Extinction Threshold Research Articles

Los centros especiales de empleo en Castilla y León, 2007-2013: efectos de la crisis en función de la personalidad jurídica que adoptan

Este trabajo surge con el propósito de realizar un estudio, económico-financiero, sobre los Centros Especiales de Empleo localizados en Castilla y León, a partir de una clasificación en función de la personalidad jurídica que adoptan y comprobar cómo les puede afectar la crisis económica iniciada a finales de 2007. Se analizan diferentes partidas recogidas en sus estados financieros y se comparan los resultados del análisis con los obtenidos en años anteriores, periodo 2007-2013, al objeto de mostrar una perspectiva más amplia de su tamaño, desarrollo, crecimiento y comportamiento. Para ello se utilizan las siguientes magnitudes económicas: activo total, cifra de negocios y resultados. Se compara la variable empleo con las subvenciones recibidas por los Centros poniendo de manifiesto como la crisis sí les afecta en función de la personalidad jurídica que adoptan, siendo las entidades más perjudicadas las asociaciones y personas físicas, pudiendo incluso ocasionar su desaparición. Además se introduce la cuenta de reversión para los Centros, que mide qué porcentaje de las ayudas públicas recibidas es devuelto a la sociedad.

Strain distribution on material surfaces during combustion regime transitions

A multi-fluid state approach is used to analyse the underlying conditions for burning mode transitions from close to the corrugated flamelet regime to distributed reactions. Turbulent (Ret ≃ 380) premixed DME/air flames were aerodynamically stabilised in a back-to-burnt opposed jet configuration with the Damköhler number varied through the mixture stoichiometry. Simultaneous Mie scattering, OH-PLIF and PIV allowed the delineation of five separate fluid states (reactants, combustion products, mixing fluid, mildly and strongly reacting fluids) with associated material surfaces. The analysis shows self-sustained flames in low strain regions with a collocated flow acceleration for higher Damköhler numbers. By contrast, in highly strained regions (e.g. beyond the twin flame extinction point) the burning mode is governed by the counter-flowing hot combustion products resulting in increased levels of vorticity and an absence of a preferential dilatation direction. The current analysis provides novel insights into combustion regime transitions by means of (i) strain rate statistics conditioned upon material surfaces and (ii) the evolution of fluid state interface probabilities as a function of the Damköhler number. The work further shows (iii) that the combustion mode influences scalar transport and that increased levels of turbulence retards the transition to non-gradient transport.

Temporal Decay in Timber Species Composition and Value in Amazonian Logging Concessions.

Throughout human history, slow-renewal biological resource populations have been predictably overexploited, often to the point of economic extinction. We assess whether and how this has occurred with timber resources in the Brazilian Amazon. The asynchronous advance of industrial-scale logging frontiers has left regional-scale forest landscapes with varying histories of logging. Initial harvests in unlogged forests can be highly selective, targeting slow-growing, high-grade, shade-tolerant hardwood species, while later harvests tend to focus on fast-growing, light-wooded, long-lived pioneer trees. Brazil accounts for 85% of all native neotropical forest roundlog production, and the State of Pará for almost half of all timber production in Brazilian Amazonia, the largest old-growth tropical timber reserve controlled by any country. Yet the degree to which timber harvests beyond the first-cut can be financially profitable or demographically sustainable remains poorly understood. Here, we use data on legally planned logging of ~17.3 million cubic meters of timber across 314 species extracted from 824 authorized harvest areas in private and community-owned forests, 446 of which reported volumetric composition data by timber species. We document patterns of timber extraction by volume, species composition, and monetary value along aging eastern Amazonian logging frontiers, which are then explained on the basis of historical and environmental variables. Generalized linear models indicate that relatively recent logging operations farthest from heavy-traffic roads are the most selective, concentrating gross revenues on few high-value species. We find no evidence that the post-logging timber species composition and total value of forest stands recovers beyond the first-cut, suggesting that the commercially most valuable timber species become predictably rare or economically extinct in old logging frontiers. In avoiding even more destructive land-use patterns, managing yields of selectively-logged forests is crucial for the long-term integrity of forest biodiversity and financial viability of local industries. The logging history of eastern Amazonian old-growth forests likely mirrors unsustainable patterns of timber depletion over time in Brazil and other tropical countries.

