• Dirichlet neighbours: revisiting Dirichlet tessellation for neighbourhood analysis

      Halls, P. J.; Bulling, Mark T.; White, Piran C. L.; Garland, Lynette; Harris, S. (2013-05-24)
      The analysis of neighbourhood for point features is reviewed in terms of requirements and available techniques; the application of Dirichlet/Voronoi/Thiessen tessellation techniques are discussed. Robin Sibson’s Natural Neighbours (Sibson, 1980, Natural neighbourhood inter- polation. In V. D. Brunner, Graphical methods for multivariate data. New York: Brunner/ Mazel) technique for Dirichlet Tessellation is discussed and benefits demonstrated in the consequently available neighbourhood descriptors of distance, bearing and nearest neigh- bours. The application of Dirichlet Tessellation derived neighbourhood analysis is shown in terms of the study of badger territories and other potential applications in the sciences and social sciences are outlined
    • Impact of biodiversity-climate futures on primary production and metabolism in a model benthic estuarine system

      Hicks, Natalie; Bulling, Mark T.; Solan, Martin; Raffaelli, D.; White, Piran C. L.; Paterson, David M. (2013-05-24)
      Background: Understanding the effects of anthropogenically-driven changes in global temperature, atmospheric carbon dioxide and biodiversity on the functionality of marine ecosystems is crucial for predicting and managing the associated impacts. Coastal ecosystems are important sources of carbon (primary production) to shelf waters and play a vital role in global nutrient cycling. These systems are especially vulnerable to the effects of human activities and will be the first areas impacted by rising sea levels. Within these coastal ecosystems, microalgal assemblages (microphytobenthos: MPB) are vital for autochthonous carbon fixation. The level of in situ production by MPB mediates the net carbon cycling of transitional ecosystems between net heterotrophic or autotrophic metabolism. In this study, we examine the interactive effects of elevated atmospheric CO 2 concentrations (370, 600, and 1000 ppmv), temperature (6°C, 12°C, and 18°C) and invertebrate biodiversity on MPB biomass in experimental systems. We assembled communities of three common grazing invertebrates ( Hydrobia ulvae, Corophium volutator and Hediste diversicolor) in monoculture and in all possible multispecies combinations. This experimental design specifically addresses interactions between the selected climate change variables and any ecological consequences caused by changes in species composition or richness. Results: The effects of elevated CO 2 concentration, temperature and invertebrate diversity were not additive, rather they interacted to determine MPB biomass, and overall this effect was negative. Diversity effects were underpinned by strong species composition effects, illustrating the importance of individual species identity. Conclusions: Overall, our findings suggest that in natural systems, the complex interactions between changing environmental conditions and any associated changes in invertebrate assemblage structure are likely to reduce MPB biomass. Furthermore, these effects would be sufficient to affect the net metabolic balance of the coastal ecosystem, with important implications for system ecology and sustainable exploitation.
    • Impact of external sources of infection on the dynamics of bovine tuberculosis in modelled badger populations

      Hardstaff, Joanne L.; Bulling, Mark T.; Marion, Glenn; Hutchings, Michael R.; White, Piran C. L. (2013-05-23)
      Background The persistence of bovine TB (bTB) in various countries throughout the world is enhanced by the existence of wildlife hosts for the infection. In Britain and Ireland, the principal wildlife host for bTB is the badger (Meles meles). The objective of our study was to examine the dynamics of bTB in badgers in relation to both badger-derived infection from within the population and externally-derived, trickle-type, infection, such as could occur from other species or environmental sources, using a spatial stochastic simulation model. Results The presence of external sources of infection can increase mean prevalence and reduce the threshold group size for disease persistence. Above the threshold equilibrium group size of 6–8 individuals predicted by the model for bTB persistence in badgers based on internal infection alone, external sources of infection have relatively little impact on the persistence or level of disease. However, within a critical range of group sizes just below this threshold level, external infection becomes much more important in determining disease dynamics. Within this critical range, external infection increases the ratio of intra- to inter-group infections due to the greater probability of external infections entering fully-susceptible groups. The effect is to enable bTB persistence and increase bTB prevalence in badger populations which would not be able to maintain bTB based on internal infection alone. Conclusions External sources of bTB infection can contribute to the persistence of bTB in badger populations. In high-density badger populations, internal badger-derived infections occur at a sufficient rate that the additional effect of external sources in exacerbating disease is minimal. However, in lower-density populations, external sources of infection are much more important in enhancing bTB prevalence and persistence. In such circumstances, it is particularly important that control strategies to reduce bTB in badgers include efforts to minimise such external sources of infection.
    • Influence of macrofaunal assemblages and environmental heterogeneity on microphytobenthic production in experimental systems

      Dyson, Kirstie E.; Bulling, Mark T.; Solan, Martin; Hernandez-Milian, Gema; Raffaelli, D.; White, Piran C. L.; Paterson, David M. (2013-05-24)
      Despite the complexity of natural systems, heterogeneity caused by the fragmentation of habitats has seldom been considered when investigating ecosystem processes. Empirical approaches that have included the influence of heterogeneity tend to be biased towards terrestrial habitats; yet marine systems offer opportunities by virtue of their relative ease of manipulation, rapid response times and the well-understood effects of macrofauna on sediment processes. Here, the influence of heterogeneity on microphytobenthic production in synthetic estuarine assemblages is examined. Heterogeneity was created by enriching patches of sediment with detrital algae (Enteromorpha intestinalis) to provide a source of allochthonous organic matter. A gradient of species density for four numerically dominant intertidal macrofauna (Hediste diversicolor, Hydrobia ulvae, Corophium volutator, Macoma balthica) was constructed, and microphyto- benthic biomass at the sediment surface was measured. Statistical analysis using generalized least squares regression indicated that heterogeneity within our system was a significant driving factor that interacted with macrofaunal density and species identity. Microphytobenthic biomass was highest in enriched patches, suggesting that nutrients were obtained locally from the sediment–water interface and not from the water column. Our findings demonstrate that organic enrichment can cause the development of heterogeneity which influences infaunal bioturbation and consequent nutrient generation, a driver of microphytobenthic production.
    • Marine biodiversity-ecosystem functions under uncertain environmental futures

