Strength Of Intraspecific Competition Research Articles

Interspecific competition in perennial sedentary organisms: An individual‐based model

AbstractWe investigated a mathematical model of the dynamics of the ecological system consisting of two competing perennial species, each of which leads a sedentary life. It is an individual‐based model, in which the growth of each individual is described. The rate of this growth is weakened by competition from neighboring individuals. The strength of the competitors' influence depends on their size and distance to them. The conditions, in which the competitive exclusion of one of the competitors and the coexistence of both competitors take place are provided. The influence of the parameters responsible for the strength of competition, the degree of competitive asymmetry, and consideration of the importance of specific elements of the spatial structure of this ecological system on the results of the competition were analyzed. Both species co‐exist when they are equal competitors. Permanent coexistence is possible only when interspecific competition is weaker than intraspecific. When interspecific competition is stronger, the coexistence of equal interspecific competitors is random. Both species have equal probability of extinction. If species are not equal competitors, the stronger one wins. This result can be modified by different strengths of intraspecific competition. The weaker interspecific competitor can permanently coexist with stronger one, when its individuals suffer stronger intraspecific competition.

Impacts of female body size on cannibalism and juvenile abundance in a dominant arctic spider.

Body size influences an individual's physiology and the nature of its intra- and interspecific interactions. Changes in this key functional trait can therefore have important implications for populations as well. For example, among invertebrates, there is typically a positive correlation between female body size and reproductive output. Increasing body size can consequently trigger changes in population density, population structure (e.g. adult to juvenile ratio) and the strength of intraspecific competition. Body size changes have been documented in several species in the Arctic, a region that is warming rapidly. In particular, wolf spiders, one of the most abundant arctic invertebrate predators, are becoming larger and therefore more fecund. Whether these changes are affecting their populations and role within food webs is currently unclear. We investigated the population structure and feeding ecology of the dominant wolf spider species Pardosa lapponica at two tundra sites where adult spiders naturally differ in mean body size. Additionally, we performed a mesocosm experiment to investigate how variation in wolf spider density, which is likely to change as a function of body size, influences feeding ecology and its sensitivity to warming. We found that juvenile abundance is negatively associated with female size and that wolf spiders occupied higher trophic positions where adult females were larger. Because female body size is positively related to fecundity in P. lapponica, the unexpected finding of fewer juveniles with larger females suggests an increase in density-dependent cannibalism as a result of increased intraspecific competition for resources. Higher rates of density-dependent cannibalism are further supported by the results from our mesocosm experiment, in which individuals occupied higher trophic positions in plots with higher wolf spider densities. We observed no changes in wolf spider feeding ecology in association with short-term experimental warming. Our results suggest that body size variation in wolf spiders is associated with variation in intraspecific competition, feeding ecology and population structure. Given the widespread distribution of wolf spiders in arctic ecosystems, body size shifts in these predators as a result of climate change could have implications for lower trophic levels and for ecosystem functioning.

The interaction effect between intraspecific competition and seed quality on the life‐history traits of the seed‐feeding beetle Acanthoscelides macrophthalmus

AbstractIt has been shown that intraspecific competition and resource quality may affect life‐history traits of insects, such as body size, fecundity, and survival. However, intraspecific competition and resource quality may interact with each other. The study of such interacting effects is crucial for understanding the influence of these ecological variables on the selection of specific life‐history traits. Here, we investigated whether the interaction between intraspecific larval competition and variation in resource quality affects adult emergence and survival, egg size, fecundity, body size, and sexual size dimorphism (SSD) of the seed‐feeding beetle Acanthoscelides macrophthalmus (Schaeffer) (Coleoptera: Chrysomelidae: Bruchinae) when infesting Leucaena leucocephala (Lam.) De Wit (Fabaceae), its host plant. In the laboratory, beetles were reared on seeds that differed in quality (e.g., different hardness, seed size, water content), in the presence or absence of larval competition. Body size and SSD did not differ between treatments (with and without competition), nor were they affected by varying resource quality. Females subjected to competition during the larval stage and females emerging from seeds of higher quality, displayed the highest fecundity. The proportion of emergent adults was higher in the absence of competition. In addition, larger eggs were laid on the low‐quality resource in the absence of competition, showing a trade‐off between egg size and egg number. Adult survival differed among treatments and resource qualities, suggesting a higher investment in adult survival for individuals emerging from seeds of low quality in the presence of competition. Whether changes in specific traits could be selected for in detriment of others will depend on the strength of intraspecific competition, the variation in resource quality, and the plasticity in the life‐history traits investigated. This needs further clarification.

