Effects Of Intraspecific Competition Research Articles

Effects of intraspecific competition on growth and photosynthesis of Atriplex prostrata

We investigated the effect of intraspecific competition on growth parameters and photosynthesis of the salt marsh species Atriplex prostrata Boucher in order to distinguish the effects of density-dependent growth inhibition from salt stress. High plant density caused a reduction of 30% in height, 82% in stem dry mass, 80% in leaf dry mass, and 95% in root dry mass. High density also induced a pronounced 72% reduction in leaf area, 29% decrease in length of mature internodes and 50% decline in net photosynthetic rate. The alteration of net photosynthesis paralleled growth inhibition, decreasing from 7.6 ± 0.9 μmol CO 2 m −2 s −1 at low density to 3.5 ± 0.4 μmol CO 2 m −2 s −1 at high density, indicating growth inhibition caused by intraspecific competition is mainly due to a decline in net photosynthesis rate. Plants grown at high density also exhibited a reduction in stomatal conductance from 0.7 ± 0.1 mol H 2O m −2 s −1 at low density to 0.3 ± 0.1 mol H 2O m −2 s −1 at high density and a reduction in transpiration rate from 6.0 ± 0.3 mmol H 2O m −2 s −1 at low density to 4.3 ± 0.3 mmol H 2O m −2 s −1 at high density. Biomass production was inhibited by an increase in plant density, which reduced the rate of photosynthesis, stomatal conductance and leaf area of plants.

Competitive Interactions Among Four Pest Species of Earth Mites (Acari: Penthaleidae)

Earth mites are major winter pests of a variety of crops and pastures in southern Australia. Competition between four earth mite species was investigated using field and shadehouse experiments. The influence of different plant hosts on the frequency and intensity of competitive interactions also were examined. This information is important, because control attempts that eradicate one species of mite could be directly followed by an increase in abundance of another earth mite species. There were strong effects of intraspecific competition on the reproductive rate of species, while interspecific interactions between Halotydeus destructor (Tucker) and Penthaleus species and between the three Penthaleus species also were detected. Competitive abilities were altered on the different plant types. On pasture, the competitive advantage swayed between Penthaleus major (Dugés), H. destructor, and Penthaleus falcatus (Qin & Halliday). Penthaleus sp. x was the strongest competitor in a mixture of wheat, Triticum aestivum (L.), and oats, Avena sativa (L.), whereas on canola, Brassica napus (L.), and bristly ox-tongue, Picris echioides (L.), P. falcatus, and H. destructor were superior competitors. These results suggest that competition is a strong force influencing the abundance of earth mites in the field and that host plant factors are important in shaping the type of interactions. This highlights the importance of identifying mite species when considering control options and suggests that effective control recommendations need to be developed for each individual species.

Competitive Interactions Among Four Pest Species of Earth Mites (Acari: Penthaleidae)

Earth mites are major winter pests of a variety of crops and pastures in southern Australia. Competition between four earth mite species was investigated using field and shadehouse experiments. The influence of different plant hosts on the frequency and intensity of competitive interactions also were examined. This information is important, because control attempts that eradicate one species of mite could be directly followed by an increase in abundance of another earth mite species. There were strong effects of intraspecific competition on the reproductive rate of species, while interspecific interactions between Halotydeus destructor (Tucker) and Penthaleus species and between the three Penthaleus species also were detected. Competitive abilities were altered on the different plant types. On pasture, the competitive advantage swayed between Penthaleus major (Dugés), H. destructor, and Penthaleus falcatus (Qin & Halliday). Penthaleus sp. x was the strongest competitor in a mixture of wheat, Triticum aestivum (L.), and oats, Avena sativa (L.), whereas on canola, Brassica napus (L.), and bristly ox-tongue, Picris echioides (L.), P. falcatus, and H. destructor were superior competitors. These results suggest that competition is a strong force influencing the abundance of earth mites in the field and that host plant factors are important in shaping the type of interactions. This highlights the importance of identifying mite species when considering control options and suggests that effective control recommendations need to be developed for each individual species.

