Positive Effect Of Density Research Articles

Density effects on native and non‐native trout survival in streams

AbstractEnvironmental stressors associated with a changing climate and non‐native fish, individually, represent significant threats to native fish conservation. These threats can exacerbate risks to native fishes when conditions interact at the trailing edge of a population's distribution. We collected capture–mark–recapture data for Rio Grande cutthroat trout (RGCT, Oncorhynchus clarkii virginalis) at the trailing edge of all cutthroat trout distributions from eight northern New Mexico populations. We used a factorial sampling design from streams characterised as “cool” or “warm” and whether RGCT were sympatric with non‐native brown trout (Salmo trutta). We tested competing hypotheses that warm temperatures, reduced flows, high densities and sympatry with brown trout would negatively impact RGCT apparent survival rates. We found the strongest evidence for a non‐native trout interaction with total trout density affecting RGCT apparent survival rates. Our results are consistent with patterns observed in northern cutthroat trout populations where non‐native salmonids negatively impacted apparent survival rates. We also found that a negative density effect was observed on allopatric RGCT and sympatric brown trout apparent survival, but a positive density effect was observed for sympatric RGCT. These results suggest higher density populations of RGCT may be more resilient to displacement by non‐native trout than low‐density populations.

Compact living or policy inaction? Effects of urban density and lockdown on the COVID-19 outbreak in the US.

The coronavirus pandemic has reignited the debate over urban density. Popular media has been quick to blame density as a key contributor to rapid disease transmission, questioning whether compact cities are still a desirable planning goal. Past research on the density-pandemic connection have produced mixed results. This article offers a critical perspective on this debate by unpacking the effects of alternative measures of urban density, and examining the impacts of mandatory lockdowns and the stringency of other government restrictions on cumulative Covid-19 infection and mortality rates during the early phase of the pandemic in the US. Our results show a consistent positive effect of density on Covid-19 outcomes across urban areas during the first six months of the outbreak. However, we find modest variations in the density-pandemic relationship depending on how densities are measured. We also find relatively longer duration mandatory lockdowns to be associated with lower infection and mortality rates, and lockdown duration's effect to be relatively more pronounced in high-density urban areas. Moreover, we find that the timing of lockdown imposition and the stringency of the government's response additionally influence Covid-19 outcomes, and that the effects vary by urban density. We argue that the adverse impact of density on pandemics could be mitigated by adopting strict lockdowns and other stringent human mobility and interaction restriction policies in a spatially targeted manner. Our study helps to inform current and future government policies to contain the virus, and to make our cities more resilient against future shocks and threats.

A Note on the Role of Cultural, Institutional, and Urbanization Features in the COVID-19 Pandemic*

A Note on the Role of Cultural, Institutional, and Urbanization Features in the COVID-19 Pandemic*

The impact of density, distance and division on intellectual property rights protection: the case of Huaihai Economic Zone, China

ABSTRACT Although intellectual property rights (IPRs) protection is often discussed at national level, it may also present spatial difference at sub-national level due to regional economic development inequality in geographic space. The paper explores the empirical impact of 3Ds (Density, Distances and Division) framework of the 2009 World Development Report on spatial heterogeneity of IPRs protection within 20 cities in Huaihai Economic Zone (HEZ), China. Endogeneity and spatial effect issues are discussed in the study. This study not only employs OLS to examine the effects of spatial heterogeneity on IPRs protection, but also uses two-stage least squares regression (2SLS) and spatial effects to address endogeneity and neighbour issues respectively. Econometric results indicate that distance plays an important role in disparities of IPRs protection, especially distances to provincial capitals. However, the role of Xuzhou, the central city of HEZ, is not highlighted. The positive effect of density can help us understand IPRs protection and its significant role proves the existence of agglomeration, but due to the small sample size, we have to treat its effect carefully. Meanwhile, the results on division demonstrate the negative impact of foreign direct investment (FDI) and the existence of low quality FDI in Huaihai Economic Zone. In addition, the above conclusions continue to be held by interaction variables, and time dummies indicate that HEZ has been committed to the development of IPRs protection in recent years. Finally, these results have a strong policy implication for economic agglomeration, economic integration and central city development in HEZ.

