Partitioning the net effect of host diversity on an emerging amphibian pathogen

Abstract

The 'dilution effect' (DE) hypothesis predicts that diverse host communities will show reduced disease. The underlying causes of pathogen dilution are complex, because they involve non-additive (driven by host interactions and differential habitat use) and additive (controlled by host species composition) mechanisms. Here, we used measures of complementarity and selection traditionally employed in the field of biodiversity-ecosystem function (BEF) to quantify the net effect of host diversity on disease dynamics of the amphibian-killing fungus Batrachochytrium dendrobatidis (Bd). Complementarity occurs when average infection load in diverse host assemblages departs from that of each component species in uniform populations. Selection measures the disproportionate impact of a particular species in diverse assemblages compared with its performance in uniform populations, and therefore has strong additive and non-additive properties. We experimentally infected tropical amphibian species of varying life histories, in single-and multi-host treatments, and measured individual Bd infection loads. Host diversity reduced Bd infection in amphibians through a mechanism analogous to complementarity (sensu BEF), potentially by reducing shared habitat use and transmission among hosts. Additionally, the selection component indicated that one particular terrestrial species showed reduced infection loads in diverse assemblages at the expense of neighbouring aquatic hosts becoming heavily infected. By partitioning components of diversity, our findings underscore the importance of additive and non-additive mechanisms underlying the DE.

Bird assemblage vulnerability depends on the diversity and biogeographic histories of islands

“…Biodiversity is widely acknowledged to significantly influence the magnitude and stability of a large array of ecosystem properties, with stronger impacts evident when larger scales (1) and multiple functions are considered (2,3). These findings provide compelling support for biodiverse systems being functionally robust, with greater stability of function (4)(5)(6), resistance to invasion (7)(8)(9)(10), and resistance to pathogen spread (11). Some of the impacts of higher levels of biodiversity on ecosystem properties that are apparent at ecological timescales (e.g., greater resistance to invasion) may influence evolutionary processes, like extinction rates.…”

Overview of chytrid emergence and impacts on amphibians

“…In the decades to follow, extensive research on many of the surviving species has confirmed the role of Bd in contributing to population declines across Australia [27], and has shown how species-specific differences in ecology and behaviour contribute to variation in disease dynamics [14,28]. Despite continued declines in many species [25], others have developed ways to persist with infection. Retallick et al [29] described a population of Taudactylus eungellensis persisting with stable infections that had experienced an epidemic die-off 10 years earlier.…”

“…Bd infects over 700 species [6], which means that potentially all species of amphibians can serve as host, vector or reservoir. Laboratory studies have shown that the species composition of tadpole communities affects disease transmission [24,25] as do studies of artificial adult communities [25]. In these cases, some species reduced infection in co-occurring species.…”

Chytrid fungus acts as a generalist pathogen infecting species-rich amphibian families in Brazilian rainforests

“…Likewise, North American ranids (another species-rich family) that are dependent on water bodies (i.e. lentic habitats) for laying eggs and tadpole development seem to be more prone to Bd infection than strictly terrestrial or directdevelopment species (see also Becker et al 2014).…”