Counterintuitive scaling between population abundance and local density: implications for modelling transmission of infectious diseases in bat populations

1. Models of host-pathogen interactions help to explain infection dynamics
in wildlife populations and to predict and mitigate the risk of zoonotic
spillover. Insights from models inherently depend on the way contacts
between hosts are modelled, and crucially, how transmission scales with
animal density. 2. Bats are important reservoirs of zoonotic disease and
are among the most gregarious of all mammals. Their population structures
can be highly heterogenous, underpinned by ecological processes across
different scales, complicating assumptions regarding the nature of
contacts and transmission. Although models commonly parameterise
transmission using metrics of total abundance, whether this is an
ecologically representative approximation of host-pathogen interactions is
not routinely evaluated. 3. We collected a 13-month dataset of
tree-roosting Pteropus spp. from 2,522 spatially referenced trees across
eight roosts to empirically evaluate the relationship between total roost
abundance and tree-level measures of abundance and density – the scale
most likely to be relevant for virus transmission. We also evaluate
whether roost features at different scales (roost-level, subplot-level,
tree-level) are predictive of these local density dynamics. 4. Roost-level
features were not representative of tree-level abundance (bats per tree)
or tree-level density (bats per m2 or m3), with roost-level models
explaining minimal variation in tree-level measures. Total roost abundance
itself was either not a significant predictor (tree-level 3-D density) or
only weakly predictive (tree-level abundance). 5. This indicates that
basic measures, such as total abundance of bats in a roost, may not
provide adequate approximations for population dynamics at scales relevant
for transmission, and that alternative measures are needed to compare
transmission potential between roosts. From the best candidate models, the
strongest predictor of local population structure was tree density within
roosts, where roosts with low tree density had a higher abundance but
lower density of bats (more spacing between bats) per tree. 6. Together,
these data highlight unpredictable and counterintuitive relationships
between total abundance and local density. More nuanced modelling of
transmission, spread and spillover from bats likely requires alternative
approaches to integrating contact structure in host-pathogen models,
rather than simply modifying the transmission function.