seedlings of 2 annual grasses and the native perennial grasses bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] A. Love), and 4 populations of squirreltail (Elymus elymoides [Raf.] Swezey; E. multisetus [J.G. Smith] M.E. Jones) in separate pots and exposed them to treatments differing in N form and availability for 17 weeks. Unexpectedly, root and shoot growth of annual grasses were equal or greater than native perennial grasses under low N availability. Annual grasses took up more NO 3 - and allocated more growth and N to shoots than the perennial grasses (P < 0.05). Perennial grasses had significantly greater root:shoot dry mass ratios than the invasive annual grasses across treatments (P < 0.05). Invasive annual and native perennial grasses both had greater (P < 0.05) shoot and root mass and allocated more N to these structures when supplied with NO 3 - relative to NH 4 +. The ecological implications of these growth and N allocation patterns in response to N availability and form provide important clues regarding the specific traits responsible for differences in competitive ability between invasive annual and native perennial grasses on semiarid rangelands.

Contrasting responses of Intermountain West grasses to soil nitrogen

The mechanisms responsible for soil-N-mediated species replacement of native perennial grasses by the invasive annual grasses cheatgrass (Bromus tectorum L.) and medusahead (Taeniatherum caput-medusae [L.] Nevski) on rangelands are not completely understood. In addition, the contributions of distinct forms of inorganic N (i.e., NH4+ and NO3-) to these shifts in species composition are currently unclear. Consequently, we conducted a greenhouse experiment to test 2 hypotheses: 1) that low N availability reduces growth (root and shoot) and N allocation of invasive annual seedlings more than native perennial species, and 2) that seedling growth and N allocation of invasive annual grasses is more responsive than native perennial grasses when supplied with NO3- relative to NH4+. We grew seedlings of 2 annual grasses and the native perennial grasses bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] A. Love), and 4 populations of squirreltail (Elymus elymoides [Raf.] Swezey; E. multisetus [J.G. Smith] M.E. Jones) in separate pots and exposed them to treatments differing in N form and availability for 17 weeks. Unexpectedly, root and shoot growth of annual grasses were equal or greater than native perennial grasses under low N availability. Annual grasses took up more NO3- and allocated more growth and N to shoots than the perennial grasses (P 0.05). Perennial grasses had significantly greater root:shoot dry mass ratios than the invasive annual grasses across treatments (P 0.05). Invasive annual and native perennial grasses both had greater (P 0.05) shoot and root mass and allocated more N to these structures when supplied with NO3- relative to NH4+. The ecological implications of these growth and N allocation patterns in response to N availability and form provide important clues regarding the specific traits responsible for differences in competitive ability between invasive annual and native perennial grasses on semiarid rangelands.

Effects of winter wheat cultivars and seed rate on the biological characteristics of naturally occurring weed flora

Summary Differential competitive ability of six winter wheat cultivars and traits that confer such attributes were investigated for a range of seed rates in the presence or absence of weeds for a naturally occurring weed flora in two successive years in split‐plot field experiments. Crop height and tillering capacity were considered suitable attributes for weed suppression, although competitiveness is a relative rather than an absolute characteristic. Maris Huntsman and Maris Widgeon were the most competitive cultivars whereas Fresco was the least competitive. Manipulation of seed rate was a more reliable factor than cultivar selection for enhancement of weed suppression, although competitiveness of cultivars Buster, Riband and Maris Widgeon was not enhanced by increased seed rate. Crop densities ranging between 125 and 270 plants m−2 were found to offer adequate weed suppression. Linear relationships were observed between individual and total weed species dry weight and reproductive structures per unit area.

Competitive Abilities of Rare and Common Plants: Comparisons UsingAcaena(Rosaceae) andChionochloa(Poaceae) from New Zealand