Stabilization and extinction of diffusion flames in an inert porous medium

Diffusion flames established in inert porous media have been reported to present temperatures lower than a comparable gaseous mixture. Therefore, the study of the flame structure, temperature and extinction limits of confined diffusion flames is of importance. In the present work we discuss extinction conditions for such flames. Using an asymptotic model that accounts for the excess/deficient enthalpy at the reaction region, we study the multiscale problem and analyze the effects of the heat exchange between gas and solid phases on the flame structure. When the heat removed from the flame by the solid matrix is large, the flame can extinguish because the lowering in the flame temperature leads to increasingly large leakage of reactants through the flame sheet. We show that this occurs when the porosity or the mass injection rate is small enough. The extinction limit associated with a small value of the mass injection rate adds to the kinetic extinction limit (which is associated with a large value of the mass injection rate) to characterize a dual-extinction-point behavior for this problem. When the porosity of the medium reaches a minimum critical value, these two distinct extinction points collapse, such that for porosities lower than the critical porosity no flame can be established inside the porous chamber. Then, it is possible to construct a flammability map for the confined diffusion flame, where the critical porosity defines an absolute flammability limit.

Dynamical behavior of an epidemiological model with a demographic Allee effect

As the Allee effect refers to small density or population size, it cannot be deduced whether or not the Allee mechanisms responsible for an Allee effect at low population density or size will affect the dynamics of a population at high density or size as well. We show using susceptible–exposed–infectious (SEI) model that such mechanisms combined with disease pathogenicity have a detrimental impact on the dynamics of a population at high population level. In fact, the eventual outcome could be an inevitable population crash to extinction. The tipping point marking the unanticipated population collapse at high population level is mathematically associated with a saddle–node bifurcation. The essential mechanism of this scenario is the simultaneous population size depression and the increase of the extinction threshold owing to disease virulence and the Allee effect. Using numerical continuation software MatCont another saddle–node bifurcation is detected, which results in the re-emergence of two non-trivial equilibria since highly pathogenic species cause their own extinction but not that of their host.

Dynamics of a stochastic multi-strain SIS epidemic model driven by Lévy noise

A stochastic multi-strain SIS epidemic model is formulated by introducing Lévy noise into the disease transmission rate of each strain. First, we prove that the stochastic model admits a unique global positive solution, and, by the comparison theorem, we show that the solution remains within a positively invariant set almost surely. Next we investigate stochastic stability of the disease-free equilibrium, including stability in probability and pth moment asymptotic stability. Then sufficient conditions for persistence in the mean of the disease are established. Finally, based on an Euler scheme for Lévy-driven stochastic differential equations, numerical simulations for a stochastic two-strain model are carried out to verify the theoretical results. Moreover, numerical comparison results of the stochastic two-strain model and the deterministic version are also given. Lévy noise can cause the two strains to become extinct almost surely, even though there is a dominant strain that persists in the deterministic model. It can be concluded that the introduction of Lévy noise reduces the disease extinction threshold, which indicates that Lévy noise may suppress the disease outbreak.

