Local extinction

Tributyltin compounds used as antifouling agents on boats have had serious toxic effects upon many mollusks, including populations of oysters and dog whelks. Females of the latter species developed a condition known as imposex, which rendered them infertile and caused local extinction of the population in shallow coastal waters. Imposex provides the basis for a valuable biomarker assay. 

Experiments confirm this mechanism. It was observed that before the extinction events set in, the speed of a limit flame propagating downward falls and the flame partially loses contact with the walls (Figure 3.1.14). In a square tube, local extinction starts in the corners, where heat loss to the walls is expected... 

Similarly, Figure 6.3.9b depicts the situation in which partial quenching of the flame results from unequal composition of the reactant mixtures issued from the inner and outer tubes, while keeping the mean velocities constant. If the equivalence ratio in the inner tube is excessively leaner or richer to exceed a typically flammable range, it would result in local extinction, thereby exhibiting a hole in the center of the premixed flame. 

Localized extinction of the flame surface can readily occur in the turbulent combustion devices, where wrinkled flames interact with turbulent eddies and gas... 

One of the most challenging aspects of modeling turbulent combustion is the accurate prediction of finite-rate chemistry effects. In highly turbulent flames, the local transport rates for the removal of combustion radicals and heat may be comparable to or larger than the production rates of radicals and heat from combustion reactions. As a result, the chemistry cannot keep up with the transport and the flame is quenched. To illustrate these finite-rate chemistry effects, we compare temperature measurements in two piloted, partially premixed CH4/air (1/3 by vol.) jet flames with different turbulence levels. Figure 7.2.4 shows scatter plots of temperature as a function of mixture fraction for a fully burning flame (Flame C) and a flame with significant local extinction (Flame F) at a downstream location of xld = 15 [16]. These scatter plots provide a qualitative indication of the probability of local extinction, which is characterized... 

Figure 7.2.5 provides a visualization of a localized extinction event in a turbulent jet flame, using a temporal sequence of OH planar LIF measurements. The OH-LIF measurements, combined with particle image velocimetry (PIV) reveal that a distinct vortex within the turbulent flow distorts and consequently breaks the OH front. These localized extinction events occur intermittently as the strength of the coupling between the turbulent flow and the flame chemistry fluctuates. The characteristics of the turbulent flame can be significantly altered as the frequency of these events increases. 

Temporal sequence of OH-LIF measurements captures a localized extinction event in a turbulent nonpremixed CH4/H2/N2 jet flame (Re 20,000) as a vortex perturbs the reaction zone. The time between frames is 125 ps. The velocity field from PIV measurements is superimposed on the second frame and has the mean vertical velocity of 9m/s subtracted. (From Hult, J. et al.. Paper No. 26-2, in 10th International Symposium on Applications of Laser Techniques to Fluid Mechanics, Lisbon, 2000. With permission.)... 

PDF calculations of turbulent nonpremixed flames with local extinction. Combustion and Flame 123, 281-307. 

One of the most successful biomarker assays is the measurement of imposex in dog whelks (Nu-cella lapillus, which have brightly-colored shells). This phenomenon is caused by tributyl tins (TBTs), widely used as antifouling coatings in boats (see Section 12.3.2). TBTs impose male characteristics on females, leading to a reduction in reproductive performance and local extinction of dog whelk populations in the most contaminated sites. 

If the point group symmetry is equivalent to orthorhombic or lower, i.e. if the optical properties are biaxial, the local extinction directions are not simply related to the director as projected onto the specimen plane. It may be possible to use light microscopy to establish whether or not a region of the specimen is optically biaxial ... 

If the microstructure is too fine, and the material is pleochroic, one can attempt to identify the azimuths for maximum and minimum absorption of linearly polarized light (171 in optically biaxial material, these need not coincide with the local extinction directions. The use of optical pleochroism as just described is one example of how studies limited to observations between crossed polars do not exhaust the information available from light microscopy. 

Classic metapopulations result from low to intermediate migration between habitat patches. Not all potential habitats necessarily contain populations. Migration between patches affects the dynamics of local populations, even including recolonization following extinction. If sufficient dispersal between patches exists, then a "rescue effect" can prevent local extinctions. Persistence of a metapopulation requires migration rates between patches, which are sufficient enough to offset local extinction rates. 

Source-sink and mainland-island metapopulations result when one or more of the local populations differ in the probability of local extinction. In a source-sink structure, the source has excess organisms that migrate to other habitat patches. The other habitat patches, sinks, do not contain the resources to maintain a growing population. In contrast to a classical metapopulation, dispersal is not equal between patches but is from the source to the sinks. In a mainland-island metapopulation, the difference is principally size, and all patches can support viable populations. Since smaller populations run a greater risk of extinction, the mainland can often provide a source for recolonization and the establishment of a new population on that patch. Conversely, islands can also act as refugia in cases where the mainland population becomes extinct. 

Any given reserve should be as close to circular as possible as this minimizes dispersal distances within the reserve. An elongated reserve may suffer local extinctions at the ends. 

As noted above, the hypothesis appears to be composed of two parts. The first is Everything is everywhere, that microorganisms are ubiquitous According to advocates of the ubiquity hypothesis. .. the vast population sizes of microorganisms drive ubiquitous dispersal. .. and make local extinction virtually impossible. Geographic isolation is therefore absent and as a result, allopatric speciation should be rare or nonexistent, which would explain the perceived low global morphospecies diversity of microbial eukaryotes. .. (Vanormelingen et al.. 

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