Abstract:It is commonly suggested that rare plant species are poor competitors. However, the study of rarity and competitive ability has, like much of the research on rare species, been hindered by insufficient sample size, scarce use of experimental methods, and lack of comparison with common species. We used a within‐taxon comparative approach to test for differences in competitive ability between rare and common species in two genera,Acaena( Rosaceae: six rare and four common species) andChionochloa( Poaceae: four rare and five common species). We grew plants in replicated experiments at two levels of soil fertility and with two levels of competition, in monoculture or mixed with the introduced grassAgrostis capillaris. Competitive response was measured as relative yield, relative tiller increase, and change in live‐leaf fraction. Rarity was measured by geographic range size, defined as the number of 10‐km grid squares occupied by each species in New Zealand. The relationship between competitive ability and geographic range size was tested with correlations. WithinAcaena, four of the rare species had high competitive ability, but there was no general relationship between competitive ability and rarity at either high or low levels of fertility. Differences in competitive ability appeared related to differences in ecological distribution rather than to rarity or commonness in a geographic sense. WithinChionochloa, common species tended to be stronger competitors than rare species, with the relationship being significant at the low level of fertility. Several of the commonChionochloaspecies have expanded their geographic ranges relatively recently, consistent with their greater competitive ability. Conversely, rareChionochloaspecies occupy specialized habitats that may be interpreted as refuges from competition. Disparate results fromAcaenaandChionochloado not support the notion that rare species are generally poor competitors. Although rare species may have low competitive ability in some cases, it should not be assumed to be a cause of rarity for all rare species.

Three‐Player Social Parasitism Games: Implications for Resource Defense and Group Formation

Abstract: Individuals that produce resources are often exploited by several individuals; such exploitation may or may not be tolerated. We modeled the decision of a resource owner to accept one scrounger (the "satellite") and of both of these to accept being joined by another (the "floater"). In general, satellites tolerated floaters when competition between them was low, while owners tolerated satellites when facilitation between satellite and floater was high. When floaters were likely to find resources without joining, owners were more likely to resist satellites. In some cases, Nash equilibria were also mutually beneficial for two of the three individuals. Our model makes the counterintuitive prediction that mutually beneficial coalitions between satellites and floaters can only arise when the net benefits arising from the other's presence are low. When facilitation between satellites and floaters is high, satellites and owners may form mutually beneficial groups and groups with division of labor, or alternatively, owners may benefit from encouraging floaters to join. Finally, our model suggests there must be differences in competitive ability or some benefit of familiarity for owners to tolerate satellites but not floaters. We discuss empirical evidence for these and other predictions of the model.

Variation Between Barley Cultivars in Early Response to Weed Competition

An experiment was conducted with three spring barley (Hordeum vulgare ssp. vulgare L.) cultivars of contrasting competitiveness to identify variations in their early competitive response to weed competition and to study canopy architecture and growth development at the early stages of cultivar growth. The aim was also to investigate whether differences in shoot morphology, growth development and competitive response could explain the differences in competitive ability against weeds. The barley cultivars were grown outdoors in boxes either in monoculture or in a mixture with white mustard (Sinapis alba L.) as a model weed. The experiment was run until the barley cultivars were in the stem elongation stage. The varieties with strong to medium competitive ability against weeds shortened the time of emergence in the presence of S. alba in contrast to the least competitive cultivar. The results also indicated that spring barley cultivars with strong competitive ability against weeds have an early stem extension as a response to weed competition and a low competitive response. Morphological traits, namely large length of the two first internodes, long main shoot in the tillering stage and a small leaf angle, may be important traits in competition for light.

Conundrums of competitive ability in plants: what to measure?

A survey of recent literature indicates that competitive ability in plants has been measured, in most studies, only in terms of the relative intensity of size suppression experienced by competitors within one growing season. Far fewer studies have recorded relative success in terms of survival and even fewer studies have recorded fecundity under competition. Differences in size suppression are usually assumed to reflect differences in relative abilities to deny resources to competitors. However, most previous studies have failed to control or account for other sources of variation in the size suppression that plants experience under competition, i.e. variation between mixtures in the resource supply/demand ratio (approach to carrying capacity), or variation in the degree of niche overlap between competitors, or variation in the intensity of concurrent facilitative interactions between competitors. For future studies, much greater caution is required in recognizing these inherent limitations of traditional measures of competitive ability and, hence, guarding against unfounded conclusions or predictions about potential for competitive success that are based on these measures. There is also a significant challenge for future studies to adopt empirical approaches for minimizing these limitations. Some initial recommendations are considered here based on an emerging view of competitive ability measured in terms of traits associated with all three conventional components of Darwinian fitness, i.e. not just growth (plant size) but also survival and fecundity allocation (offspring production per unit plant size per unit time). According to this model, differences in competitive ability imply differences in the ability, despite intense competition (i.e. low resource supply/demand ratio), to recruit offspring into the next generation and thereby limit offspring recruitment by other plants. The important traits of competitive ability, therefore, are not only those that allow a plant to deny resources to competitors, suppress their sizes and hence, maximize the plant's own size, but also those traits that allow the plant to withstand suppression from competition enough to persist, both as an individual (through survival) and across generations (through descendants).

