Extinction Threshold Research Articles

On the Reproduction Number of a Gut Microbiota Model.

A spatially structured linear model of the growth of intestinal bacteria is analysed from two generational viewpoints. Firstly, the basic reproduction number associated with the bacterial population, i.e. the expected number of daughter cells per bacterium, is given explicitly in terms of biological parameters. Secondly, an alternative quantity is introduced based on the number of bacteria produced within the intestine by one bacterium originally in the external media. The latter depends on the parameters in a simpler way and provides more biological insight than the standard reproduction number, allowing the design of experimental procedures. Both quantities coincide and are equal to one at the extinction threshold, below which the bacterial population becomes extinct. Optimal values of both reproduction numbers are derived assuming parameter trade-offs.

Fundamental properties of cooperative contagion processes

We investigate the effects of cooperativity between contagion processes that spread and persist in a host population. We propose and analyze a dynamical model in which individuals that are affected by one transmissible agent A exhibit a higher than baseline propensity of being affected by a second agent B and vice versa. The model is a natural extension of the traditional susceptible-infected-susceptible model used for modeling single contagion processes. We show that cooperativity changes the dynamics of the system considerably when cooperativity is strong. The system exhibits discontinuous phase transitions not observed in single agent contagion, multi-stability, a separation of the traditional epidemic threshold into different thresholds for inception and extinction as well as hysteresis. These properties are robust and are corroborated by stochastic simulations on lattices and generic network topologies. Finally, we investigate wave propagation and transients in a spatially extended version of the model and show that especially for intermediate values of baseline reproduction ratios the system is characterized by various types of wave-front speeds. The system can exhibit spatially heterogeneous stationary states for some parameters and negative front speeds (receding wave fronts). The two agent model can be employed as a starting point for more complex contagion processes, involving several interacting agents, a model framework particularly suitable for modeling the spread and dynamics of microbiological ecosystems in host populations.

Continental-scale, data-driven predictive assessment of eliminating the vector-borne disease, lymphatic filariasis, in sub-Saharan Africa by 2020

BackgroundThere are growing demands for predicting the prospects of achieving the global elimination of neglected tropical diseases as a result of the institution of large-scale nation-wide intervention programs by the WHO-set target year of 2020. Such predictions will be uncertain due to the impacts that spatial heterogeneity and scaling effects will have on parasite transmission processes, which will introduce significant aggregation errors into any attempt aiming to predict the outcomes of interventions at the broader spatial levels relevant to policy making. We describe a modeling platform that addresses this problem of upscaling from local settings to facilitate predictions at regional levels by the discovery and use of locality-specific transmission models, and we illustrate the utility of using this approach to evaluate the prospects for eliminating the vector-borne disease, lymphatic filariasis (LF), in sub-Saharan Africa by the WHO target year of 2020 using currently applied or newly proposed intervention strategies.Methods and ResultsWe show how a computational platform that couples site-specific data discovery with model fitting and calibration can allow both learning of local LF transmission models and simulations of the impact of interventions that take a fuller account of the fine-scale heterogeneous transmission of this parasitic disease within endemic countries. We highlight how such a spatially hierarchical modeling tool that incorporates actual data regarding the roll-out of national drug treatment programs and spatial variability in infection patterns into the modeling process can produce more realistic predictions of timelines to LF elimination at coarse spatial scales, ranging from district to country to continental levels. Our results show that when locally applicable extinction thresholds are used, only three countries are likely to meet the goal of LF elimination by 2020 using currently applied mass drug treatments, and that switching to more intensive drug regimens, increasing the frequency of treatments, or switching to new triple drug regimens will be required if LF elimination is to be accelerated in Africa. The proportion of countries that would meet the goal of eliminating LF by 2020 may, however, reach up to 24/36 if the WHO 1% microfilaremia prevalence threshold is used and sequential mass drug deliveries are applied in countries.ConclusionsWe have developed and applied a data-driven spatially hierarchical computational platform that uses the discovery of locally applicable transmission models in order to predict the prospects for eliminating the macroparasitic disease, LF, at the coarser country level in sub-Saharan Africa. We show that fine-scale spatial heterogeneity in local parasite transmission and extinction dynamics, as well as the exact nature of intervention roll-outs in countries, will impact the timelines to achieving national LF elimination on this continent.

