The effect of rock surface aspect on growth, size structure and competition in the lichen Rhizocarpon geographicum

Rhizocarpon geographicum is a crustose lichen found frequently on rock surfaces of southern aspect and less frequently on rock surfaces of northern aspect in Gwynedd, North Wales. This study tested the hypothesis that the radial growth of R. geographicum thalli predicts aspect distribution. Thalli of all sizes, however, exhibited significantly greater radial growth over 18 months on northwest compared with southeast facing surfaces. The hypothesis that a more intense competitive environment on northwest facing surfaces may explain the aspect distribution of R. geographicum was then tested. The size frequency distributions of thalli revealed a higher proportion of thalli in the smallest size class and a more restricted thallus size range on the northwest facing surfaces. In addition, thallus mortality appeared to be greater on northwest facing surfaces. Significantly more associated lichen species were present on rock surfaces of northern aspect at sites where R. geographicum was present. The mean frequency of the associated lichen species, however, was significantly lower on surfaces of northern aspect where R. geographicum was present. In addition, two common foliose species at these sites were demonstrated experimentally to overgrow thalli of R. geographicum. It is concluded that the growth of R. geographicum over the study period did not predict aspect distribution and that differences in the competitive environments on northwest and southeast surfaces may be an important factor determining aspect distribution. © 2002 Elsevier Science B.V. All rights reserved.

Crayfish assemblage shifts in a large drought-prone wetland: the roles of hydrology and competition

1. Faster growing, larger and/or more aggressive crayfish species are predicted to dominate permanent waterbodies. We tested this prediction using a 9 year dataset for two species of crayfish (Procambarus alleni and Procambarus fallax) co-existing in a sub-tropical flowing slough in southern Florida. Using a series of laboratory and mesocosm experiments we also compared life history traits and performance of the respective species to test mechanisms that could explain dominance shifts in the local crayfish assemblages. 2. Over the 9-year period, P. alleni densities were the greatest in shallower, shorterhydroperiod areas bordering the slough, while P. fallax densities were higher in deeper, longer-hydroperiod central areas. These areas were separated by 0.8–2 km of continuous wetland with no apparent barriers to movement between them. 3. Density of P. fallax was not strongly affected by any measures of hydrological variation, while P. alleni density increased with more severe drought conditions. Following the strongest droughts, P. alleni colonized areas in the centre of the slough where they had been absent or scarce in wetter years. 4. We conducted experiments to compare growth rates, drought tolerance, and competitive dominance of these species. P. alleni survived drought conditions better, had higher growth rates, and was the dominant competitor for space and food. While drought probably limits P. fallax in the drier slough habitats, neither drought sensitivity nor interspecific competition with P. fallax can explain decreases of P. alleni with wetter conditions. 5. Our results indicate that a competition-colonization tradeoff cannot explain the crayfish compositional dynamics in this wetland because P. alleni is both the best competitor and the best at surviving in and colonizing areas with the strongest droughts. Future attention should focus on the potential for selective effects of predators that co-vary with hydrology. 6. The traits (large size, fast growth, competitive dominance) exhibited by P. alleni, which is absent in long-hydroperiod wetlands, are those exhibited by dominant crayfish in permanent lakes and streams containing fish. Although these traits make crayfish less vulnerable to fish in some lakes and streams, life-history models of community structure across permanence gradients suggest the opposite traits should be favoured for co-existence with fish.

Relative roles of dispersal dynamics and competition in determining the isotopic niche breadth of a wetland fish

1. The niche variation hypothesis predicts that among-individual variation in niche use will increase in the presence of intraspecific competition and decrease in the presence of interspecific competition. We sought to determine whether the local isotopic niche breadth of fish inhabiting a wetland was best explained by competition for resources and the niche variation hypothesis, by dispersal of individuals from locations with different prey resources or by a combination of the two. We analysed stable isotopes of carbon and nitrogen as indices of feeding niche and compared metrics of within-site spread to characterise site-level isotopic niche breadth. We then evaluated the explanatory power of competing models of the direct and indirect effects of several environmental variables spanning gradients of disturbance, competition strength and food availability on among-individual variation of the eastern mosquitofish (Gambusia holbrooki). 2. The Dispersal model posits that only the direct effect of disturbance (i.e. changes in water level known to induce fish movement) influences among-individual variation in isotopic niche. The Partitioning model allows for only direct effects of local food availability on among-individual variation. The Combined model allows for both hypotheses by including the direct effects of disturbance and food availability. 3. A linear regression of the Combined model described more variance than models limited to the variables of either the Dispersal or Partitioning models. Of the independent variables considered, the food availability variable (per cent edible periphyton) explained the most variation in isotopic niche breadth, followed closely by the disturbance variable (days since last drying event). 4. Structural equation modelling provided further evidence that the Combined model was best supported by the data, with the Partitioning and the Dispersal models only modestly less informative. Again, the per cent edible periphyton was the variable with the largest direct effect on niche variability, with other food availability variables and the disturbance variable only slightly less important. Indirect effects of heterospecific and conspecific competitor densities were also important, through their effects on prey density. 5. Our results support the Combined hypotheses, although partitioning mechanisms appear to explain the most diet variation among individuals in the eastern mosquitofish. The results also support some predictions of the niche variation hypothesis, although both conspecific and interspecific competition appeared to increase isotopic niche breadth in contrast to predictions that interspecific competition would decrease it. We think this resulted from high diet overlap of co-occurring species, most of which consume similar macroinvertebrates.