Habitat degradation and loss as key drivers of regional population extinction

Habitat quality is a fundamental driver of species distributions and population outcomes but is often difficult to measure and compare alongside measures of habitat amount and fragmentation. Consequently, habitat quality is often omitted from many landscape-level habitat analyses or more indirectly or subjectively represented in resulting habitat management or conservation planning. Yet, the implications of this conceptual and planning omission are poorly understood. We lack general theory that identifies the conditions under which habitat quality is expected to play a vital role in characterizing local and regional population responses. Using a factorial simulation design, we examined the independent contributions of habitat quality, amount, and fragmentation to population persistence to identify the conditions under which habitat quality might be expected to play a more important role than those of habitat amount or fragmentation. We generated a wide range of fractal landscapes, independently varying in habitat amount, fragmentation, and quality in QRule. We simulated interactive animal movement, habitat selection, and persistence for r and K strategist species with short and long dispersal abilities using spatially explicit individual-based models developed in HexSim. Population abundance and extinction risk were recorded through time for each landscape-species combination and used to quantify the relative influence of habitat amount, fragmentation, and landscape quality on population outcomes. We found that habitat degradation influenced extinction risk through a wide range of landscape conditions and species attributes. The most severe extinction responses were observed in scenarios of combined habitat loss and degradation, suggesting that the interactive effects of these variables may greatly affect persistence. Landscape quality modified extinction risks associated with habitat amount-fragmentation thresholds, and we found evidence for quality-based extinction thresholds as habitat was degraded. The strength of landscape-level quality on extinction risk outcomes suggests that habitat degradation should be further investigated as a major driver of population responses to landscape change. A more inclusive paradigm may be required to elucidate the general influences landscape change on population extinction. Habitat degradation, along with habitat loss and fragmentation, should be explicitly considered when assessing the implications of landscape change on population extinction.

Interactions of Earth's atmospheric oxygen and fuel moisture in smouldering wildfires

Vegetation, wildfire and atmospheric oxygen on Earth have changed throughout geological times, and are dependent on each other, determining the evolution of ecosystems, the carbon cycle, and the climate, as found in the fossil record. Previous work in the literature has only studied flaming wildfires, but smouldering is the most persistent type of fire phenomena, consuming large amounts of biomass. In this study, the dependence of smouldering fires in peatlands, the largest wildfires on Earth, with atmospheric oxygen is investigated. A physics-based computational model of reactive porous media for peat fires, which has been previously validated against experiments, is used. Simulations are conducted for wide ranges of atmospheric oxygen concentrations and fuel moisture contents to find thresholds for ignition and extinction. Results show that the predicted rate of spread increases in oxygen-rich atmospheres, while it decreases over wetter fuels. A novel nonlinear relationship between critical oxygen and critical moisture is found. More importantly, we show that compared to previous work on flaming fires, smouldering fires can be ignited and sustained at substantially higher moisture contents (up to 100% MC vs. 40% for 21% oxygen level), and lower oxygen concentrations (down to 13% vs. 16%). This defines a new atmospheric oxygen threshold for wildfires (13%), even lower than previously thought in Earth Sciences (16%). This finding should lead to reinterpretation of how the char remains observed in the fossil record constrain the lower concentration of oxygen in Earth's atmosphere in geological timescale.

Epidemiological, evolutionary, and economic determinants of eradication tails

Despite modern medical interventions, infectious diseases continue to generate huge socio-economic losses. The benefits of eradicating a disease are therefore high. While successful with smallpox and rinderpest, many other eradication attempts have failed. Eradications require huge and costly efforts, which can be sustained only if sufficient progress can be achieved. While initial successes are usually obtained more easily, progress often becomes harder as a disease becomes rare in the eradication endgame. A long eradication tail of slowly decreasing incidence levels can frustrate eradication efforts, as it becomes unclear whether progress toward eradication is still being made and how much more needs to be invested to push the targeted disease beyond its extinction threshold. Realistic disease dynamics are complicated by evolutionary responses to interventions and by interactions among different temporal and spatial scales. Models accounting for these complexities are required for understanding the shapes of eradication tails. In particular, such models allow predicting how hard or costly eradication will be, and may even inform in which manner progress has to be assessed during the eradication endgame. Here we outline a general procedure by analyzing the eradication tails of generic SIS diseases, taking into account two major ingredients of realistic complexity: a group-structured host population in which host contacts within groups are more likely than host contacts between groups, and virulence evolution subject to a trade-off between host infectivity within groups and host mobility among groups. Disentangling the epidemiological, evolutionary, and economic determinants of eradication tails, we show how tails of different shapes arise depending on salient model parameters and on how the extinction threshold is approached. We find that disease evolution generally extends the eradication tail and show how the cost structure of eradication measures plays a key role in shaping eradication tails.

