Modelling repeated epidemics with general infection kernels

An integral equation approach is taken to explore the characteristics of a general infectious disease in a homogeneous population. It is shown that the final size of the epidemic depends on the basic reproduction ratio for the infection and the initial number of susceptibles. A discrete map for the susceptible population from epidemic generation to epidemic generation is formed to consider the long term behaviour of the disease in a population of constant size

Relaxational dislocation damping due to dislocation-dislocation intersections with application to magnesium single crystals

Relaxational dislocation damping due to dislocation-dislocation intersections with applications to magnesium single crystal

Analysis of the gain distribution across the active region of InGaAs-InAlGaAs multiple quantum well lasers

Spectral gain measurements for two InGaAs-InAlGaAs multiple width quantum well structures, with inverse-configured active regions, have been presented. One structure consisted of wide quantum wells near the p-side and narrow quantum wells near the n-side of the active region. The other structure consisted of narrow quantum wells near the p-side of the active region with wider quantum wells near the n-side. It is shown that, for the same operating conditions, the structure with wide quantum wells on the p-side of the active region provided a 15% broader gain spectrum in comparison to the structure with narrow quantum wells on the p-side of the active region. The analysis of the results shows non-uniform carrier distribution across the active region of the structures, where the structure with wide quantum wells near the p-side of the active region provided 65% more gain in comparison to the structure with narrow quantum wells near the p-side of the active region. The gain distribution results have been compared with that obtained for the phosphorous quaternary structures in other literature and have shown there is some evidence to suggest that the gain distribution is more uniform in aluminium quaternary than phosphorous quaternary material

A model for evolution and extinction

We present a model for evolution and extinction in large ecosystems. The model incorporates the effects of interactions between species and the influences of abiotic environmental factors. We study the properties of the model by approximate analytic solution and also by numerical simulation, and use it to make predictions about the distribution of extinctions and species lifetimes that we would expect to see in real ecosystems. It should be possible to test these predictions against the fossil record. The model indicates that a possible mechanism for mass extinction is the coincidence of a large coevolutionary avalanche in the ecosystem with a severe environmental disturbance.Comment: Postscript (compressed etc. using uufiles), 16 pages, with 15 embedded figure

The threatened status of restricted-range coral reef fish species

Coral reefs are the most diverse ecosystem in the sea. Throughout the world they are being overfished, polluted and destroyed, placing biodiversity at risk. To date, much of the concern over biodiversity loss has centred on local losses and the possibility of global extinction has largely been discounted. However, recent research has shown that 24% of reef fish species have restricted ranges (< 800 000 km(2)), with 9% highly restricted (< 50 000 km(2)). Restricted-range species are thought to face a greater risk of extinction than more widespread species since local impacts could cause global loss. We searched for information on status in the wild and characteristics of 397 restricted-range reef fish species. Fish body size, habitat requirements and usefulness to people were compared with those of a taxonomically-matched sample of more widespread species. We found that on average species with restricted ranges were significantly smaller (mean total length 19.1 cm versus 24.4 cm), tended to have narrower habitat requirements and were less used by people. Greater habitat specificity will tend to increase extinction risk while, if real, more limited usefulness (equivalent to exploitation) may reduce risk. Fifty-eight percent of restricted-range species were considered common/abundant in the wild and 42% uncommon/rare. Population status and threats to 319 species for which data were available were assessed according to the categories and criteria of the IUCN red list of threatened animals. A number of species were found to be rare, were exploited and had highly restricted ranges overlapping areas where reef degradation is particularly severe, placing them at a high risk of extinction. Five species were listed as Critically Endangered, two of them possibly already extinct in the wild, one as Endangered and 172 as Vulnerable. A further 126 species fell into Lower Risk categories and 11 were considered Data Deficient. Given the intensity of impacts to reefs, the broad geographical areas affected and the large numbers of restricted-range species, global extinctions seem likely. Urgent management action is now crucial for the survival of several species of reef fishes