Many roads to reservoirs? How susceptibility and shedding shape host competence in amphibians

Host competence—the ability to acquire, harbor, and transmit
infections—drives pathogen spread and persistence in multi-host
communities. Evaluating species-specific competence is critical for
predicting transmission, particularly for generalist fungal pathogens like
Batrachochytrium dendrobatidis (Bd). Despite its central role in disease
dynamics, we lack an epidemiologically grounded competence metric that
rigorously accounts for how infection intensity affects a host's
competence. This knowledge gap limits our ability to compare mechanisms
across species and assess their roles in pathogen persistence. To address
these challenges, we developed a novel, load-dependent competence metric
using host-pathogen Integral Projection Models (IPMs) that integrates
variation in susceptibility, within-host pathogen growth, and pathogen
shedding dynamics. We applied this metric to lab-based challenge
experiments with three common North American amphibians (Notophthalmus
viridescens, Rana clamitans, and Rana catesbeianus) that persist
endemically with Bd. Using dose-response assays and repeated pathogen
shedding measurements across species, we asked: i) is there a consistent,
non-linear relationship between infection intensity and pathogen shedding
across species? and ii) which load-based traits best predict host
competence? We quantified four of five components of host
competence—susceptibility, pathogen growth, pathogen survival, and
load-dependent shedding—and used these to parameterize species-specific
IPMs, integrating competence into a single relative metric across species.
We found that Bd shedding increased non-linearly with infection intensity,
contradicting the standard assumption that Bd shedding is linearly related
to infection intensity. N. viridescens and R. catesbeianus were the most
competent hosts but through distinct pathways: high susceptibility in N.
viridescens and elevated shedding rates in R. catesbeianus. In contrast,
density dependent reductions in pathogen growth and shedding limited R.
clamitans competence. Thus, species-level competence is not determined by
a single trait, but emerges from interactions among multiple load-based
processes. Our results demonstrate that variation in competence emerges
from distinct, species-specific processes across multiple dimensions of
competence. By linking individual infection dynamics to population-level
transmission potential, our integrative framework provides a more
mechanistic approach to predicting host contributions to community-level
pathogen persistence