      Bulling, Mark T.; Hicks, Natalie; Murray, L.; Paterson, David M.; Raffaelli, D.; White, Piran C. L.; Solan, Martin (2013-05-24)
      Anthropogenic activity is currently leading to dramatic transformations of ecosystems and losses of biodiversity. The recognition that these ecosystems provide services that are essential for human well-being has led to a major interest in the forms of the biodiversity – ecosystem functioning relationship. However, there is a lack of studies examining the impact of climate change on these relationships and it remains unclear how multiple climatic drivers may affect levels of ecosystem functioning. Here, we examine the roles of two important climate change variables, temperature and concentration of atmospheric carbon dioxide, on the relationship between invertebrate species richness and nutrient release in a model benthic estuarine system. We found a positive relationship between invertebrate species richness and the levels of release of NH 4 -N into the water column, but no effect of species richness on the release of PO 4 -P. Higher temperatures and greater concen- trations of atmospheric carbon dioxide had a negative impact on nutrient release. Importantly, we found significant interactions between the climate variables, indicating that reliably predicting the effects of future climate change will not be straightforward as multiple drivers are unlikely to have purely additive effects, resulting in increased levels of uncertainty.
    • Modelling the impact of vaccination on tuberculosis in badgers.

      Hardstaff, Joanne L.; Bulling, Mark T.; Marion, Glenn; Hutchings, Michael R.; White, Piran C. L.; University of York, Environment Department (2013-04-10)
      SUMMARY Tuberculosis (TB) in livestock, caused by Mycobacterium bovis, persists in many countries. In the UK and Ireland, efforts to control TB through culling of badgers (Meles meles), the principal wildlife host, have failed and there is significant interest in vaccination of badgers as an alternative or complementary strategy. Using a simulation model, we show that where TB is self-contained within the badger population and there are no external sources of infection, limited-duration vaccination at a high level of efficacy can reduce or even eradicate TB from the badger population. However, where sources of external infection persist, benefits in TB reduction in badgers can only be achieved by ongoing, annual vaccination. Vaccination is likely to be most effective as part of an integrated disease management strategy incorporating a number of different approaches across the entire host community.
    • Species effects on ecosystem processes are modified by faunal responses to habitat composition.

      Bulling, Mark T.; Solan, Martin; Dyson, Kirstie E.; Hernandez-Milian, Gema; Luque, Patricia; Pierce, Graham J.; Raffaelli, D.; Paterson, David M.; White, Piran C. L.; University of York, Environment Department (2008-12)
      Heterogeneity is a well-recognized feature of natural environments, and the spatial distribution and movement of individual species is primarily driven by resource requirements. In laboratory experiments designed to explore how different species drive ecosystem processes, such as nutrient release, habitat heterogeneity is often seen as something which must be rigorously controlled for. Most small experimental systems are therefore spatially homogeneous, and the link between environmental heterogeneity and its effects on the redistribution of individuals and species, and on ecosystem processes, has not been fully explored. In this paper, we used a mesocosm system to investigate the relationship between habitat composition, species movement and sediment nutrient release for each of four functionally contrasting species of marine benthic invertebrate macrofauna. For each species, various habitat configurations were generated by selectively enriching patches of sediment with macroalgae, a natural source of spatial variability in intertidal mudflats. We found that the direction and extent of faunal movement between patches differs with species identity, density and habitat composition. Combinations of these factors lead to concomitant changes in nutrient release, such that habitat composition effects are modified by species identity (in the case of NH4-N) and by species density (in the case of PO4-P). It is clear that failure to accommodate natural patterns of spatial heterogeneity in such studies may result in an incomplete understanding of system behaviour. This will be particularly important for future experiments designed to explore the effects of species richness on ecosystem processes, where the complex interactions reported here for single species may be compounded when species are brought together in multi-species combinations.
    • Using model systems to address the biodiversity-ecosystem functioning process

      Bulling, Mark T.; White, Piran C. L.; Raffaelli, D.; Pierce, Graham J. (2013-06-11)
      Declines in biodiversity resulting from anthropogenic disturbance to ecosystems have focused attention on the role of biodiversity in ecosystem functioning. However, the high level of complexity of ecosystems has made this a difficult topic to investigate. Much simpler model systems incorporating small-scale, spatially delimited, artificial assemblages of species have been widely used recently to address the link between biodiversity and ecosystem functioning (BEF). Their simplicity lends tractability to these systems, but has also resulted in much criticism in the literature over their relevance. Here, we examine the strengths and limitations of model systems and examine how useful these systems might be in addressing several issues that are likely to represent future challenges to understanding BEF: spatial scale, multiple trophic levels, variation, environmental stochasticity and the choice of representative combinations of species. We find that model systems have already played an important role in enhancing our understanding of BEF and are likely to continue this role in the future. However, they do have important limitations, and it is essential to take these into account when putting results into the broader context of ecosystems and to improve the level of integration of results with those from other methodologies.