Non-equilibrium relaxation in a stochastic lattice Lotka–Volterra model

We employ Monte Carlo simulations to study a stochastic Lotka–Volterra model on a two-dimensional square lattice with periodic boundary conditions. If the (local) prey carrying capacity is finite, there exists an extinction threshold for the predator population that separates a stable active two-species coexistence phase from an inactive state wherein only prey survive. Holding all other rates fixed, we investigate the non-equilibrium relaxation of the predator density in the vicinity of the critical predation rate. As expected, we observe critical slowing-down, i.e., a power law dependence of the relaxation time on the predation rate, and algebraic decay of the predator density at the extinction critical point. The numerically determined critical exponents are in accord with the established values of the directed percolation universality class. Following a sudden predation rate change to its critical value, one finds critical aging for the predator density autocorrelation function that is also governed by universal scaling exponents. This aging scaling signature of the active-to-absorbing state phase transition emerges at significantly earlier times than the stationary critical power laws, and could thus serve as an advanced indicator of the (predator) population’s proximity to its extinction threshold.

Biodiversity Education & the Anthropocene: An Indicator of Extinction or Recovery

The importance of extant biodiversity, concerns regarding the rising Anthropocene extinction rates, and commitments made by signatories to biodiversity conventions each increase demands for timely data. However, as species and conservation indicators become more complex, the less accessible they are to educators. New pedagogies are needed so that students can generate their own data for studies of biodiversity and extinction. I present a simple indicator of species diversity that examines declines in species’ populations and whether or not these species subsequently recovered or faced extinction. Using such data, 14 threatened species are used as examples of the time taken for each species to reach a point of either recovery or extinction. The learning and pedagogical context for this information is reviewed, student use of the data demonstrated, and the lesson evaluated according to its learning objectives.

Role of extrinsic noise in the sensitivity of the rod pathway: rapid dark adaptation of nocturnal vision in humans.

Rod-mediated 500 nm test spots were flashed in Maxwellian view at 5 deg eccentricity, both on steady 10.4 deg fields of intensities (I) from 0.00001 to 1.0 scotopic troland (sc td) and from 0.2 s to 1 s after extinguishing the field. On dim fields, thresholds of tiny (5') tests were proportional to √I (Rose-DeVries law), while thresholds after extinction fell within 0.6 s to the fully dark-adapted absolute threshold. Thresholds of large (1.3 deg) tests were proportional to I (Weber law) and extinction thresholds, to √I. rod thresholds are elevated by photon-driven noise from dim fields that disappears at field extinction; large spot thresholds are additionally elevated by neural light adaptation proportional to √I. At night, recovery from dimly lit fields is fast, not slow.

The role of orature in African socio-cultural space

The spoken words are more natural and perhaps better understood than the written letters, which are invented to capture and to re-enact human mind, intention, emotion, opinion, view, experience and prediction among other things. Oral narratives, all over the world, set the pace for the written literature. It is an integral part of every human culture. However, the sophistication of the modern world, particularly the globalisation, has directly or indirectly affected the potency of oral narratives in Africa. Having discovered the neglect of orature in Africa as a whole and in the Yoruba land in particular, this article seeks to explain the essentials of this aspect of life which is at a dangerous point of extinction. The study sets its search light on certain elements of African oral literature such as folklores, myths, legends, proverbs and oral poetry which emanate from the Yoruba community. Using the theme study method/approach, the article aims at reversing the dying fortune of the oral narratives being a tool to achieving sustainable socio-cultural transformation among the African people. Key words: Orature, folklores, myths, legends, proverbs, socio-cultural space, sustainable development.

