Neutron Optics

The description of neutron optical phenomena within the framework of dynamical diffraction theory is described. The coherent wave and optical potential are introduced, and an expression for the complex neutron refractive index in terms of the scattering length density and attenuation coefficient is obtained. The extension to magnetic media and polarized neutrons is covered. Neutron reflectivity is defined, and the wavevector dependence of the reflectivity profile is derived by a transfer matrix method and an optical method. Exact results are compared with the Born approximation. The technique of neutron imaging is described, including neutron radiography and computed tomography. Several optical phenomena that occur in Bragg diffraction from near-perfect crystals, including Pendellösung oscillations, and primary and secondary extinction.

Increasing network stability towards large food webs

Stability is a key attribute of complex food webs that has been for a long time in the focus of studies. It remained an intriguing question how large and complex food webs are persisting if smaller and simple ones tend to be more stable at least from a mathematic perspective. Presuming that with the increasing size of food webs their stability also grows, we analyzed the relationship between number of nodes in food webs and their stability based on 450 food webs ranging from a few to 200 nodes. Our results show that stability increases non-linearly with food web size based both on return times after disturbance and on robustness calculated from secondary extinction rates of higher trophic levels. As a methodologic novelty we accounted for food web generation time in the return time calculation process. Our results contribute to the explanation of large and complex food web persistence: in spite of the fact that with increasing species number the stability of food webs decreases at small node numbers, there is a constant stability increase over a large interval of increasing food web size. Therefore, in food web stability studies, we stress the use of food web generation times.

Local extinction of the Asian tiger mosquito ( Aedes albopictus ) following rat eradication on Palmyra Atoll

The Asian tiger mosquito, Aedes albopictus, appears to have been extirpated from Palmyra Atoll following rat eradication. Anecdotal biting reports, collection records, and regular captures in black-light traps showed the species was present before rat eradication. Since then, there have been no biting reports and no captures over 2 years of extensive trapping (black-light and scent traps). By contrast, the southern house mosquito, Culex quinquefasciatus, was abundant before and after rat eradication. We hypothesize that mammals were a substantial and preferred blood meal for Aedes , whereas Culex feeds mostly on seabirds. Therefore, after rat eradication, humans and seabirds alone could not support positive population growth or maintenance of Aedes . This seems to be the first documented accidental secondary extinction of a mosquito. Furthermore, it suggests that preferred host abundance can limit mosquito populations, opening new directions for controlling important disease vectors that depend on introduced species like rats.

Nullifying the extinction effect in XRD characterization of fibre textures

To gain accuracy and, hence, physical reality of the data acquired by XRD measurements of fibre textures, a technique is elaborated to achieve experimental values, which are free of extinction effects. Its elaboration is based on combining basic definitions of the extinction theory and texture analysis. This technique is applicable to characterization of metal coatings that appear infinitely thick for X-rays. A nickel sample representing <100> + <221> texture components is used as a model. Resultant derived series of data on pole-density distribution of the {200} diffraction pole figure shows that the data corresponding to the main <100> texture component are strongly affected by extinction. On the contrary, due to definitions that require reduction of the intensity distribution to multiples of random density, the extinction-free values of the volume fraction of texture components do not differ substantially from those calculated by standard methods. Evidently, any of the standard methods for volume fraction measurements provides reasonable data if secondary extinction is even disregarded.

The Role of Stochastic Simulations to Extend Food Web Analyses

Food webs are schematic representations of who eats whom in ecosystems. They are widely used in linking process to pattern (e.g., degree distribution and vulnerability) and investigating the roles played by particular species within the interaction web (e.g., centrality indices and trophic position). First, I present the dominator tree, a topological structure reducing food web complexity into linear pathways that are essential for energy delivery. Then, I describe how the dominance relations based on dominator trees extracted from binary food webs may be modified by including interaction strength. Consequences related to the skewed distribution of weak links towards the trophic chain are discussed to explain higher risks of secondary extinction that characterize top predators dominated by basal species. Finally, stochastic simulations are introduced to suggest an alternative approach to static analyses based on food web topology. Ranking species importance using stochastic-based simulations partially contradicts the predictions based on network analyses.