The influence of landscape and environmental factors on ranavirus epidemiology in a California amphibian assemblage

<p>1. A fundamental goal of disease ecology is to determine the landscape and environmental<br />
processes that drive disease dynamics at different biological levels to<br />
guide management and conservation. Although ranaviruses (family Iridoviridae)<br />
are emerging amphibian pathogens, few studies have conducted comprehensive<br />
field surveys to assess potential drivers of ranavirus disease dynamics.</p>

<p>2. We examined the factors underlying patterns in site-level ranavirus presence and<br />
individual-level ranavirus infection in 76 ponds and 1,088 individuals representing<br />
five amphibian species within the East Bay region of California.</p>

<p>3. Based on a competing-model approach followed by variance partitioning, landscape<br />
and biotic variables explained the most variation in site-level presence. However, biotic<br />
and individual-level variables explained the most variation in individual-level infection.</p>

<p>4. Distance to nearest ranavirus-infected pond (the landscape factor) was more important<br />
than biotic factors at the site level; however, biotic factors were most influential<br />
at the individual level. At the site level, the probability of ranavirus presence<br />
correlated negatively with distance to nearest ranavirus-positive pond, suggesting<br />
that the movement of water or mobile taxa (e.g., adult amphibians, birds, reptiles)<br />
may facilitate the movement of ranavirus between ponds and across the landscape.</p>

<p>5. Taxonomic richness associated positively with ranavirus presence at the site<br />
level, but vertebrate richness associated negatively with infection prevalence in<br />
the host population. This might reflect the contrasting influences of diversity on<br />
pathogen colonisation versus transmission among hosts.</p>

<p>6. Amphibian host species differed in their likelihood of ranavirus infection: American<br />
bullfrogs (Rana catesbeiana) had the weakest association with infection while roughskinned<br />
newts (Taricha granulosa) had the strongest. After accounting for host species<br />
effects, hostswith greater snout–vent length had a lower probability of infection.</p>

<p>7. Our study demonstrates the array of landscape, environmental, and individual level<br />
factors associated with ranavirus epidemiology. Moreover, our study helps<br />
illustrate that the importance of these factors varies with biological level.</p>