Interspecific competition among urban cockroach species

AbstractThe aim of this study was to quantify and to compare the effects of intraspecific and interspecific competition among pairs of urban cockroaches [Blatta orientalis L., Periplaneta americana (L.), and Periplaneta australasiae Fabricius (Dictyoptera: Blattodea)] in relation to the limitation of resources such as shelter or food. Our approach was to assess whether the presence of one species affected the resource exploitation of another. A reduction in access to shelters or to food for one species revealed dominant/subordination relationships among species and induced spatial segregation in shelters and temporal segregation during food exploitation. The fragmentation of available resources facilitated spatial segregation and the access of more individuals of the subordinate species. Individuals of each species were aware of the presence of non‐conspecifics. The outcome of interspecific competition under laboratory conditions should help us to understand how segregated spatial distributions occur under natural conditions in urban areas.

Competitive regulation of plant allometry and a generalized model for the plant self-thinning process

Taking into account the individual growth form (allometry) in a plant population and the effects of intraspecific competition on allometry under the population self-thinning condition, and adopting Ogawa’s allometric equation 1 y = 1 ax b + 1 c as the expression of complex allometry, the generalized model describing the change mode of r (the self-thinning exponential in the self-thinning equation, log M= K+ rlog N, where M is mean plant mass, K is constant, and N is population density) was constructed. Meanwhile, with reference to the changing process of population density to survival curve type B, the exponential, r, was calculated using the software MATHEMATICA 4.0. The results of the numerical simulation show that (1) the value of the self-thinning exponential, r, is mainly determined by allometric parameters; it is most sensitive to change of b of the three allometric parameters, and a and c take second place; (2) the exponential, r, changes continuously from about −3 to the asymptote −1; the slope of −3/2 is a transient value in the population self-thinning process; (3) it is not a ‘law’ that the slope of the self-thinning trajectory equals or approaches −3/2, and the long-running dispute in ecological research over whether or not the exponential, r, equals −3/2 is meaningless. So future studies on the plant self-thinning process should focus on investigating how plant neighbor competition affects the phenotypic plasticity of plant individuals, what the relationship between the allometry mode and the self-thinning trajectory of plant population is and, in the light of evolution, how plants have adapted to competition pressure by plastic individual growth.

Effect of intra-specific competition on development and growth of Alopecums myosuroides Hudson

Observations were made on the effects of different levels of plant density and nitrogen nutrition on the development and seed production of a weed (Alopecurus myosuroides Hudson). Experiments were conducted in two greenhouses with one plant per pot in order to avoid competition for nitrogen absorption. The density effect was studied by increasing the number of pots per unit surface. The results indicated that the leaf and tiller appearance rates were modified by nitrogen availability on plant density. The lack of nitrogen and higher plant density increased the leaf and tiller appearance rate. Growth and seed production was strongly influenced by the plant density, and the increased nitrogen availability at a given density did not modify the values of growth parameters. Some parameters as flowering time or seed viability were not modified in our experiment conditions. Possibilities of using these results in cropping systems to improve the control of A. myosuroides are discussed.

Intraspecific competition and spatial heterogeneity alter life history traits in an individual-based model of grasshoppers*1

An individual-based model (IBM) was developed to examine the effects of intraspecific competition and spatial structuring of food on life history traits of grasshoppers inhabiting temperate-zone grasslands. Each individual carried real-valued genes which determined size of offspring and size at maturity, and which were passed on to its offspring. Size at maturity was a plastic trait, depending on an individual’s growth rate, whereas size of offspring was a fixed trait. Individuals with more successful combinations of traits produced more offspring that eventually came to dominate the population. Populations were food limited and intraspecific competition was either exploitative or size-based interference. Growth rates and fecundity depended on food quality, which declined within season as a function of day of year and the proportion consumed by grasshoppers. Three different spatial distributions of food quality were examined: uniform, random, and clumped. Optimal egg size was larger under interference competition and spatially clumped resources. Reaction norms of size at maturity were strongly affected by type of competition, and, by spatial distribution of resources within exploitative competition, but not under interference competition. The IBM shows promise as a means of analyzing life history evolution in grasshoppers in relation to processes that arise from localized interactions between individuals.