The Role of Local Governments in Conservation of Endemic Species: Case of Taurus Frog (Rana Holtzi)

All living and non-living things are encompassed by the environment. Local governments play importantrole in conservation of environment. It is the responsibility of local governments to ensure a safe andlivable environment for all living things. However, some living species have come to the point ofextinction as a result of some people's unconscious behaviors. One of these living species is the TaurusFrog living in Taurus Mountains in Nigde City of Turkey. This study emphasizes the role and importanceof local governments in conservation of the Taurus Frog (Rana Holtzi), one of endemic species.

The PM2.5 threshold for aerosol extinction in the Beijing megacity

Particulate pollution has remained at a high level in the megacity of Beijing in the past decade. The PM2.5, PM10, aerosol optical depth (AOD), Angstrom exponent(α), and PM2.5/PM10 ratio (the proportion of PM2.5 in PM10) in Beijing were 70±6 μg m−3, 128±6 μg m−3, 0.57 ± 0.05, 1.10 ± 0.08, 45 ± 4%, respectively, from 2005 to 2014. The annual means of PM concentration, AOD, α, and PM2.5/PM10 ratio decreased slightly during this decade, meanwhile PM concentration increased in the winter. Furthermore, we found there were thresholds of PM2.5 concentration for aerosol extinction. When the PM concentration was lower than a certain threshold, AOD decreased quickly with the decline of PM concentration. To make the improvement of the particle pollution more noticeable, the PM concentration should be controlled under the threshold. The annual averaged threshold is 63 μg m−3, and the threshold values reached the maximum of 74 μg m−3 in spring, ranged from 54 to 56 μg m−3 in the three other seasons. The threshold values ranged from 55 to 77 μg m−3 under other relevant factors, including air masses directions and relative humidity.

Informing decisions on an extremely data poor species facing imminent extinction

AbstractSome of the species that are believed to have the highest probability of extinction are also amongst the most poorly known, and this makes it extremely difficult to decide how to spend scarce resources. Assessments of conservation status made on the basis of loss or degradation of habitat and lack of records may provide compelling indications of a decline in geographical range and population size, but they do not help identify where conservation action might be best targeted. Methods for assessing the probability of extinction and for modelling species’ distributions exist, but their data requirements often exceed the information that is available for some of the most urgent conservation cases. Here we use all available information (localities, expert information, climate and landcover) about a high-priority Vietnamese bird species (Edwards's pheasant Lophura edwardsi) to assess objectively the probability of its persistence, and where surveys or other conservation action should be targeted. It is clear that the species is on the threshold of extinction and there is an urgent need to survey Bach Ma National Park (including the extension) and to consider surveying Ke Go Nature Reserve. This approach has potential to help identify where conservation action should be targeted for other Critically Endangered species for which there is an extreme scarcity of information.

Quantification of combustion regime transitions in premixed turbulent DME flames

The current study quantifies the probability of encountering up to five fluid states (reactants, combustion products, mixing fluid, fluids with low and high reactivity) in premixed turbulent DME flames as a function of the Damköhler number. The flames were aerodynamically stabilised in a back-to-burnt opposed jet configuration featuring fractal grid generated multi-scale turbulence (Re≃ 18,400 and Ret > 370). The chemical timescale was varied via the mixture stoichiometry resulting in a wide range of Damköhler numbers (0.08 ≤ Da ≤ 5.6). The mean turbulent strain (≥ 3200 s−1) exceeded the extinction strain rate of the corresponding laminar flames for all mixtures. Simultaneous Mie scattering, OH-PLIF and PIV were used to identify the fluid states and supporting computations show that the thermochemical state (e.g. OH and CH concentrations) at the twin flame extinction point correlates well with flames in the back-to-burnt geometry at the corresponding rate of heat release. For mixtures where the bulk strain (≃ 750 s−1) was similar to (or less than) the extinction strain rate, fluids with low and high reactivity could accordingly be segregated by a threshold based on the OH concentration at the extinction point. A sensitivity analysis of the distribution between the fluid states was performed. The flow conditions were further analysed in terms of Damköhler and Karlovitz numbers. The study provides (i) the evolution of multi-fluid probability statistics as a function of the Damköhler number, including (ii) the flow direction across fluid interfaces and OH gradients, (iii) mean flow field statistics, (iv) conditional velocity statistics and (v) a tentative combustion regime classification.

