Reliability demonstration for safety-critical systems

This paper suggests a new model for reliability demonstration of safety-critical systems, based on the TRW Software Reliability Theory. The paper describes the model; the test equipment required and test strategies based on the various constraints occurring during software development. The paper also compares a new testing method, Single Risk Sequential Testing (SRST), with the standard Probability Ratio Sequential Testing method (PRST), and concludes that: • SRST provides higher chances of success than PRST • SRST takes less time to complete than PRST • SRST satisfies the consumer risk criterion, whereas PRST provides a much smaller consumer risk than the requirement

Change and causes of change in the vascular plant flora of Ireland: 1970–1999

The analysis of decadal change in regional floras has received fresh impetus in recent years with studies often detecting anthropogenic effects and biotic homogenization. Here we analyse the changing flora of the island of Ireland (i.e., both the Republic and Northern Ireland) using data derived from the New Atlas of the British Flora (Preston et al., 2002) to compare changing plant status from the previous national plant atlas of Perring and Walters (1962). We compare the number of 10 x 10 km grid squares occupied by individual plant species and enter these into a regression analysis. The null hypothesis assumes that all the species will be located on the line of best fit hence the resulting residuals are used as an Index of Change. This Index was entered into correlation analysis with independent environmental indicators to interpret the potential causes of any change. Despite some potential sampling bias in the data the findings point to several factors being implicated in change with eutrophication being a major correlate of change. This finding is discussed in relation to land use change and agricultural intensification over the period of the survey

Turnover dynamics of breeding land birds on islands: is island biogeographic theory ‘true but trivial’ over decadal time-scales?

The theory of island biogeography has revolutionised the study of island biology stimulating considerable debate and leading to the development of new advances in related areas. One criticism of the theory is that it is ‘true but trivial’, i.e., on the basis of analyses of annual turnovers of organisms on islands, it has been posited that stochastic turnover mainly comprises rare species, or repeated immigrations and extinctions thereof, and thereby contribute little to the overall ecological dynamics. Here, both the absolute and relative turnover of breeding land birds are analysed for populations on Skokholm, Wales, over census intervals of 1, 3, 6, 12, and 24 years. As expected, over short census intervals (≤6 years), much of the turnover comprised repeated colonisations and extinctions of rare species. However, at longer intervals (12 and 24 years), a sizeable minority of species (11% of the total recorded) showed evidence of colonisation and/or extinction events despite sizeable populations (some upwards of 50 pairs). These results suggest that a longer-term view is required to take into account turnover involving more common species

Vegetation cover and grasslands in the vicinity accelerate development of carabid beetle assemblages on restored landfill sites

There is increasing evidence that rare and scarce carabid species of conservation importance are frequent in brownfield sites such as restored landfill. However, this potential has largely been unexplored and was investigated here by examining carabid species composition, richness, and abundance in relation to habitat quality and landscape structure on landfill sites in comparison to paired reference sites of existing wildlife value. Sampling was conducted by collecting carabids in ten pitfall traps set along two 100 m transects on each of nine restored landfill and their paired reference sites in the East Midlands region of the UK. A total of 1014 individuals representing thirty seven carabid species were found during April to September in 2007 and 2008. On the landfill sites, generalist species were common, while no nationally rare or scarce species were found. Neither species richness nor diversity of carabid species was found to be different from that of the reference sites. Seeding during restoration was found to have a strong positive effect on richness and diversity, with seeded landfill sites tending to be similar to reference sites in terms of carabid species composition. Marked differences in diversity and richness were also attributed to variation in the amount of local vegetation cover, with presence of grassland in the surrounding landscape having a positive effect on carabid assemblages. We suggest that initial seeding may be an appropriate conservation strategy to improve beetle diversity and richness, coupled with management in terms of cutting to increase the potential of these sites for carabid conservation at the landscape scale

Do British birds conform to Bergmann's and Allen's rules? An analysis of body size variation with latitude for four species

Capsule: An analysis of body mass and wing length for four bird species shows trends broadly in line with predictions from Bergmann's and Allen's rules but with species- and sex-specific trends in terms of body size variation with latitude in Britain. Aims: To analyse body size characteristics for bird species with latitude in Britain and to test Bergmann’s and Allen’s rules (over a range of c. 740 km). Methods Body mass and wing length for four bird species (Blackbird Turdus merula, House Sparrow Passer domesticus, Robin Erithacus rubecula, and Song Thrush Turdus philomelos) were analysed using principal components regression analysis to investigate trends with latitude, longitude, or by sex and Julian day. Results: Evidence was found for latitudinal gradients in body mass for male Blackbird, female House Sparrow (both increasing in size northwards), and female Robin (decreasing in size northwards) and in wing length for female Robin and male Song Thrush (decreasing and increasing northwards, respectively). Conclusion: Trends were broadly in line with predictions from Bergmann's and Allen's rules except for Robin which had trends opposite to those expected. Differences in trends between sexes suggest a role for an interplay between natural and sexual selection with latitude that deserves further consideration