Essays in Firm Responses to Demand and Competition Shocks

The aim of this dissertation is to examine, model and estimate firm responses to

demand shocks by focusing on specific industries where demand shocks are well

identified. Combining reduced-form evidence and structural analysis, this dissertation

extends the economic literature by focusing on within-firm responses of firms

to two important demand shocks that are identifiable in empirical settings. First,

I focus on how firms respond to a decrease in effective demand due to competition

shocks coming from globalization. By considering China's accession to the World

Trade Organization in 2001 and its impact on the apparel industry, the aim of these

chapters is to answer how firms react to the increase in Chinese import competition,

what is the mechanism behind these responses, and how important they are in explaining

the survival of the Peruvian apparel industry. Second, I study how suppliers'

survival probability relates to the sudden disruption of their main customer-supplier

relationships with downstream manufacturers, conditional on suppliers' own idiosyncratic

characteristics such as physical productivity.

Tutoria da anatomia mamária feminina utilizando uma rede neural artificial interactive activation and competition orientada a serviço

Dissertação (mestrado)—Universidade de Brasília, Faculdade Gama, Programa de Pós-Graduação em Engenharia Biomédica, 2015.

Essays on Financial Constraints, R&D Investments, and Competition

This dissertation consists of two chapters of theoretical studies that investigate the effect of financial constraints and market competition on research and development (R&D) investments. In the first chapter, I explore the impact of financial constraints on two different types of R&D investments. In the second chapter, I examine the impact of market competition on the relationship between financial constraints and R&D investments. In the first chapter, I develop a dynamic monopoly model to study a firm’s R&D strategy. Contrary to intuition, I show that a financially constrained firm may invest more aggressively in R&D projects than an unconstrained firm. Financial constraints introduce a risk that a firm may run out of money before its project bears fruit, which leads to involuntary termination on an otherwise positive-NPV project. For a company that relies on cash flow from assets in place to keep its R&D project alive, early success can be relatively important. I find that when the discovery process can be expedited by heavier investment (“accelerable” projects), a financially constrained company may find it optimal to “over”-invest in order to raise the probability of project survival. The over-investment will not happen if the project is only “scalable” (investment scales up payoffs). The model generates several testable implications regarding over-investment and project values. In the second chapter, I study the effects of competition on R&D investments in a duopoly framework. Using a homogeneous duopoly model where two unconstrained firms compete head to head in an R&D race, I find that competition has no effect on R&D investment if the project is not accelerable, and the competing firms are not constrained. In a heterogeneous duopoly model where a financially constrained firm competes against an unconstrained firm, I discover interesting strategic interactions that lead to preemption by the constrained firm in equilibrium. The unconstrained competitor responds to its constrained rival’s investment in an inverted-U shape fashion. When the constrained competitor has high cash flow risk, it accelerates the innovation in equilibrium, while the unconstrained firm invests less aggressively and waits for its rival to quit the race due to shortage of funds.

Evaluating non-price effects of regulatory and competition enforcement interventions

This thesis provides an ex-post evaluation of the effects of regulatory and competition policy enforcement interventions on non-price dimensions of competition. Chapter 1 examines the effects of a merger between two large Dutch supermarket chains on the variety and composition of product assortment. Chapter 2 and Chapter 3 investigate, both theoretically and empirically, the effects of access regulation in fixed telecoms markets on incentives to invest in superior infrastructure technologies. Non-price effects, together with price effects, are crucial to shed light on the extent of competition in a market and assess the effectiveness of regulatory and competition authorities' interventions. When evaluating non-price effects, however, it is harder to draw conclusions on the overall impact on consumers' welfare.