Evaluating the local habitat history deepens the understanding of the extinction debt for endangered plant species in semi-natural grasslands

Habitat losses occur non-randomly within human-modified landscapes, resulting in high spatial heterogeneity of local habitat histories. Although local habitat history can modulate the existence of extinction debt (i.e., the number of populations predicted to become extinct) in a landscape, its role in detecting extinction debt has not been examined explicitly. We aimed to compare the detectability of extinction debt among populations of an endangered semi-natural grassland species, Echinops setifer (Compositae), in the grassland landscape of Mt. Aso, Japan. We classified populations into three groups that differed in local habitat history: stable (habitat loss ≤30% since the 1930s), moderately decreased (30% 90%). We then evaluated whether the effects of habitat areas during the 1930s and 2000s varied among groups to explain population size by GLMMs and estimated coefficient of explanatory variable by Bayesian MCMC methods. Within the groups, stable group showed significant positive relationships with both past and current habitat areas. The moderately decreased group only showed significant positive relationships with past habitat areas, indicating the existence of extinction debt in these populations. The severely decreased group only showed significant positive relationships with current habitat areas, indicating that they may have already paid their extinction debt because the rate of grassland loss exceeded the extinction threshold. Even within the same landscape, extinction debt varied in response to local habitat history. In spatially heterogeneous landscapes, evaluation of effects of local habitat history can elucidate the habitat-based extinction risks for plant populations.

Threshold for extinction and survival in stochastic tumor immune system

This paper mainly investigates the stochastic character of tumor growth and extinction in the presence of immune response of a host organism. Firstly, the mathematical model describing the interaction and competition between the tumor cells and immune system is established based on the Michaelis-Menten enzyme kinetics. Then, the threshold conditions for extinction, weak persistence and stochastic persistence of tumor cells are derived by the rigorous theoretical proofs. Finally, stochastic simulation are taken to substantiate and illustrate the conclusion we have derived. The modeling results will be beneficial to understand to concept of immunoediting, and develop the cancer immunotherapy. Besides, our simple theoretical model can help to obtain new insight into the complexity of tumor growth.

Ecological and genetic basis of metapopulation persistence of the Glanville fritillary butterfly in fragmented landscapes

Ecologists are challenged to construct models of the biological consequences of habitat loss and fragmentation. Here, we use a metapopulation model to predict the distribution of the Glanville fritillary butterfly during 22 years across a large heterogeneous landscape with 4,415 small dry meadows. The majority (74%) of the 125 networks into which the meadows were clustered are below the extinction threshold for long-term persistence. Among the 33 networks above the threshold, spatial configuration and habitat quality rather than the pooled habitat area predict metapopulation size and persistence, but additionally allelic variation in a SNP in the gene Phosphoglucose isomerase (Pgi) explains 30% of variation in metapopulation size. The Pgi genotypes are associated with dispersal rate and hence with colonizations and extinctions. Associations between Pgi genotypes, population turnover and metapopulation size reflect eco-evolutionary dynamics, which may be a common feature in species inhabiting patch networks with unstable local dynamics.

Threshold Dynamics of a Stochastic Chemostat Model with Two Nutrients and One Microorganism

A new stochastic chemostat model with two substitutable nutrients and one microorganism is proposed and investigated. Firstly, for the corresponding deterministic model, the threshold for extinction and permanence of the microorganism is obtained by analyzing the stability of the equilibria. Then, for the stochastic model, the threshold of the stochastic chemostat for extinction and permanence of the microorganism is explored. Difference of the threshold of the deterministic model and the stochastic model shows that a large stochastic disturbance can affect the persistence of the microorganism and is harmful to the cultivation of the microorganism. To illustrate this phenomenon, we give some computer simulations with different intensity of stochastic noise disturbance.

