This thesis aims to assess the population-level risk posed by Batrachochytrium dendrobatidis (Bd), a pathogen responsible for worldwide declines and extinctions, to the amphibians of a series of infected Pyrenean Lakes. The principle focus was on the common midwife toad (Alytes obstetricans), which, in parts of its range has suffered population declines and even local extinction as a result of exposure to Bd.
Infection severity thresholds may play an important role in the risk of decline a population faces, but such thresholds are only useful if individual-level infection estimates are accurate. I found that widely-used data obtained via epidermal swabs are not accurate indications of the full infection burden of an individual, as assessed by a novel full-skin digest procedure I developed (Chapter Three). Swab-based infection thresholds should therefore be applied with caution. However, infection intensity in dead and moribund individuals was significantly higher than that of visually healthy individuals, suggesting that infection severity is an important predictor of mortality.
Population monitoring is necessary for understanding whether long-term infection translates into changes to host population size. I used capture-mark-recapture (CMR) techniques on larval A. obstetricans and found evidence of possible disease-induced declines in two populations, but an increase in abundance in another, suggesting that the pathogen may exist in an endemic state in some host populations (Chapter Four).
To assess disease-induced mortality across populations, I compared mortality rates monitored in laboratory-held individuals, to those estimated in the field (Chapter Five). This highlighted inter-site differences within my study populations and provided evidence of host persistence with low-level Bd infection. The presence of hosts in the field with low-level infection may indicate host population persistence is possible.
To broaden my thesis from a single host to the amphibian community, I analysed temporal data on infection in three species within a single focal lake (Chapter Six). I show that environmental effects may affect the prevalence of infection in two sympatric species, suggesting infection is likely influenced by biotic and abiotic variables and may change over time.
Overall, these data lend valuable insights into not only the individual level responses to infection, but that of the population and even begin to elucidate community level effects. These data will be useful in developing conservation policy for these, and other, Alytes populations.Open Acces
