Strategic conservation action for frogs

A common trait of the most successful pathogen species is their broad host range. Despite a wealth of research on Batrachochytrium dendrobatidis, it remains unclear exactly how reservoir media and sympatric host species affect the disease dynamics of many declining species. Batrachochytrium dendrobatidis (Bd) has caused widespread declines in amphibians and may infect other taxa such as crustaceans and fish, yet this pathogen does not cause disease in every host it infects. Understanding the ecological role that non-declining species play in shaping population dynamics and transmission of Bd is paramount to informing conservation efforts for targeted species afflicted by disease and understanding mechanisms of decline. Brannelly et al. (2017) follow-up on their previous work to describe Bd dynamics of a widespread and common frog species that occurs at important sites for conservation in south-eastern Australia. The non-declining frog Crinia signifera commonly have a high prevalence and infection load of Bd across survey sites, and Brannelly et al. (2017) demonstrate that this result applies to a large area and long time period. Previous research by this team has demonstrated that the infection prevalence of Bd in a declining sympatric species (Pseudophryne pengilleyi) is higher at sites where C. signifera occur, suggesting that an amplification effect on Bd is produced by this reservoir species (Scheele et al., 2017). Their work further demonstrates that in some cases it may be of equal importance to understand the factors driving the abundance and disease dynamics of reservoir hosts, as the factors that affect a target species directly. Taken together, this strongly suggests that reintroducing threatened species at sites without C. signifera may translate to a useful conservation technique and should be a priority in adaptive management in this system. This concept can be easily applied to other conservation strategies where pathogens are a threat, by exploring the possibility of prioritizing conservation efforts away from sites with reservoir species that harbor high infection loads. The alpine system provides a valuable model to answer some related questions that still linger in the Bd puzzle. Specifically, suggestions that Bd may persist in the landscape for some time without biotic hosts remain unresolved (Johnson & Speare, 2003; Berger et al., 2005; Mitchell et al., 2008). Yet, the contribution of non-biotic reservoirs to infection loads of host species appears to be low if the prevalence of Bd infection in P. pengilleyi decreases from 41.4% to 2.6% in the absence of C. signifera (Scheele et al., 2017). This appears to differ from the comparatively more tolerant Batrachochytrium salamandrivorans, which is demonstrably capable of maintaining non-motile spores in water and soil (Stegen et al., 2017) and then reinfecting its hosts. In many places, amphibian communities will have more than one non-declining species and Bd reservoir effects will be more complex with dilutor host species and interspecific interactions within the community (Stockwell et al., 2016) such that the absence of a single reservoir host may have less of an effect on Bd dynamics overall. The identification of C. signifera as a competent reservoir species may also be important to our current understanding of Bd dynamics in other regions. Previous research in Western Australia identified high infection loads and prevalence of Bd in the congeneric and non-declining Crinia georgiana (Riley, Berry & Roberts, 2013) and suggested that Bd has not caused frog declines in the region. Brannelly et al. (2017) demonstrate the distinction between an effect of Bd on competent species such as C. signifera that persist with high Bd prevalence and infection load, alongside susceptible sympatric species such as P. pengilleyi that may simultaneously decline while exposed to identical conditions. It follows that if Bd does not negatively affect one non-declining species, we cannot apply this generality to other sympatric species in the area. Indeed, each species needs to be considered separately to identify its vulnerability to Bd. As we look ahead to a world where increasing pathogen emergence poses a key threat to biodiversity, unraveling mechanisms that help reduce pathogen transmission and growth in targeted species is very valuable. The prospects after the initial Bd-induced declines appeared dire, but now, system-specific conservation strategies that can reduce disease effects, or pre-emptively buffer disease outbreaks (Bower et al., 2017), could mean the difference between persistence and extinction of species. In some cases, recovery is occurring without intervention (McKnight et al., 2017), and therefore any strategy that can assist in the short-term may have long-lasting consequences. The mechanism of a broad host range that makes Bd so successful, is a feature we can use to our advantage in conservation, by focusing efforts with this knowledge in mind. The alpine region and other areas with low amphibian species richness are well suited to this strategy when tackling species threatened by Bd, and this general concept applies to other systems and pathogens. Improving our conservation strategy around reintroductions of threatened species will lead to a more efficient use of resources and higher prospects for recovery.