Monarch butterfly spatially discrete advection model

We study the population cycles of the Monarch butterfly using one of the simplest systems incorporating both migration and local dynamics. The annual migration of the Monarch involves four generations. Members of Generations 1–3 (occasionally 4) migrate from the over-wintering site in Central Mexico to breeding grounds that extend as far north as the Northern United States and Southern Canada. A portion of the Generation 3 and all members of the Generation 4 butterflies begin their return to the over-wintering grounds in August through October where they enter reproductive diapause for several months. We developed a simple discrete-time island chain model in which different fecundity functions are used to model the reproductive strategies of each generation. The fecundity functions are selected from broad classes of functions that capture the effects of either contest or scramble intraspecific competition in the Monarch population. The objectives of our research are multiple and include the study of the generationally dependent intraspecific competition and its effect on the pool size of migrants as well as the persistence of the overall butterfly populations. The stage structure used in modeling the Monarch butterfly dynamics and their generationally dependent reproductive strategies naturally support fluctuating patterns and multiple attractors. The implications of these fluctuations and attractors on the long-term survival of the Monarch butterfly population are explored.

Combined effects of intraspecific competition and parasitoid attacks on the dynamics of a host population: a stage‐structured model

Although it is well known that intraspecific competition affects host‐parasitoid dynamics, previous studies have adopted oversimplified descriptions of competition. We developed and analysed a host‐parasitoid stage‐structured model in which host intraspecific competition is accounted for by a flexible density dependence function. We focused particularly on the influence of two factors generally neglected in previous studies: the strength of competition and possible differences in the effect of intraspecific competition between healthy and parasitized hosts. We showed, by means of a combination of analytical studies and simulations, that the presence of parasitoids in the system markedly changes the effects of competition on population dynamics. In particular, position in the “contest”‐“scramble” spectrum, which plays a crucial role in single‐population dynamics, may have a smaller effect in populations subjected to parasitism. We also showed that stability is increased when healthy hosts are more susceptible to competition than parasitized hosts. The effects of host intraspecific competition and its characteristics on dynamics strongly depend on the characteristics of parasitism and on the distribution of attacks in particular. Moreover, interactions between top‐down (parasitism) and bottom‐up (intraspecific competition) effects may be crucial for the regulation of host‐parasitoid systems.

Spatial structure of Aspidosperma polyneuron in two semi‐deciduous forests in Southeast Brazil

Abstract. Description of the spatial structure of vegetation is the first step in generating hypotheses regarding underlying processes. It is also important to take into account variation in spatial structure between different localities to be able to distinguish between life history traits and the influence of environment on population processes. Correlogram analysis was used to examine spatial recruitment and mortality patterns of Aspidosperma polyneuron, an emergent tree species, at Reserva Municipal de Santa Genebra (SG), a forest fragment in southeast Brazil. The spatial structure of individuals was compared with two areas with similar physiognomy, one in the same forest and another at Mata do Ribeirão Cachoeira (RC), a semi‐deciduous forest located ca. 14 km away. Seedling, sapling and adult mortality occurred in clumps, while seedling recruitment occurred mostly in one region of the area. The spatial pattern varied among the three study areas, and the differences were higher between fragments than within SG. The adults showed a negative correlation in space with saplings and seedlings in one of the SG areas, which could be a consequence of intraspecific competition, or the influence of density or distance dependent natural enemies. However, this pattern was not observed in the other areas. The results suggest that the mechanisms governing the spatial population dynamics of A. polyneuron vary among nearby and similar localities.