Effects of an eco-silica source based activator on functional alkali activated lightweight composites

Abstract In this paper, alkali activated slag-fly ash lightweight composites with moderate densities between around 1200 and 1500 kg/m3 are prepared and characterized. An eco-olivine nano-silica is applied to prepare sustainable silicate based activators to replace commercial sodium silicates. Na2O contents of 2.0, 3.5 and 5.0 wt% are investigated in order to reach a suitable balance between performance, costs and application. The results show the positive effect of density and Na2O content on strength, while strength increment between Na2O dosage of 3.5% and 5.0% is limited. The reduction of Na2O content shows a dramatic delay of the reaction process up to around 3 d, but shows negligible effect on the typical Si-O bonds. An increased Na2O content benefits the formation of reaction products, including the contents of hydrotalcite and carbonates. Besides, the thermal conductivity and acoustical absorption properties of the lightweight products are characterized; the phase transition zones between lightweight aggregate and binder matrix are evaluated by SEM. The calculation on the carbon footprint shows an evident advantage of using alkali activated materials to replace Portland cement, also the utilization of olivine nano-silica further reduces the carbon emission of the activator by around 25%.

Data-driven competitive facilitative tree interactions and their implications on nature-based solutions

Spatio-temporal data are more ubiquitous and richer than even before and the availability of such data poses great challenges in data analytics. Ecological facilitation, the positive effect of density of individuals on the individual's survival across a stress gradient, is a complex phenomenon. A large number of tree individuals coupled with soil moisture, temperature, and water stress data across a long temporal period were followed. Data-driven analysis in the absence of hypothesis was performed. Information theoretic analysis of multiple statistical models was employed in order to quantify the best data-driven index of vegetation density and spatial scale of interactions. Sequentially, tree survival was quantified as a function of the size of the individual, vegetation density, and time at the optimal spatial interaction scale. Land surface temperature and soil moisture were also statistically explained by tree size, density, and time. Results indicated that in space both facilitation and competition co-exist in the same ecosystem and the sign and magnitude of this depend on the spatial scale. Overall, within the optimal data-driven spatial scale, tree survival was best explained by the interaction between density and year, sifting overall from facilitation to competition through time. However, small sized trees were always facilitated by increased densities, while large sized trees had either negative or no density effects. Tree size was more important predictor than density in survival and this has implications for nature-based solutions: maintaining large tree individuals or planting species that can become large-sized can safeguard against tree-less areas by promoting survival at long time periods through harsh environmental conditions. Large trees had also a significant effect in moderating land surface temperature and this effect was higher than the one of vegetation density on temperature.

Harvesting a red-listed species: determinant factors for willow ptarmigan harvest rates, bag sizes, and hunting efforts in Norway

A main objective of game management on public lands in Norway is to ensure that harvest levels are sustainable while securing hunting access to the public. The willow ptarmigan is a popular but red-listed small game species, and it is challenging for managers to optimize harvest levels based on uncertain population estimates and limited knowledge of the factors influencing hunting impact. We studied how willow ptarmigan hunting effort, catch per unit effort, and harvest rates were influenced by ptarmigan density, topography, vegetation, and infrastructure. Data were collected during 2013–2016 from 162 hunting blocks covering 26,828 km2 state-owned land in the counties Nordland and Troms, northern Norway. Hunting effort averaged 1.27 days/km2 (SD = 2.08), and it was the highest in areas close to roads and cabins. We found an opposite effect on the catch per unit effort (CPUE, mean = 0.77 ptarmigan/day, SD = 0.48), which was lower close to roads. There was a marked positive effect of density on CPUE, whereas terrain steepness (slope) had a negative effect. On average, harvest removed < 10% of the autumn population in the hunting areas during the study period. This indicates a relatively low and partially compensatory hunting mortality in most years and areas. Although this study shows a low harvest rate, we recommend managers to survey ptarmigan populations, hunting effort, and harvest, especially in easily accessible areas close to infrastructure and in periods of low population density.

Density-dependent dispersal strategies in a cooperative breeder.

Dispersal is a key ecological process that influences the dynamics of spatially and socially structured populations and consists of three stages-emigration, transience, and settlement-and each stage is influenced by different social, individual, and environmental factors. Despite our appreciation of the complexity of the process, we lack a firm empirical understanding of the mechanisms underlying the different stages. Here, using data from 65 GPS-collared dispersing female coalitions of the cooperatively breeding meerkat (Suricata suricatta), we present a comprehensive analysis of the effects of population density, mate availability, dispersing coalition size, and individual factors on each of the three stages of dispersal in a wild population. We expected a positive effect of density on dispersal due to increased kin competition at high densities. We further anticipated positive effects of mate availability, coalition size, and body condition on dispersal success. We observed increasing daily emigration and settlement probabilities at high population densities. In addition, we found that emigration and settlement probabilities also increased at low densities and were lowest at medium densities. Daily emigration and settlement probabilities increased with increasing female coalition size and in the presence of unrelated males. Furthermore, the time individuals spent in the transient stage increased with population density, whereas coalition size and presence of unrelated males decreased dispersal distance. The observed nonlinear relationship between dispersal and population density is likely due to limited benefits of cooperation at low population densities and increased kin competition at high densities. Our study provides empirical validation for the theoretical predictions that population density is an important factor driving the evolution of delayed dispersal and philopatry in cooperative breeders.