Extended EDC local extinction model accounting finite-rate chemistry for MILD combustion

An extended Eddy Dissipation Concept (EDC) local extinction model is proposed to take into account the effects of finite-rate chemistry, normally occurring in Moderate to Intense Low oxygen Dilution (MILD) combustion, on the extinction limits. Local extinction is predicted when the local fine structure residence time is below a local critical value that is determined theoretically in the present study. The proposed model has been evaluated against experimental data reported for CH4/H2 jet-in-hot and diluted coflow flames. Comparison with the standard EDC extinction model is also presented. Results show that prediction of extinction threshold in MILD combustion conditions is attainable only through the application of the extended EDC extinction model on a well-resolved turbulence–chemistry interaction field. The effect of penetrating surrounding air into the reaction zone with subsequent flame cooling at downstream is also captured by the proposed extinction model. Despite its simplicity, the extended EDC extinction model is able to describe many features of localized extinction under MILD combustion as well as conventional combustion conditions.

A numerical study of the diffusive-thermal instability of opposed nonpremixed tubular flames

The diffusive-thermal (D-T) instability of opposed nonpremixed tubular flames near extinction is investigated using two-dimensional (2-D) direct numerical simulations together with the linear stability analysis. Two different initial conditions (IC), i.e. the perturbed IC and the C-shaped IC are adopted to elucidate the effects of small and large amplitude disturbances on the formation of flame cells, similar to conditions found in linear stability analysis and experiments, respectively. The characteristics of the D-T instability of tubular flames are identified by a critical Damköhler number, DaC, at which the D-T instability first occurs and the corresponding number of flame cells for three different tubular flames with different flame radii. It is found that DaC predicted through linear stability analysis shows good agreement with that obtained from the 2-D simulations performed with two different ICs. The flame cell number, Ncell, from the 2-D simulations with the perturbed IC is also found to be equal to an integer close to the maximum wavenumber, kmax, obtained from the linear stability analysis. However, Ncell from the 2-D simulations with the C-shaped IC is smaller than kmax and Ncell found from the simulations with the perturbed IC. This is primarily because the strong reaction at the edges of the horseshoe-shaped cellular flame developed from the C-shaped IC is more likely to produce larger flame cells and reduce Ncell. It is also found that for cases with the C-shaped IC, once the cellular instability occurs, the number of flame cells remains constant until global extinction occurs by incomplete reaction manifested by small Da. It is also verified through the displacement speed, Sd, analysis that the two edges of the horseshoe-shaped cellular flame are stationary and therefore do not merge due to the diffusion–reaction balance at the edges. Moreover, large negative Sd is observed at the local extinction points while small positive or negative Sd features in the movement of flame cells as they adjust their location and size towards steady state.

Species extinction thresholds in the face of spatially correlated periodic disturbance.

The spatial correlation of disturbance is gaining attention in landscape ecology, but knowledge is still lacking on how species traits determine extinction thresholds under spatially correlated disturbance regimes. Here we develop a pair approximation model to explore species extinction risk in a lattice-structured landscape subject to aggregated periodic disturbance. Increasing disturbance extent and frequency accelerated population extinction irrespective of whether dispersal was local or global. Spatial correlation of disturbance likewise increased species extinction risk, but only for local dispersers. This indicates that models based on randomly simulated disturbances (e.g., mean-field or non-spatial models) may underestimate real extinction rates. Compared to local dispersal, species with global dispersal tolerated more severe disturbance, suggesting that the spatial correlation of disturbance favors long-range dispersal from an evolutionary perspective. Following disturbance, intraspecific competition greatly enhanced the extinction risk of distance-limited dispersers, while it surprisingly did not influence the extinction thresholds of global dispersers, apart from decreasing population density to some degree. As species respond differently to disturbance regimes with different spatiotemporal properties, different regimes may accommodate different species.

Simulation of population response to ionizing radiation in an ecosystem with a limiting resource – Model and analytical solutions