Competitive abilities of introduced and native grasses

Differencesin competitive ability may explain the maintenance of existing plantpopulationsand the invasion of new areas by plant species. We used field experiments toexamine the competitive responses of Agropyron cristatum(L.) Gaertn., an introduced C3 grass, and Boutelouagracilis (HBK.) Lag., a native C4 grass, and thecompetitive effects of Agropyron-dominated vegetation andsuccessional prairie. We also tested whether the outcome of competitiveinteractions varied with water availability. In each vegetation type,transplants of each species were grown under two levels of competition(presenceor absence of neighboring vegetation) and three levels of water availability(high, medium, or low). Transplant survival, growth, and biomass allocationpatterns were measured. Water availability had no effect on the measuredvariables, suggesting that both species were limited by another resource.Growthrates were affected more by competition, while survival and root: shoot ratiowere affected more by transplant species identity. In the successional prairie,neighboring vegetation suppressed the growth of Agropyrontransplants less than that of Bouteloua transplants,suggesting that Agropyron has a stronger ability to resistcompetitive suppression in that vegetation type. The spread ofAgropyron into surrounding vegetation may relate to itsability to resist competitive suppression. In theAgropyron-dominated vegetation, neighboring vegetationsuppressed the growth of both species by the same extent. However, competitionaccounted for more variation in transplant growth inAgropyron-dominated vegetation than in successionalprairie, suggesting that Agropyron has strong competitiveeffects which hinder plant growth and prevent other species from establishinginAgropyron fields.

Differences in competitive ability among races of Colletotrichum graminicola in mixtures

Mixtures of races of Colletotrichum graminicola, causing sorghum (Sorghum bicolor) anthracnose and differing in their virulence range, were inoculated for five and six generations on the susceptible sorghum cultivar BR009 (Tx623), in two experiments in a greenhouse. In each generation a sample of 50 single spore isolates was obtained and inoculated on a standard differential set to determine the proportion of each race in the mixture. Isolates of the race 30A, with the narrowest virulence range, predominated over isolates of the more complex races 31B, 31C and 31E indicating the existence of differences in the survival ability among races of this pathogen.

The Approximately Ideal, More or Less Free Distribution

We present the minimum set of requirements necessary and sufficient to represent the foraging behaviour of an animal, and its utilisation of food, in order to explore the emergent properties of behaviour that allow animals to reduce their hunger. We present an individual-based model of foraging that provides a simple quantification of the requirements, which is sufficiently simple to yield some analytical results. Complex interactions beyond the scope of analysis have been explored through simulating animals foraging in regenerating patchy environments. In most cases the populations pass into equilibrium distributions which appear to be stable. The equilibria always approximate closely to the ideal free distribution, although typically with a small degree of undermatching. (Undermatching is the term applied to the departure from the ideal free distribution caused by a smaller proportion of the population than expected occupying areas with a higher than average regeneration rate). The model therefore implies that the distribution, hitherto accounted for in terms of ESSs may, in fact, be simply an effect of the animal's utilization of the food it collects to reduce its hunger. The model defines a specific feeling rate, v, the rate at which an animal can feed on a unit of food. This is a function of three parameters, v1, the specific feeding rate when alone, v∞, the rate, possibly zero, at which it can feed in the presence of an indefinitely large number of conspecifics, and n1/2, the number of conspecifics that cause v to take the value (v1+v∞)/2. Exploitation competition in the absence of interference is represented by setting v1=v∞. Differences in competitive ability in exploitation have been represented by simulating animals with a range of values of v1, those with the larger values, feeding more rapidly, being the more effective competitors, and those with the lower values being the less effective. Interference competition is represented by setting v1>v∞ and social facilitation by v1<v∞. Individual differences in the strength of interaction are represented by different values of n1/2. In competition, the animals with the larger values of n1/2 are the more effective competitors: in facilitation, they are the less effective facilitators. The addition of physiological and behavioural detail makes very little alteration to the emergent equilibria, always close to the ideal free distribution, almost always showing undermatching.