Temperature-Specific Competition between Invasive Mosquitofish and an Endangered Cyprinodontid Fish

Condition-specific competition is widespread in nature. Species inhabiting heterogeneous environments tend to differ in competitive abilities depending on environmental stressors. Interactions between these factors can allow coexistence of competing species, which may be particularly important between invasive and native species. Here, we examine the effects of temperature on competitiveinteractions between invasive mosquitofish, Gambusia holbrooki, and an endemic Iberian toothcarp, Aphanius iberus. We compare the tendency to approach heterospecifics and food capture rates between these two species, and examine differences between sexes and species in aggressive interactions, at three different temperatures (19, 24 and 29uC) in three laboratory experiments. Mosquitofish exhibit much more aggression than toothcarp. We show that mosquitofish have the capacity to competitively displace toothcarp through interference competition and this outcome is more likely at higher temperatures. We also show a reversal in the competitive hierarchy through reduced food capture rate by mosquitofish at lower temperatures and suggest that these two types of competition may act synergistically to deprive toothcarp of food at higher temperatures. Males of both species carry out more overtly aggressive acts than females, which is probably related to the marked sexual dimorphism and associated mating systems of these two species. Mosquitofish may thus impact heavily on toothcarp, and competition from mosquitofish, especially in warmer summer months, may lead to changes in abundance of the native species and displacement to non-preferred habitats. Globally increasing temperatures mean that highly invasive, warm-water mosquitofish may be able to colonize environments from which they are currently excluded through reduced physiological tolerance to low temperatures. Research into the effects of temperature on interactions between native and invasive species is thus of fundamental importance

INTERFERENCE PERIODS AMONG WEEDS AND SOYBEAN RR TM CROPS IN THE WESTERN CENTER AREA OF THE BRAZILIAN STATE OF PARANÁ

ABSTRACT One of the factors that can influence soybeans yield is the interference imposed by weeds. This research has aimed to determine the critical period of weed interference on cv. INT 6100 RRTM soybeans. The experiment was conducted under field conditions at Campo Mourão County, Parana State, in the 2013/2014 harvest, using randomized blocks, arranged in a 2 x 8 factorial, with four replications. In the first factor, the coexistence (period before weed interference) and control (total period of weed interference prevention) periods were assessed. The second factor consisted of management times of weed species (0, 7, 14, 28, 35, 49, 56 and 130 days after emergence - DAE). The evaluations performed were density and shoot dry matter of the weed community, height, number of pods, thousand grain weight and soybean yield. Among the weed species in soybean crops, there was predominance of eudicotyledonous ones (82%). The yield results allowed establishing, for cv. INT 6100 RRTM soybeans at Campo Mourão County, Parana State, a critical period for preventing interference between 24-38 DAE.

EFFECTS of MINERAL NUTRITION on INTER- and INTRASPECIFIC INTERFERENCE of PEANUT (Arachis hypogaea L.) and HAIRY BEGGARTICKS (Bidens pilosa L.)

The present work was carried out to study the effects of mineral nutrition on peanut (Arachis hypogaea L) cv. IAC Runner-886 and hairy beggarticks (Bidens pilosa L) growth, when submitted to inter- and intraspecific competition. The treatments consisted of two peanut plants per pot, two hairy beggarticks per pot and one plant of each species per pot. The plants were nourished with Hoagland and Arnon (1950) complete solution, or without potassium, or without phosphorus or without nitrogen. Sixty days after planting, no inter- or intraspecific competition effect on growth characteristics of peanut was verified and nutrition was not a limiting factor to the culture. No interaction between competition and nutrition effects was observed for both species. The weed suffered more negative effects from intraspecific competition and nutrition. The absence of N had a pronounced effect compared to the other elements, resulting in a reduction in all the evaluated characteristics. The deficiency of nutrients and competition affected the weed more than the crop, showing that peanut was more competitive than hairy beggarticks.

Competition and fixation of cohorts of adaptive mutations under Fisher geometrical model

One of the simplest models of adaptation to a new environment is Fisher's Geometric Model (FGM), in which populations move on a multidimensional landscape defined by the traits under selection. The predictions of this model have been found to be consistent with current observations of patterns of fitness increase in experimentally evolved populations. Recent studies investigated the dynamics of allele frequency change along adaptation of microbes to simple laboratory conditions and unveiled a dramatic pattern of competition between cohorts of mutations, i.e., multiple mutations simultaneously segregating and ultimately reaching fixation. Here, using simulations, we study the dynamics of phenotypic and genetic change as asexual populations under clonal interference climb a Fisherian landscape, and ask about the conditions under which FGM can display the simultaneous increase and fixation of multiple mutations-mutation cohorts-along the adaptive walk. We find that FGM under clonal interference, and with varying levels of pleiotropy, can reproduce the experimentally observed competition between different cohorts of mutations, some of which have a high probability of fixation along the adaptive walk. Overall, our results show that the surprising dynamics of mutation cohorts recently observed during experimental adaptation of microbial populations can be expected under one of the oldest and simplest theoretical models of adaptation-FGM.