Study of Kashmiri Folklore in current Scenario

Kashmir has a remarkable and rich tradition in folk literature. Kashmiri folklore relates to the socio-cultural, historical and literary context of the Kashmir valley. Folk literature of Kashmir includes spoken, sung and written forms of folk narratives, mythological stories, fairy tale, romantic tales, anecdote, jokes, numskull tale form an essential component of its folklore. However, the contemporary generation has little or no knowledge about folk narratives. The journey of media has seen different eras and phases as the need for information has ever been increasing. It was this need that led to the unification of different media channels that pushed the traditional media nearly to the extinction point. In this era of technology, folk forms offer no diversity to satisfy the needs of the audience. The sources of entertainment and recreation, available to the people, today are abundant. This study looks at the various aspects of Kashmir folklore that bring forth its different dimensions and genres and the impact of technology on folk forms of Kashmir.

Study of ignition and extinction of small-scale fires in experiments with an emulating gas burner

The objective of this study is to explore mechanisms for ignition and extinction for condensed-phase fuels via the use of a gas-fueled burner. Flames were generated with a porous 25mm circular burner using mixtures of methane and propane with nitrogen. The procedure was to specify a set of mass fluxes of nitrogen-fuel mixture that corresponded to the flash- fire- and extinction points and for the minimum mass flux where steady burning was achieved. The results show an increase in the critical mass flux with a decreased heat of combustion. The data fall into two regimes depending on the mixture flow rate; one buoyancy-driven (Fr<1) and one induced by momentum jet forces. The buoyancy-driven regime is geometrically consistent with the definitions of flash and fire points under natural convection conditions. The results for the momentum regime align reasonably with existing stagnant layer theory. Extinction theory is also suggested to give approximate results for the fire point. This argument is based on similar flame geometries for fire point and extinction and theoretical reasoning. An anchor point is proposed as the end point of ignition. Produced anchor point data result in a flammability diagram, below which quasi-steady burning occurs.

High-resolution tide projections reveal extinction threshold in response to sea-level rise.

Sea-level rise will affect coastal species worldwide, but models that aim to predict these effects are typically based on simple measures of sea level that do not capture its inherent complexity, especially variation over timescales shorter than 1year. Coastal species might be most affected, however, by floods that exceed a critical threshold. The frequency and duration of such floods may be more important to population dynamics than mean measures of sea level. In particular, the potential for changes in the frequency and duration of flooding events to result in nonlinear population responses or biological thresholds merits further research, but may require that models incorporate greater resolution in sea level than is typically used. We created population simulations for a threatened songbird, the saltmarsh sparrow (Ammodramus caudacutus), in a region where sea level is predictable with high accuracy and precision. We show that incorporating the timing of semidiurnal high tide events throughout the breeding season, including how this timing is affected by mean sea-level rise, predicts a reproductive threshold that is likely to cause a rapid demographic shift. This shift is likely to threaten the persistence of saltmarsh sparrows beyond 2060 and could cause extinction as soon as 2035. Neither extinction date nor the population trajectory was sensitive to the emissions scenarios underlying sea-level projections, as most of the population decline occurred before scenarios diverge. Our results suggest that the variation and complexity of climate-driven variables could be important for understanding the potential responses of coastal species to sea-level rise, especially for species that rely on coastal areas for reproduction.

The Hippocratic need for adequate supports while merging programs: at first do no harm

In response to a paper calling for the re-engagement of agricultural education with the sciences and science education, this essay is supportive but argues to proceed with caution: one that at first does no harm. I offer a supplementary lens and story of change at Cornell University as a cautionary and motivational tale. I concur with the authors who want to redirect faculty, departments, and societies of agricultural educators to become more tightly aligned with the sciences. The need for and ability to better contribute to the critical discussions on contemporary agriculture issues (e.g., urban farming, GMOs, sustainability, local economies) is timely and important. The trick, however, is to do this without weakening the agricultural education community to the point of extinction. Drawing on Institutional Theory, I offer an enhanced perspective highlighting the importance of resource, conformity, and legitimacy. I explore the nature of competing pressures: the strong and restrictive ties to isolated communities and the pressure to reengage a community with a more socially and intellectually central world. I want to emphasize how these strong ties are typically beneficial to the organizations of interest, despite what it may look like to others outside that sector. It is argued that agricultural education will become increasingly “vulnerable and isolated” should it not cross the street and at least partially connect with contemporary science, social science, and new policy and ethical concerns.