Intraspecific competition and spatial heterogeneity alter life history traits in an individual-based model of grasshoppers

An individual-based model (IBM) was developed to examine the effects of intraspecific competition and spatial structuring of food on life history traits of grasshoppers inhabiting temperate-zone grasslands. Each individual carried real-valued genes which determined size of offspring and size at maturity, and which were passed on to its offspring. Size at maturity was a plastic trait, depending on an individual’s growth rate, whereas size of offspring was a fixed trait. Individuals with more successful combinations of traits produced more offspring that eventually came to dominate the population. Populations were food limited and intraspecific competition was either exploitative or size-based interference. Growth rates and fecundity depended on food quality, which declined within season as a function of day of year and the proportion consumed by grasshoppers. Three different spatial distributions of food quality were examined: uniform, random, and clumped. Optimal egg size was larger under interference competition and spatially clumped resources. Reaction norms of size at maturity were strongly affected by type of competition, and, by spatial distribution of resources within exploitative competition, but not under interference competition. The IBM shows promise as a means of analyzing life history evolution in grasshoppers in relation to processes that arise from localized interactions between individuals.

Differentiation in populations of Hordeum spontaneum Koch along a gradient of environmental productivity and predictability: Intra- and interspecific competitive responses

We compared intra- and interspecific competitive responses of wild barley, Hordeum spontaneum, from four populations originating in distinct environments in Israel. The environments ranged along two parallel gradients of rainfall amount and predictability, from low (desert) to moderate (semi-steppe batha) to high (Mediterranean grassland and mountain, the latter also experiencing frost stress). The target barley plants grew under one of five densities (0, 4, 8, 16, and 32 surrounding plants per bucket) of either barley from the same population or oats (Avena sterilis) from a neutral population. The traits examined included estimates of fitness, reproductive traits, and resource allocation. The effect of intraspecific competition was stronger than interspecific competition at a high increment of neighbor density (from 4 to 32 neighbors). There was no difference in interspecific competitive responses of plants originating in the four environments at any neighbor density increments, but intraspecific competi...

Consequences of Intraspecific Competition and Environmental Variation for Selection in the Mustard Sinapsis arvensis: Contrasting Ecological and Evolutionary Perspectives

Consequences of Intraspecific Competition and Environmental Variation for Selection in the Mustard Sinapsis arvensis: Contrasting Ecological and Evolutionary Perspectives

The Interrelationships of Winter Wheat Cultivars, Crop Density and Competition of Naturally Occurring Weed Flora

ABSTRACT The effects of intraspecific and interspecific competition on a wide range of winter wheat cultivars were investigated in two consecutive split plot field experiments. Significant reductions of grain yield at greatly reduced seed rates were observed in the first experiment, whereas increasing crop density up to 380 plants m−2 in the second experiment failed to produce a significant yield response due to compensation through increased ears and grains per plant at lower crop densities. Appreciable weed suppression and acceptable grain yield can be achieved at crop densities between 150 and 270 plants m−2. Reductions in final yield due to weed competition occurred in both experiments; 11.7 and 13.6% for the first and second experiment, respectively, with the onset of weed competition occurring from tillering in the first experiment and from stem elongation in the second. The possibility of enhancing crop competitiveness for weed suppression and improved grain yield is discussed.

Intraspecific competition controls spatial distribution and social organisation of the coral-dwelling goby Gobiodon histrio

The role of competition in regulating reef fish populations has been controversial. Here, we test the effect of intraspecific competition on the spatial distribution and social organisation of Gobiodon histrio, an obligate coral-dwelling goby that inhabits the branching coral Acropora nasuta. At Lizard Island (Great Barrier Reef), social groups of G. histrio consisted of either a single individual or a breeding pair and occupied 98% of A. nasuta colonies, indicating that corals are a limiting resource. Furthermore, there was a positive relationship between fish size and coral size, with small, single G. histrio mostly occupying small corals ( 15 to 20 cm diameter). A manipulative experiment involving small and large corals demonstrated that this positive relationship between fish size and habitat size was due to size- based competition for large corals. Small and large G. histrio exhibited a similar preference for large corals (using large corals in approximately 80% of trials), but small fish were usually excluded from large corals in the presence of a larger fish. Small fish were more likely to use large corals in the pres- ence of a single adult (38% of trials) than in the presence of an adult pair (8% of trials), suggesting that small fish may be able to enter a large coral following the loss of one individual in a breeding pair. The presence of a threshold coral size for the formation of breeding pairs means that an individual's ability to compete for large corals will influence its reproductive success.