Influence of the microstructure on the subsurface and surface damage during lubricated rolling–sliding wear of sintered and sinterhardened 1.5%Mo–2%Cu–0.6%C steel: theoretical analysis and experimental investigation

ABSTRACTThe influence of the microstructure on density on the resistance to subsurface and surface damage, during lubricated rolling–sliding wear of a 1.5%Mo–2%Cu–0.65%C sintered and sinterhardened steel, was investigated. The nucleation of subsurface contact fatigue cracks and the occurrence of surface plastic deformation due to surface stresses were studied through a theoretical analysis and confirmed through wear tests. The expected positive effect of density and sinterhardening was confirmed. Moreover, the resistance of the steel to the two damage mechanisms investigated depends on the microstructural heterogeneity. The theoretical model for subsurface and for surface damage may be used to predict the resistance of the material but, depending on the distribution of the microstructural constituents, either a local or a mean approach in defining the properties of the metallic matrix has to be used.

Variation in Allee effects: evidence, unknowns, and directions forward

AbstractAllee effects, positive effects of population size or density on per‐capita fitness, are of broad interest in ecology and conservation due to their importance to the persistence of small populations and to range boundary dynamics. A number of recent studies have highlighted the importance of spatiotemporal variation in Allee effects and the resulting impacts on population dynamics. These advances challenge conventional understanding of Allee effects by reframing them as a dynamic factor affecting populations instead of a static condition. First, we synthesize evidence for variation in Allee effects and highlight potential mechanisms. Second, we emphasize the “Allee slope,” i.e., the magnitude of the positive effect of density on the per‐capita growth rate, as a metric for demographic Allee effects. The more commonly used quantitative metric, the Allee threshold, provides only a partial picture of the underlying forces acting on population growth despite its implications for population extinction. Third, we identify remaining unknowns and strategies for addressing them. Outstanding questions about variation in Allee effects fall broadly under three categories: (1) characterizing patterns of natural variability; (2) understanding mechanisms of variation; and (3) implications for populations, including applications to conservation and management. Future insights are best achieved through robust interactions between theory and empiricism, especially through mechanistic models. Understanding spatiotemporal variation in the demographic processes contributing to the dynamics of small populations is a critical step in the advancement of population ecology.

Positive density-dependent reproduction regulated by local kinship and size in an understorey tropical tree.

Global pollinator declines and continued habitat fragmentation highlight the critical need to understand reproduction and gene flow across plant populations. Plant size, conspecific density and local kinship (i.e. neighbourhood genetic relatedness) have been proposed as important mechanisms influencing the reproductive success of flowering plants, but have rarely been simultaneously investigated. We conducted this study on a continuous population of the understorey tree Miconia affinis in the Forest Dynamics Plot on Barro Colorado Island in central Panama. We used spatial, reproductive and population genetic data to investigate the effects of tree size, conspecific neighbourhood density and local kinship on maternal and paternal reproductive success. We used a Bayesian framework to simultaneously model the effects of our explanatory variables on the mean and variance of maternal viable seed set and siring success. Our results reveal that large trees had lower proportions of viable seeds in their fruits but sired more seeds. We documented differential effects of neighbourhood density and local kinship on both maternal and paternal reproductive components. Trees in more dense neighbourhoods produced on average more viable seeds, although this positive density effect was influenced by variance-inflation with increasing local kinship. Neighbourhood density did not have significant effects on siring success. This study is one of the first to reveal an interaction among tree size, conspecific density and local kinship as critical factors differentially influencing maternal and paternal reproductive success. We show that both maternal and paternal reproductive success should be evaluated to determine the population-level and individual traits most essential for plant reproduction. In addition to conserving large trees, we suggest the inclusion of small trees and the conservation of dense patches with low kinship as potential strategies for strengthening the reproductive status of tropical trees.

Influence of larval density on some biological and demographic aspects of Chrysomya megacephala (Diptera: Calliphoridae).