Combustion stability and hetero-/homogeneous chemistry interactions for fuel-lean hydrogen/air mixtures in platinum-coated microchannels

The hetero-/homogeneous combustion and stability limits of fuel-lean hydrogen/air mixtures (equivalence ratio φ=0.40) were investigated numerically in a platinum-coated planar microchannel with a length of 10mm and a height of 1mm. A two-dimensional numerical model was used for both the gas and the solid, which included elementary heterogeneous and homogeneous reaction mechanisms, detailed transport, heat conduction in the solid, surface radiation heat transfer, and external losses via a heat transfer coefficient h. Pressures of 1 and 5bar and solid thermal conductivities ks = 1 and 16Wm-1K−1 were analyzed, while stability maps were constructed in terms of the critical extinction heat transfer coefficient hcr versus inlet velocity UIN (or mass throughput). For a given solid thermal conductivity, there existed a crossover mass throughput above (below) which the stability envelope was broader at 5bar (1bar). Simulations with a surface perfectly stirred reactor (SPSR) model qualitatively reproduced the crossover points, which originated from a shift in the pressure dependence of the catalytic reactivity of hydrogen. For the low solid thermal conductivity ks = 1 Wm-1K−1, a non-monotonic dependence of the stability limits on the mass throughput was shown, with local minima created below the crossover point. The stability limits of hydrogen were solely determined by catalytic chemistry, as it sustained combustion at temperatures down to 320–380K, at which gas-phase chemistry was frozen. Away from the critical extinction points, both catalytic and gas-phase reaction pathways were controlling. The diffusional imbalance of hydrogen, which led to catalytically-induced superadiabatic surface temperatures, and the suppression of the surface superadiabaticity by gaseous chemistry resulted in rich combustion phenomena, such as increasing peak wall temperatures with increasing heat transfer coefficients h. Critical extinction heat transfer coefficients for hydrogen were three to four orders of magnitude higher than those reported for methane and propane fuels in a similar channel geometry.

Resource situation investigation and analysis about endangered Chinese herbal medicine Daphnes Cortex

In order to figure out the status and distribution of the wild and cultivated resources of traditional Chinese medicine Daphnes Cortex, its suitable habitat and endangering factors were analyzed to provide the basis for its rational use, protection and cultivation.Our research group tooka resources survey in Shanxi, Gansu, Sichuan and Qinghai provinces, which include 23 counties. Investigation and sampling investigation combined with interview were carried out. The total reserve of resources was estimated through route-quadrat method in combination with the vegetation and soil-type map area method. The results indicated that there was no obvious change between the present distribution ranges of the wild Daphnes Cortex and its historical records, but the density of the population has undergone major changes. The wild reserves resources has declined seriously, even on the verge of exhaustion in some regions. According to the survey results, the current total reserve of the wild Daphnes Cortex in the four provinces was no more than 600 tons. Simultaneously, we only found the cultivated resource in a mountain at an altitude of about 2 800 m in Kang county of Gansu province, which cropping scope was about 33 000 m². The cultivated resource can't provide medicinal products at present, because their growing period is too short to have curative effect. Destructive excavation and the longer growth cycle result in a sharp decline of the wild resources reserves, even to the point of extinction. Artificial cultivation of product will become the main source of medicinal resources in the future. Therefore, we must protect its suitable habitat, formulate rational harvesting policy, strengthen the supervision of government departments, collect and establish the germplasm nursery and seed bank. On the basis, we must carry out studies into seed-selecting and breeding as well as rapid propagation and growth technology at once.

Locally dispersing populations in heterogeneous dynamic landscapes with spatiotemporal correlations. I. Block disturbance

