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Predicting impacts of anthropogenic disturbance and biodiversity loss on emerging infectious diseases

Final Report Summary - DISTURBED (Predicting impacts of anthropogenic disturbance and biodiversity loss on emerging infectious diseases)

Recent reviews investigating the role played by biodiversity in disease emergence and transmission have revealed an intriguing paradox: whilst biodiversity loss appears to frequently increase rates of disease transmission, areas of naturally high biodiversity may also serve as a source pool for new pathogens that may subsequently emerge. However, current evidence suggests that preserving ecosystems and their biodiversity should generally reduce the emergence of infectious diseases in natural systems. Correspondingly, the current rates of biodiversity loss at the global scale may be considered as having potentially significant consequences for emerging disease in natural environments. Consequently, the aim of this research was to determine how human-induced disturbances on freshwater systems underpin disease emergence using large-scale, field-based approaches using freshwater ponds. Environmental disturbances included habitat loss, nutrient enrichment (eutrophication) and introduced species. Impacts of these disturbances on freshwater biodiversity included diversity indices of species, functional groups and trophic levels. Disease emergence was assessed through the parasite communities of fish populations, identifying how the assessed disturbances impacted the diversity indices, and how this then affects the rates of fish disease emergence. These data were then be used in predictive models relating environmental change with emerging infectious diseases. The research objectives were thus to: (i) Determine how environmental disturbances caused by human activities impacts the diversity of species, functional groups and trophic levels in freshwater ponds; (ii) identify how the diversity of species, functional groups and trophic levels affects the emergence and transmission of infectious diseases in freshwater ponds; and (iii) evaluate the pathological and ecological consequences for fish hosts and populations of increased emergence and transmission rates of infectious diseases in freshwater ponds. This enabled the testing of two hypotheses: 1. Fish populations in highly disturbed ponds will have significantly higher rates of emergence and transmission of infectious diseases compared with undisturbed (pristine) ponds; 2. Fish populations in highly disturbed ponds with high rates of disease emergence will have significantly higher parasite burdens and associated impacts compared with pristine ponds. To complete the objectives and test the hypotheses, the research used two approaches: 1. A large-scale, empirical field study using a series of freshwater ponds with high replication (n = 25) that enabled a range of environmental disturbances to be classified across the ponds, with determination of how these then impacted pond biodiversity and rates of fish disease emergence; and 2. An experimental approach using mesocosms that were manipulated and controlled to allow more precise measurements to be taken on rates of environmental disturbance and disease emergence. The field study was completed on 25 small ponds (<0.5 ha) in North Norfolk, England through samples collected in April 2012 and October 2012 (to account for seasonal effects). At each pond, the physico-chemical properties of the water at each pond, measures of electrical conductivity (µS cm–1), pH, oxygen level (mg L–1) and Secchi depth (m) were taken; water samples were collected for quantifications of phosphates and nitrates; and the biological communities of macrophytes, zooplankton, benthic invertebrates and fish were sampled. The fish were used within parasitological indices to determine their parasite prevalence, loading, diversity and functional diversity. The other data were analysed using appropriate tests that provided variables suitable for testing against the parasite data within statistical models. Testing using multivariate mixing models revealed a range of outputs that enabled testing of the hypotheses. Key outputs included parasite load was a function of fish functional diversity and species diversity rather than water quality, but parasite functional diversity decreased as metrics of water quality decreased. The experimental study used three treatments, each replicated three times: control (no nutrient enrichment), mild nutrient enrichment and high nutrient enrichment. The experiment ran between May 2012 and October 2013, and started with known numbers and lengths of roach Rutilus rutilus being introduced to each mesocosm, with three-spined stickleback Gasterosteus aculeatus already present. Water quality indices were monitored monthly to ensure the nutrient enrichment was consistent within and between the treatments, and the same parameters of physic-chemical properties, water quality and biological samples were taken. At the conclusion of the experiment, the fish were recaptured and a full parasitological survey undertaken of each. As per the field experiment, testing using multivariate mixing models revealed a range of outputs that enabled testing of the hypotheses. Key outputs were parasite loading of roach increased with nutrient enrichment (as per hypothesis 2), whereas stickleback parasite diversity and parasite functional diversity were higher in the control than the treatments, a finding consistent with the field study in general. Our results demonstrate that the relationship between disease emergence, biodiversity and environmental change is complex, and they revealed some key relationships that substantially increase our understanding of the relationships between pathogens, their host communities and their environment. Due to this complexity, it is only now that the data are close to being ready for publication. Once published, they will thus demonstrate that this research has generated innovative knowledge on the issue of disease emergence through the use of multidisciplinary approaches that will provide a highly valuable contribution to understandings of the mechanisms of disease emergence.