Effects of intraspecific competition on yield of early potato grown in mediterranean environment

Since the environment of the Mediterranean regions allows offseason production, potatoes are planted in autumn-winter and harvested in spring. During this period, potatoes are subjected to low temperatures and short day lengths which modify the growth characteristics of plants. For this reason, our analysis of competition response was conducted to better clarify the biological relationship between yield and plant density. Field trials were conducted in Sicily (south Italy), a highly representative area of early potato crop in the Mediterranean Basin, with the aim of studying effects of intraspecific competition on tuber yield and yield components. Ten planting densities (ranging from 3.0 to 8.0 plants m−2) were studied on cv. Spunta using “tuberpieces” with a different number of eyes (one eye or all the eyes in 1996; one eye, two eyes, or all the eyes in 1997). Intraspecific competition reduced the tuber yield of individual plants, which became gradually less evident with increasing plant density. Competition affected the number of tubers per plant in the lower plant populations only (from 3.0 to 5.8 plants m−2), whereas effects on average tuber weight were at times more marked in the higher populations (from 5.8 to 8.0 plants m−2) and at other times in the lower densities (from 3.0 to 5.8 plants m−2). As a consequence of increased plant density, and notwithstanding the higher intraspecific competition, the yield of tubers per unit area increased linearly. Regardless of the number of eyes per tuber-piece, when passing from the lower to the higher plant density, yield increased from 34.0 to 54.11 ha−1 in 1996 and from 39.9 to 56.7 t ha−1 in 1997.

The ‘ghost of settlement past’ determines mortality and fecundity in the barnacle, Semibalanus balanoides

Intraspecific competition is common in many organisms. For many sessile marine invertebrates high settlement densities can lead to competition with conspecifics that can affect the probability of mortality, morphology, and reproductive output. This work aims to determine the affect of recruit density on shell morphology and fecundity and the temporal nature of intraspecific competition for the acorn barnacle, Semibalanus balanoides. Two tile arrays were used; experiment 1 was deployed at the start of the settlement season (43,264 recruits, 4.1% survivorship to adult), experiment 2 was deployed in the middle of the season (5,454 recruits, 42.8% survivorship). Experiment 2 displayed density dependent mortality between recruits and juveniles and adults, whereas no relationship was found for exp. 1. Experiment 2 had higher egg production per individual, but also higher egg production per area of tile compared to exp. 1. A negative relationship was found between recruit density and egg production, however during the summer period, this relationship switched to a positive relationship between juvenile density and egg production, due to high mortality on tiles with high recruit density. Cumulative population density (CPD) was calculated as the cumulative mean density of barnacles per cm‐2 of tile and was used as an index of intraspecific competition. There was a linear negative relationship between mass of eggs per individual and CPD. However, CPD over the summer period more fully explained egg production per individual. A time window was identified when cumulative intraspecific competitive effects were shown to have a strong negative effect on egg production and an increase in mortality, causing a flip from density independent fecundity to density dependent fecundity. These results suggest that recruit density is an important driver of subsequent population processes.

Effect of intraspecific competition by Aspergillus flavus on aflatoxin formation in suspended disc culture