Locally dispersing populations are generally favorably affected by increasing the scale of habitat heterogeneity because they can exploit contiguous patches of suitable habitat. Increasing the spatial scale of landscape disturbances (such as by applying a pesticide to control an unwanted species) drives down population density because of reasons including dispersal-limited recolonization and the resulting increase in temporal variability. Here, we examine how population density changes as the spatial scale of landscape disturbance increases: does it increase due to increases in spatial correlations in landscape habitat type, or does it decrease due to the various spatial and temporal effects of larger-scale disturbances? We use simulations, mean field approximations, pair approximations, landscape-improved pair approximations (LIPA), and block probabilities to investigate a model of a locally dispersing species on a dynamic landscape with spatiotemporally structured heterogeneous habitat. Pesticide is applied at a given spatial scale, leaving habitat unsuitable for some time before dissipating and allowing the habitat to revert to a suitable state. We found that increasing the spatial scale of disturbances (while keeping the overall disturbance rate fixed) can increase population density, but generally only when landscape turnover is slow relative to population dynamics and when the population is somewhat close to its extinction threshold. Applying control measures at larger spatial scales may allow them to be more effective with the same overall treatment rate. The optimal spatial strategy for applying disturbances depends on both habitat availability as well as the turnover rate of the control measure being used. For the large-scale habitat dynamics in our model, it is possible to analytically calculate spatial correlations in habitat types over arbitrary scales. However, including exact habitat correlations at the triplet scale but approximating population correlations at that scale still neglects information needed to accurately predict simulation results, showing that larger-scale correlations in the population distribution have an important effect on dynamics.

Evaluating the impacts of wood production and trade on bird extinction risks

Biodiversity loss can be accelerated by human consumption in regions that are far removed from habitat degradation because of economic globalization, but no study has directly quantified the effects of global trade on extinction risks at a global scale with consideration for species differences. We propose a novel biodiversity footprint index based on bird extinction risks to evaluate the effects of global wood production and trade on biodiversity. Using 536 endangered bird species threatened by wood harvesting and logging, we calculated the “quasi-extinction” probabilities, that is, the probabilities that population sizes become lower than an extinction threshold after habitat loss based on initial population sizes and forest habitat loss rates. We then used bilateral wood trade data to link the biodiversity impacts in wood exporters to wood importers. We found that if recent trends in forest cover loss continue until 2100, bird species in Brazil would be the most rapidly and heavily affected by wood production and trade, followed by those in Indonesia; these two countries alone would account for about half of all global bird extinctions. Large-scale wood importers (i.e., China, Japan, and the United States) significantly elevate overseas extinction risks and, simultaneously, reduce domestic impacts, indicating a heavy responsibility of these countries for global biodiversity loss. We also conducted a scenario analysis, which showed that the total projected number of extinct species would not decrease if each country produced the amount of wood materials necessary to meet current consumption levels. This is because bird extinction risks in tropical wood importers, such as Mexico and the Philippines, as well as Japan and China will increase if these countries increase domestic wood production. Our biodiversity footprint index is useful to identify countries whose bird species are highly affected by wood production and trade, and to quantify the role of wood trade in bird species extinctions. Additional scenario analyses are needed to establish effective patterns of wood production and consumption for bird biodiversity conservation.

Study on the combustion limit, near-limit extinction boundary, and flame regimes of low-Lewis-number CH4/O2/CO2 counterflow flames under microgravity

To obtain an accurate and comprehensive understanding on the fuel-lean combustion limit, near-limit extinction boundary and flame regimes, experimental and numerical investigations using premixed counterflow flames were conducted for low-Lewis-number mixtures. Counterflow flame experiments were conducted under microgravity (2.2≦a≦12.9s−1) and under normal gravity (35≦a≦150s−1) using fuel-lean CH4/O2/CO2 mixtures (Le=0.75) where a is the stretch rate. The mole fraction ratio of O2 to CO2 in the mixtures was 0.40. Microgravity experiments at a=2.7–3.2s−1 showed that transitions from planar flames to ball-like flames occurred near extinction for the investigated mixture. In addition to planar flames and ball-like flames, multi-dimensional flames such as cellular flames and twin-curved flames were also observed at a=2.2–5.5s−1 in microgravity experiments. In conjunction with experimental results under normal gravity, experimental flame regimes for overall stretch rates were obtained for the first time and the region where ball-like flames were observed showed a qualitative agreement with our previous study based on the 3-D transient computations with the diffusive-thermal model. Extinction points obtained by microgravity experiments were found to scatter at very low stretch rates where multi-dimensional flames such as ball-like flames and cellular flames were observed, indicating the existence of a flame regime at lower stretch rates leaner than the planar counterflow flame extinction boundary. In addition, flame bifurcation at low stretch rates and at ϕ=0.58 and 0.60 were experimentally observed for the present low-Lewis-number mixture, indicating the validity of the previous computational and theoretical studies on the G-shaped extinction curve of planar counterflow flames.