The objective of this work was to study the influence of larval density on some aspects of the population ecology & survival of Chrysomya megacephala, analyzing demographic aspects of adults kept under experimental conditions. A positive effect of density on survival and pre-ovipostion period was observed, while no simultaneous influence of density in sex ratio. The results showed that the demographic parameter for life expectancy at intrinsic growth rate (e0), total fecundity (TF), net fecundity (NF) and men reproductive age (T) increased with increasing the density of population larvae. Other population parameters such as the intrinsic growth rate (r), the finite growth rate (λ) and the time to double population (DT) were also calculated.

Organizational Form Emergence

In this study, the authors provide an assessment of the ecological theory of organizational form emergence and focus on the positive density effect associated with legitimation. The argument comes in two steps. First, organizational ecology seeks to understand cross-population similarities in search of general patterns in form emergence processes. Using summative meta-analysis, the authors show that the average effect of density dependence on population-level legitimation is positive, but this average effect hides large variation across different populations. Second, in the spirit of recent revisions of this theory, the authors introduce two concepts that can be linked to industries or populations to explain this unaccounted-for variation: perceived simplicity of organizational goals and tangibility of offerings. Using formative meta-analysis, the study reveals that both population-level characteristics increase legitimation. Density effects on organizational founding rates tend to be stronger in manufacturing and for-profit industries, which are arguably settings featuring higher simplicity of goals and larger tangibility of offerings, respectively. On the basis of this set of findings, the authors conclude with a plea for a population-level theory of ecological differences, developing a comparative research strategy that is distinct from the current emphasis on single-population studies.

Growth networks

We map the relationship between products in global trade and the products a country exports as a network to devise a measure of the density of links between the products in a country's export basket and a measure of network proximity from a country's export basket to products that a country does not export. The density measure is a proxy for synergies between the products in a country's export basket. The network proximity measure is an indicator of how difficult it is likely to be for a given country to move from its current product specialization to new products. We find that density and network proximity are together of importance for a poor country to move to higher income products and experience higher growth rates. Higher network proximity is associated with a greater likelihood of experiencing growth acceleration, but the positive effect of density tapers off at higher values.

Simultaneous positive and negative density‐dependent dispersal in a colonial bird species

Contradictory patterns of density-dependent animal dispersal can potentially be reconciled by integrating the conspecific attraction hypothesis with the traditional competition hypothesis. We propose a hypothesis that predicts a U-shaped relationship between density and both natal and breeding dispersal distance. Using 10 years of observations on a breeding colony of the Blue-footed Booby (Sula nebouxii), the hypothesis was confirmed by documenting simultaneous positive and negative density-dependent dispersal distances in natal and breeding dispersal of males and breeding dispersal of females within the colony. Point-pattern analyses demonstrated that the breeding sites of Blue-footed Boobies were highly aggregated in all years within a large study area, and aggregation presumably resulted in heterogeneity in patch density throughout the colony. As predicted, at moderate to high densities, dispersal distances showed positive density dependence, with individuals moving to lower density patches. In contrast, at low to moderate densities, dispersal distances showed negative density dependence, with individuals moving to higher density patches. In both sexes of the 1994 cohort, the higher the mean density in patches used by an individual over the long term (up to age 11 years), the fewer fledglings it produced. A positive effect of density on long-term reproductive success was not detected, possibly because birds that failed during pair formation or incubation were not sampled. Density of conspecifics may be an important influence on habitat selection of breeders, and dispersal may tend to carry individuals to patches where pair formation opportunities are better and negative effects of competition on reproductive success are reduced.

Density effects at multiple scales in an experimental plant population

1 In sessile organisms, each individual in a population experiences a different local conspecific density, and this density itself will vary depending on the scale at which it is measured (converging on a 'mean field' population density only at coarse scales). 2 The effects of density on any ecological process (e.g. competition, herbivory, mortality) may therefore be scale-specific, and different processes may respond to density at different spatial scales. 3 A field experiment was set up in which focal Silene latifolia plants were subjected to a range of densities at each of four spatial scales (squares with sides of 16 cm, 80 cm, 4 m and 20 m); data were collected on growth, herbivory, flowering and survival. The Akaike information criterion was used to determine which scale displayed the strongest relationship between density and each response variable. 4 The variables showed peak responses to density at different scales and with various relationships (positive, negative and non-monotonic). Some (e.g. herbivory by mammals) showed contrasting responses at different scales. 5 Growth rate showed surprisingly little response to conspecific density, presumably owing to balancing effects of intra-specific and inter-specific competition from the surrounding vegetation. However, 25% of variation in first-year survival was explained by a model including a positive effect of density at the 80-cm scale. 6 Herbivory was mainly due to slugs and showed negative responses to density at the 4-m and 20-m scales, in accordance with hypotheses about resource limitation, mobility and spatial scaling. Mammal herbivory responded with contrasting patterns at the 80-cm and 20-m scales. Congruence between the responses of herbivory and survival suggest that herbivory was an important cause of mortality. 7 Plant population patterns may thus reflect density effects interacting at different characteristic scales. Generalizations about density-dependence in spatially structured populations may be misleading if they fail to recognize that density is scale-dependent.