Intraspecific competition increases toxicant effects in outdoor pond microcosms

Competition is a ubiquitous factor in natural populations and has been reported to alter the ecological impact of xenobiotics. We investigated conditions that mirror the natural variation of environmental factors. For this, different treatments were applied to 96 outdoor pond microcosms by shading the ponds and harvesting the communities. Then, the effect of esfenvalerate (0.03, 0.3, and 3 μg/L) on populations of Daphnia spp. was investigated. The pesticide effect and the sensitivity of Daphnia spp. in the context of a zooplankton community was increased by intraspecific competition 11 days after contamination. This relationship was most pronounced at 0.03 and 0.3 μg/L esfenvalerate, which were the concentrations that led to partial mortality. In contrast, interspecific interaction did not significantly alter the effect of the toxicant on Daphnia spp. Modelled concentration-response curves showed that the negative effects of the pesticide differed by a factor of up to 100 depending on the strength of intraspecific competition. In addition, a wider range of concentrations led to negative effects at high levels of intraspecific competition than at low levels. We argue that increased intraspecific competition reduces the availability of resources at the individual level and thereby increases the effect of contaminants. This knowledge about the interaction between competition and the response to toxicants is important in assessing the effects of these factors under field conditions.

Multiple life stages with multiple replicated density levels are required to estimate density dependence for plants

Negative density dependence resulting from intraspecific competition can regulate plant populations by limiting demographic rates (survival, growth, fecundity). However, the strength of intraspecific competition can vary within and among populations due to spatial or temporal environmental heterogeneity, or genetic differences. Quantification of variation under a relatively constant environment is needed to assess the inherent potential for density dependence to vary. This knowledge will help adjust data collection effort required for parameterisation of density dependence. Our review of published plant demographic studies revealed that only half of the studies included the whole life‐cycle in the analysis of density dependence. Approximately half of the studies manipulated density, while the rest examined density dependence from observed densities in the field. Regardless of the design used, density dependence was estimated from a small number of replicates. To investigate inherent variation in density dependence during the life‐cycle, and the effect of low replication on density dependence estimates, we combined an experimental approach with simulations for an invasive herb Senecio madagascariensis. We found significant negative density dependence for five out of six examined demographic rates in a constant environment, with the strength of density dependence increasing during the life‐cycle. An exception was plant growth, in which the direction of density dependence varied from positive to negative depending on the life stage. Simulations showed substantial deviation for density dependence parameterised from a small number of replicates even when environmental variation was minimal. This suggests that data collection procedures currently used to assess the effect of density on plant demographic rates may produce inaccurate estimates, increasing uncertainty in demographic models. Due to variation in the direction and strength of density dependence during the life‐cycle, multiple life stages with multiple replicated density levels are required to parameterise density dependence for demographic rates.

Aphid individual performance may not predict population responses to elevated CO2 or O3

AbstractChanges in atmospheric composition affect plant quality and herbivore performance. We used the Aspen Free Air CO2 Enrichment (FACE) facility to investigate the impacts of elevated carbon dioxide (CO2) and ozone (O3) on the performance of the aphid Cepegillettea betulaefoliae Granovsky feeding on paper birch (Betula papyrifera Marsh.). In Year 1, we simultaneously measured individual performance and population growth rates, and in Year 2 we surveyed natural aphid, predator and parasitoid populations throughout the growing season. Aphid growth and development (relative growth rate (RGR), development time, adult weight, embryo number and the birth weight of newborn nymphs) were unaffected by CO2 and O3. Aphid fecundity decreased on trees grown at elevated CO2, O3 and CO2+O3. Neither nymphal performance nor adult size were reliable indicators of future fecundity at elevated CO2 and/or O3. Aphid populations protected from natural enemies were unaffected by elevated CO2, but increased significantly at elevated O3. Individual fecundity in elevated CO2 and O3 atmospheres did not predict population growth rates, probably because of changes in the strength of intraspecific competition or the ability of the aphids to induce nutrient sinks. Natural aphid, predator and parasitoids populations (Year 2) showed few significant responses to CO2 and O3, although CO2 and O3 did affect the timing of aphid and natural enemy peak abundance. Elevated CO2 and O3 affected aphid and natural enemy populations independently: no CO2× O3 interactions were observed. We conclude that: (1) aphid individual performance did not predict population responses to CO2 and O3 and (2) elevated CO2 and O3 atmospheres are unlikely to affect C. betulaefoliae populations in the presence of natural enemy communities.