A dynamic mathematical model is formulated, predicting the development of radiation effects in a generic animal population, inhabiting an elemental ecosystem ‘population-limiting resource’. Differential equations of the model describe the dynamic responses to radiation damage of the following population characteristics: gross biomass; intrinsic fractions of healthy and reversibly damaged tissues in biomass; intrinsic concentrations of the self-repairing pool and the growth factor; and amount of the limiting resource available in the environment. Analytical formulae are found for the steady states of model variables as non-linear functions of the dose rate of chronic radiation exposure. Analytical solutions make it possible to predict the expected severity of radiation effects in a model ecosystem, including such endpoints as morbidity, mortality, life shortening, biosynthesis, and population biomass. Model parameters are selected from species data on lifespan, physiological growth and mortality rates, and individual radiosensitivity. Thresholds for population extinction can be analytically calculated for different animal species, examples are provided for generic mice and wolf populations. The ecosystem model demonstrates a compensatory effect of the environment on the development of radiation effects in wildlife. The model can be employed to construct a preliminary scale ‘radiation exposure-population effects’ for different animal species; species can be identified, which are vulnerable at a population level to chronic radiation exposure.

How similar can co-occurring species be in the presence of competition and ecological drift?

If two species live on a single resource, the one with a slight advantage will out-compete the other: complete competitors cannot coexist. This is known as the competitive exclusion principle. If no extinction occurs, it is because evolutionary adaptation to slightly different niches takes place. Therefore, it is widely accepted that ecological communities are assembled by evolutionary differentiation and progressive adaptation of species to different niches. However, some ecologists have recently challenged this classic paradigm highlighting the importance of chance and stochasticity. Using a synthetic framework for community dynamics, here we show that, while deterministic descriptors predict coexistence, species similarity is limited in a more restrictive way in the presence of stochasticity. We analyse the stochastic extinction phenomenon, showing that extinction occurs as competitive overlap increases above a certain threshold well below its deterministic counterpart. We also prove that the extinction threshold cannot be ascribed only to demographic fluctuations around small population sizes. The more restrictive limit to species similarity is, therefore, a consequence of the complex interplay between competitive interactions and ecological drift. As a practical implication, we show that the existence of a stochastic limit to similarity has important consequences in the recovery of fragmented habitats.

A mechanistic approach to understanding range shifts in a changing world: What makes a pioneer?

A species’ range can be thought of as a manifestation of the ecological niche in space. Within a niche, evolution has resulted in traits that maximize fitness. Across millennia, natural oscillations in temperature have caused shifts in the geographic location of appropriate habitat and with corresponding changes in species’ ranges. Contemporary climate change and human disturbance may lead to rapid range expansion or contractions with largely unknown consequences. Birds provide an excellent case study of this phenomenon with some taxa expanding range and others contracting even to the point of extinction. What leads some populations to expand while others contract? Are there physiological and behavioral attributes of “pioneers” at the forefront of a range shift/expansion?The concept of allostasis provides a framework with which to begin to evaluate when a species will be able to successfully expand into new habitat. This tool allows the integration of normal energetic demands (e.g. wear and tear of daily and seasonal routines) with novel challenges posed by unfamiliar and human altered environments. Allostasis is particularly attractive because it allows assessment of how individual phenotypes may respond differentially to changing environments. Here, we use allostasis to evaluate what characteristics of individuals and their environment permit successful range expansion. Understanding variation in the regulatory mechanisms that influence response to a novel environment will be fundamental for understanding the phenotypes of pioneers.

Long-Distance Rescue and Slow Extinction Dynamics Govern Multiscale Metapopulations.

Rare long-distance dispersal is known to be critical for species dynamics, but how the interplay between short- and long-distance colonization influences regional persistence in a fragmented habitat remains poorly understood. We propose a metapopulation model that combines local colonization within habitat islands and long-distance colonization between islands. We study how regional occupancy dynamics are affected by the multiscale colonization process. We find that the island size distribution (ISD) is a key driver of the long-term occupancy dynamics. When the ISD is heterogeneous-that is, when the size of islands is variable-we show that extinction dynamics become very slow. We demonstrate that this behavior is unrelated to the well-known extinction debt near the critical extinction threshold. Hence, this finding questions the equivalence between extinction debt and critical transitions in the context of metapopulation collapse. Furthermore, we show that long-distance colonization can rescue small islands from extinction and sustain a steady regional occupancy. These results provide novel theoretical and practical insights into extinction dynamics and persistence in fragmented habitats and are thus relevant for the design of conservation strategies.