The ability of two non-aflatoxin producing strains of Aspergillus flavus to interfere with aflatoxin production by a toxigenic A. flavus strain was examined using a replacement series with suspended disc culture method. Individual glass fiber discs, affixed to a pin suspended from the caps of scintillation vials, were inoculated with medium containing A. flavus conidial mixtures in different proportions (aflatoxin producer:non-producer = 100:0, 80:20, 60:40, 20:80 and 0:100 by vol) at a constant total density (1 x 10(5) spores ml(-1)). Reductions in the total conidial density of these strains when grown alone, had little effect on fungal growth (mycelium dry weight) or aflatoxin production. Significant (P < 0.0001) reductions in aflatoxin B1 were recorded when non-toxigenic strains represented any proportion of the inoculum mixture. Aflatoxin yield values were less than (P < 0.0001) expected from the input ratios for toxigenic vs. non-toxigenic conidial inoculum within the replacement series. Aflatoxin yields were also reduced (P < 0.001), with a corresponding increase in fungal growth (P < 0.001), when conidia from aflatoxin producing strains were mixed in equal proportions. This suggests that the substantial inhibition of aflatoxin yield for inoculum mixtures results from the failure of spore germlings to establish a cooperative mycelial network.

Effects of intraspecific and interspecific density on the demography of a perennial herb, Sanicula europaea

We examined the effects of intraspecific and interspecific competition on demographic processes in the perennial herb Sanicula europaea by manipulating the density of neighbouring plants. We followed the response in terms of survival, growth and reproduction and in terms of seedling recruitment. The demographic data from all phases of the life cycle enabled us to assess also the overall effects of treatments on population growth rate (λ) by transition matrix models. We also decomposed the differences in λ between control and treatments, using life table response experiments (LTRE). To study the effects of competition on recruitment in more detail and to evaluate the role of seed availability, we sowed seeds at different densities with or without vegetation removal.Vegetative growth and flowering frequency of established individuals was not significantly affected by removal treatments, which suggest no, or a delayed response to released competition. Neighbour removal had no effect on seedling emergence but enhanced recruitment through a higher seedling survival. Conspecific and simultaneous conspecific and heterospecific removal of plants led to an increase in population growth rate (λ), whereas heterospecific removal alone led to a decrease. Emergence of seedlings and fate of vegetative established individuals contributed most to differences in λ between the control and the different treatments. Seed addition enhanced seedling emergence but, as seedling and juvenile survival were density dependent, densities of established individuals appear not to be seed limited.In S. europaea removal treatments had different effects on established individuals and recruitment. This suggests that studies quantifying the effects of competition over the entire life cycle and performed in a natural environment are necessary to assess the importance of competition in perennial plant populations.

Spatial and temporal variations in mortality of the biennial plant, Lysimachia rubida: effects of intraspecific competition and environmental heterogeneity

Summary We studied mortality, growth and spatial distribution of individuals of Lysimachia rubida to elucidate causes and consequences of temporal and spatial variations in mortality. This short‐lived monocarpic biennial grows in open dry habitats on rocky coastal cliffs of the subtropical Bonin (Ogasawara) Islands, Japan. All individuals of two successive cohorts that appeared in a 8 × 8‐m quadrat were mapped, and their fate and size were recorded at 2–3 month intervals from March 1998 to July 2000. We analysed spatial and temporal changes in the relative importance of plant size, local densities of neighbouring plants and ground surface conditions to determine plant mortality. We also examined spatially non‐random mortality and its spatial scales. Patchy seed dispersal from scattered mother plants resulted in a clumped distribution of seedlings. During the growth cycle of L. rubida, we recognized two growth stages that differed in the relative importance of the factors affecting plant mortality. At the earlier growth stage, after germination in March until July, mortality was caused mainly by local density. Smaller plants within areas of high local densities were more likely to die. The spatial patterns of surviving and dead plants tended to be regular and aggregated, respectively, in comparison with those expected from random mortality. The spatial scale of aggregation of the dead plants was 5–25 cm, corresponding to the distance of direct interactions between neighbours. At the later growth stages, in October until May of the following year, mortality was affected mainly by ground surface conditions. Both surviving and dead plants were spatially aggregated at spatial scales of 20–100 cm. Plants tended to survive in microhabitats of smaller gravel, which had a greater water‐holding capacity. This study also showed the consequence of spatial variations in mortality on population structures of L. rubida. Patchy seed dispersal and subsequent temporal and spatial variations in mortality are likely to contribute to the scattered distribution of L. rubida individuals observed at the study site.