Analysis of radial growth responses to changes in stand density for four tree species

Enhancement of radial-growth by reducing stand density is a common silvicultural practice. Traditional methods of analysis based on stand level variables, such as stand mean diameters, overestimate the effects of reduced density on radial-growth due to ‘technical growth’, i.e. increases in the mean diameter caused by the thinning operation itself through removal of small trees. A new method for analyzing radial-growth responses in forest stands based on samples of the largest trees within populations is proposed, and the hypotheses tested that: (1) reducing stand density enhances radial-growth; (2) radial-growth response varies with site and changes with the release period, i.e. the period of time after thinning; (3) radial-growth response varies among tree species. Based on studies of Picea abies (L.) Karst. it is suggested that water supply influences radial-growth responses significantly. Tree species differ greatly in their ability to enhance radial-growth when stand density is reduced. For Pinus sylvestris L. and Acer pseudoplatanus L. the responses are very limited. P. abies (L.) Karst. showed the greatest response to thinning while Fagus sylvatica L. showed an intermediate response. These differences may be ascribed to differences in strengths of intraspecific competition, strategies of photosynthate allocation or inherent growth inertia. The method of analysis applied in this study can be used to quantify radial-growth responses of tree species relative to the length of release periods and site.

Analysis of radial growth responses to changes in stand density for four tree species

Enhancement of radial-growth by reducing stand density is a common silvicultural practice. Traditional methods of analysis based on stand level variables, such as stand mean diameters, overestimate the effects of reduced density on radial-growth due to ‘technical growth’, i.e. increases in the mean diameter caused by the thinning operation itself through removal of small trees. A new method for analyzing radial-growth responses in forest stands based on samples of the largest trees within populations is proposed, and the hypotheses tested that: (1) reducing stand density enhances radial-growth; (2) radial-growth response varies with site and changes with the release period, i.e. the period of time after thinning; (3) radial-growth response varies among tree species. Based on studies of Picea abies (L.) Karst. it is suggested that water supply influences radial-growth responses significantly. Tree species differ greatly in their ability to enhance radial-growth when stand density is reduced. For Pinus sylvestris L. and Acer pseudoplatanus L. the responses are very limited. P. abies (L.) Karst. showed the greatest response to thinning while Fagus sylvatica L. showed an intermediate response. These differences may be ascribed to differences in strengths of intraspecific competition, strategies of photosynthate allocation or inherent growth inertia. The method of analysis applied in this study can be used to quantify radial-growth responses of tree species relative to the length of release periods and site.

Plant‐based host preference and larval competition of the tachinid parasitoid Epicampocera succincta

AbstractWe investigated the causes of among‐plant variation in the parasitism rate of Pieris melete larvae (Lepidoptera: Pieridae) by the parasitoid fly Epicampocera succincta (Diptera: Tachinidae). The rate of parasitism by E. succincta was much higher on the bitter cress Cardamine appendiculata than on any other cruciferous plants. Adult female flies were found to be more attracted to C. appendiculata than to other cruciferous plants for searching for hosts. The parasitoid appeared to be unable to distinguish P. melete from an alternative host, Pieris rapae larvae, that coexisted with P. melete on most crucifer plants. Similarly, E. succincta failed to avoid P. rapae parasitized by the braconid wasp Cotesia glomerata. C. glomerata is a superior competitor to E. succincta if occupying the same host, killing the host before the E. succincta larva can grow and depriving the larva of the chance to survive. E. succincta attacked P. rapae larvae on most cruciferous plants, many of which were already occupied by C. glomerata; only on Cardamine appendiculata was E. succincta free from interspecific competition, because only P. melete was found on this particular plant. However, the strong preference for the hosts on C. appendiculata incurred heavy intraspecific competition among larvae on this plant, killing as great a proportion of larvae as interspecific competition did on the other plants. The balance between the strength of intraspecific competition on the preferred plant and of interspecific competition on the other plants appeared to maintain plant preference by E. succincta.