Electron mobility in dense argon gas at several temperatures

The mobility /spl mu/ of excess electrons in dense Argon gas was measured as a function of the applied electric field E and of the gas density N at several temperatures in the range 142.6 T/sub c/, shows the well known anomalous positive density effect, i.e., /spl mu//sub 0/N increases with increasing N. Below T,, however, /spl mu//sub 0/N does not show the expected effect, but features a broad maximum. This appears to be a crossover behavior between the positive density effect shown for T > T, and the small negative effect previously observed for T /spl ap/ 90 K However, the data at all temperatures confirm the interpretation of the anomalous density effect as being essentially due by the density-dependent quantum shift of the electron ground state kinetic energy in a disordered medium as a result of multiple scattering (MS) processes, although other MS processes influence the experimental outcome.

Effects of density on the vital rates of a modular seaweed

The effects of density on the vital rates of the modular seaweed Ascophyllum nodosum (L.) Le Jol. were investigated in 2 areas of the northern Atlantic Ocean: the Isle of Man, in the Irish Sea, and Tjarno, on the Swedish west coast. The densities of plants (genets) were manipulated in the field. The impact of density on survival, growth and reproduction of plants >2 cm was estimated over 1y r. Percentage survival of A. nodosum was high and apparently density-independent. This result is in accordance with other studies in land and marine systems where low mortality after establishment appears common and overpopulation is avoided by a balance in module density. The growth of A. nodosum was negatively density-dependent. The response of the modules (shoots) within plants was asymmetrical. Furthermore, the inception of new shoots was enhanced at low densities on the Isle of Man. Apart from intraspecific competition, other density-dependent processes affected the perfor- mance of the alga: the probability of breakage to smaller sizes increased with density in the Swedish area. As a result, the impact of density on the net plant growth was more patent at Tjarno than on the Isle of Man. In relation to reproduction, the results of this study suggest a positive effect of density on the annual reproductive effort of the alga. Nevertheless, the manipulation was carried out after the initiation of reproductive structures. Increasing density clearly reduced plant growth, and reproduc- tion is size-dependent in A. nodosum. This indicates that, in successive reproductive periods, repro- duction may be negatively affected by density. In contrast to the results of another previous work, our study shows a negative impact of density on the net growth of A. nodosum and indicates the impor- tance of module length structure in the demographical processes of the species. Not only the effect of density but also the significance of density dependence in the dynamics of the species probably vary in these areas with very different physical domains.

Modelling and prediction of travel distance by car

A theoretical basis and empirical modelling and prediction results for Swedish travelling distances by car are given. The results form the basis for an assessment of the economic consequences of Swedish petrol price adjustments by region and travel type. Truncated regression models are estimated using a general distribution class specification and a robust estimator based on symmetric trimming. Predictors and prediction variances are given. The study confirms the hypothesis of a positive income effect for work and business travel distances. The effect is negative and significant for recreation distances. Travel distances tend to be shorter in less densely populated regions, i.e. a positive density effect. Males have significantly longer work and shopping trips. The differences between estimators and corresponding predictions are small. The financial support from the Swedish Transport Research Board (Dnr 21/88–82) is gratefully acknowledged.

The effects of prostaglandins on the beating activity of cultured heart cells

The effects on the beating behavior of cultured rat heart cells of fourteen prostaglandins of the A, B, D, E, and F series were investigated, together with adrenaline and ouabain, at dose levels of 10 −7 and 10 −5M. Single heart cell beating activity was monitored photo-electrically and five parameters of beating behavior measured. Only PGF 2a markedly increased rate while PGF 2B reduced it. Maintenance of a stable rate (rate range) was minimally affected by prostaglandins with PGF 2β possibly reducing, and PGF 1α and 2-decarboxy E 1 possibly increasing, rate range. PGF 1α and F 2α both statistically reduced the percentage of cells beating while the other prostaglandins had no effect. Most of the prostaglandins either produced no change, or reduced, indices of contractile force (optical density changes with contractions and its first derivative (dOD/dt)). Only the negative chronotropic agent PGF 2β positive density effect. In conclusion, except for PGF 2α, prostaglandins generally have limited actions on the beating activity of cultured heart cells.