An availability study for a SME

A case study of an availability analysis for a small commercial company is presented. The analysis was carried out to meet a customer requirement for the availability of an electronic ground-based system in a benign environment. Availability calculations were based on failure data provided and an explanation of the methodology and problems encountered and dealt with are discussed. The methodology includes failure classification according to MIL-HDBK-781A and how it may be used to promote and develop internal processes. A commentary on the background to reliability/availability specification is provided and a number of recommendations for monitoring reliability and availability are given

The curious case of Skokholm: equilibrium, non-equilibrium and a phase shift in an island landbird assemblage

Previous work has indicated that the landbirds of Skokholm island (Wales) are not in equilibrium as defined in MacArthur–Wilson's classic theory of island biogeography. This study takes a new dataset with over six decades of data and investigates equilibrium on Skokholm using cluster analysis to identify periods of turnover stability. The attributes of the identified periods were investigated in relation to the MacArthur–Wilson model using analyses of change in numbers of species, S, from one year to the next and measures of variability in S quantified for each of the periods identified together with a consideration of the dynamics in the numbers of species by habitat groupings. Cluster analysis identified four main periods of which two middle periods appeared to be in equilibrium but with a phase shift in-between. The first and last periods showed non-equilibrium dynamics but plots of species by habitat groupings suggested that this was due to habitat changes going on at those times. This decadal long dataset indicates that the landbirds of Skokholm exhibit periods of both equilibrium and non-equilibrium with the latter attributable to habitat change. The apparent phase shift in the equilibrium number of species was unexpected within the framework of island biogeographic theory and not easily explained using the current MacArthur–Wilson framework. There is a need to integrate the theory of island biogeography with more recent work on alternative stable states, tipping points, and phase (or regime) shifts, together with equilibrium and non-equilibrium dynamics, into a single framework

How well is current plant trait composition predicted by modern and historical forest spatial configuration?

There is increasing evidence to suggest that a delayed response of many forest species to habitat loss and fragmentation leads to the development of extinction debts and immigration credits in affected forest habitat. These time lags result in plant communities which are not well predicted by present day landscape structure, reducing the accuracy of biodiversity assessments and predictions for future change. Here, species richness data and mean values for five life history characteristics within deciduous broadleaved forest habitat across Great Britain were used to quantify the degree to which aspects of present day forest plant composition are best explained by modern or historical forest patch area. Ancient forest specialist richness, mean rarity and mean seed terminal velocity were not well predicted by modern patch area, implying the existence of a degree of lag in British forest patches. Mean seedbank persistence values were more closely related to modern patch area than historical, particularly in larger patches. The variation in response for different mean trait values suggests that species respond to landscape change at different rates depending upon their combinations of different trait states. Current forest understorey communities are therefore likely to consist of a mixture of declining species whose extinction debt is still to be paid, and faster colonising immigrant species. These results indicate that without management action, rare and threatened species of plant are likely to be lost in the future as a result of changes in forest spatial configuration that have already taken place. The lag seen here for rare specialist plants suggests however that there may still be scope to protect such species before they are lost from forest patches

Vegetation diversity of conventional and organic hedgerows in Denmark

Many attempts have been made to reduce the impact of modern conventional farming on the environment and semi-natural ecosystems. One of them is organic farming, known primarily for the absence of pesticides and artificial fertilisers. The objective of this study was to study and test the differences in the spontaneous vegetation of comparable hedgerows in the same area situated within organic and conventional farming systems. The hedge bottom vegetation was surveyed during August 2001 in 13 hedgerows of each farming system. Farming type had not changed on either side of the hedgerows for the lifetime of the hedges (10-14 years). Sampling was associated with a set of 16 measured environmental variables. In the two farming systems hedgerows were comparable in terms of landscape, age, soil type, nutrient status and width. A mixed analysis of variance found no significant difference in measured soil and radiation variables between farming types. Farming types only differed in the use of pesticides. Significant differences between farming types in plant species diversity at alpha, beta and gamma levels were found. Also more species that normally occur in semi-natural habitats were found on organic farms. There was an overlap in species composition between farming type, but a slightly higher species turnover on conventional farms. The ordination axes were highly correlated with calibrated Ellenberg values of fertility, light and soil moisture. Soil fFertility and farming type were important factors to explain variation in species composition. Organic farming had a significantly reduced impact on hedge bottom vegetation compared to conventional farming. Higher extinction rates due to pesticide drift and immigration rates due to pesticide drift rates oin conventional farminsg may be responsible for the significantly higher species diversity and different species composition in hedges on organic farms. The differences in species diversity and plant types are briefly discussed