Long-term and fine-scale coexistence of closely related species

Models of coexistence often assume that competitive exclusion takes place at fine scale, but that disturbances (non-equilibrium models) or heterogeneity (spatial models) are necessary for long-term coexistence. As an alternative, very slow exclusion among competitively equivalent species has been proposed, but questioned on the ground that in the long run even the smallest difference in competitive ability will express itself in the loss of species. In this paper we demonstrate long-term coexistence among closely-related mire plant species (notablySphagnum mosses). In any bog in the boreal region, most species that can tolerate the nutrient-poor, acidic and partly anaerobic conditions are present. Thus, the ratio between the actual and regional species pool is 1 (or very close to 1), and selection of species from the regional pool seems hardly affected by interspecific competition or dispersal limitation. The evidence for coexistence comes from experiments and permanent plots (time scale 1–10 yrs), remapping (40–60 yrs) and stratigraphy (100–2000 yrs), and the spatial scale is cm-ha. At the cm-scale there were no cases of competitive exclusion, even when species were transplanted into each others microsites, and for 10 × 10-cm plots published stratigraphic data showed that combinations with 2–4 species lasted for 200–300 yrs on average, and in some cases almost 1000 yrs. Most temperate ecosystems will not remain without substantial changes in conditions for such a long time. Thus, exclusion among similar species can be so slow that it may well explain the coexistence for the whole “life-time” of a community.

Water‐Seeded Rice Cultivars Differ in Ability to Interfere with Watergrass

The repeated use of a limited number of herbicides has led to the development of resistant weed species in California rice (Oryza sativa L.). Competitive rice cultivars could help reduce herbicide dependency and decrease selective pressure for resistance. We conducted field experiments to determine if differences in competitive ability existed between two semidwarf cultivars of water‐seeded rice. Cultivars M‐202 and A‐301 were grown on Stockton clay soil at two seed rates (84 and 168 kg ha−1) in 1996 and four seed rates (56, 112, 168, and 224 kg ha−1) in 1997. Molinate (S‐ethyl hexahydro‐1H‐azepine‐1‐carbothioate) and propanil [N‐(3,4‐dichlorophenyl)propanamide] were used to control a mixed infestation of watergrass [Echinochloa oryzoides (Ard.) Fritsch, E. phyllopogon (Stapf) Koss] in 1996 and 1997, respectively. Molinate (0 and 4.5 kg a.i. ha−1) was applied in 1996, and propanil (0, 1, 2 and 4 kg a.i. ha−1) was applied in 1997. M‐202 was taller, produced more leaf dry weight, had greater light interception, and reduced watergrass biomass more than A‐301 in both years of the study. The more competitive M‐202 also had higher yields than A‐301 under weedy and weed‐free conditions. Watergrass growth was not affected by rice seed rates in either year. This study suggests that herbicide rates could be reduced and weed control could be improved if more competitive cultivars were developed for water‐seeded rice.

Hatching date influences age at first reproduction in the black-headed gull.

In long-lived colonial birds, age at recruitment is an important life-history character. Variation in this parameter may reflect differences in several factors, including competitive ability and breeding strategies. Further, these differences may be due to timing of hatching (for instance through differences in competitive ability). We investigated the age of first-time breeders in relation to hatching date in a black-headed gull Larus ridibundus colony situated in central France, from 1979 to 1993. Age at first breeding was estimated for four groups of individuals (total n=550) according to their hatching date, using a recent capture-recapture methodology which allowed us to estimate recruitment rate without the limiting assumptions of methods relying on simple return rates. The age at first breeding was negatively correlated with the hatching date of individuals: individuals hatched earlier in the season started breeding at a younger age than individuals born later. Proportionally more 2-year-old late-hatched individuals were seen breeding on small peripheral colonies than young early-hatched individuals. This difference disappeared after age 3years. These results strongly suggest that individuals hatched late in the season start to breed on peripheral colonies before recruiting to their natal colony. A difference of few weeks in hatching date has consequences which can last for several years.

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.

A Test for a Differential Colonization and Competitive Ability in Two Generalist Predators

A Test for a Differential Colonization and Competitive Ability in Two Generalist Predators