Host density has limited effects on pathogen invasion, disease‐induced declines and within‐host infection dynamics across a landscape of disease

Abstract

1. Host density is hypothesized to be a major driver of variability in the responses and outcomes of wildlife populations following pathogen invasion. While the effects of host density on pathogen transmission have been extensively studied, these studies are dominated by theoretical analyses and small‐scale experiments. This focus leads to an incomplete picture regarding how host density drives observed variability in disease outcomes in the field. 2. Here, we leveraged a dataset of hundreds of replicate amphibian populations that varied by orders of magnitude in host density. We used these data to test the effects of host density on three outcomes following the arrival of the amphibian‐killing fungal pathogen Batrachochytrium dendrobatidis (Bd): the probability that Bd successfully invaded a host population and led to a pathogen outbreak, the magnitude of the host population‐level decline following an outbreak and within‐host infection dynamics that drive population‐level outcomes in amphibian–pathogen systems. 3. Based on previous small‐scale transmission experiments, we expected that populations with higher densities would be more likely to experience Bd outbreaks and would suffer larger proportional declines following outbreaks. To test these predictions, we developed and fitted a Hidden Markov Model that accounted for imperfectly observed disease outbreak states in the amphibian populations we surveyed. 4. Contrary to our predictions, we found minimal effects of host density on the probability of successful Bd invasion, the magnitude of population decline following Bd invasion and the dynamics of within‐host infection intensity. Environmental conditions, such as summer temperature, winter severity and the presence of pathogen reservoirs, were more predictive of variability in disease outcomes. 5. Our results highlight the limitations of extrapolating findings from small‐scale transmission experiments to observed disease trajectories in the field and provide strong evidence that variability in host density does not necessarily drive variability in host population responses following pathogen arrival. In an applied context, we show that feedbacks between host density and disease will not necessarily affect the success of reintroduction efforts in amphibian‐Bd systems of conservation concern.

Warmer temperatures reduce the transmission of a virus in a gregarious forest insect

“…The relationship of this species with its NPV appears to be well adapted to variable and increasing environmental warming. Our results support the recent conclusions of Mihaljevic et al (2020) and Wilber et al (2022) that small‐scale transmission studies are unlikely to be sufficient to explain larger‐scale epizootics. Our experiment only used two greenhouses, and while these were very similar in terms of site and conditions, there could have been other, unmeasured factors within each greenhouse that affected the outcome and need to be explored in further studies.…”

Reintroduction of resistant frogs facilitates landscape-scale recovery in the presence of a lethal fungal disease

“…Specifically, we compared frog genomes sampled in 4 populations that have not yet experienced a Bd-caused epizootic ("naive") (45) versus in 5 populations that experienced a Bd epizootic during the past several decades and have since recovered to varying degrees ("recovering"; Figure 5) (14,33). Bd-exposure histories of the 9 study populations are based on 10-20 years of VES and Bd surveillance using skin swabbing (e.g., 14,45,46). Naive populations are characterized by large numbers of adults (i.e, typically 1000s), Bd prevalence that is generally 0% except during occasional Bd failed invasions (during which Bd loads remain very low, 46), and no history of Bd epizootics since we first surveyed these populations in the late 1990s and early 2000s (45).…”

“…We also did not directly consider the dynamics of Bd in this model. We made this decision because (i) translocated populations are infected with Bd at high prevalence (41), and (ii) host density does not seem to play a significant role in multi-year Bd infection dynamics in this system (46).…”

“…We also did not directly consider the dynamics of Bd in this model. We made this decision because (i) translocated populations are infected with Bd at high prevalence (23), and (ii) host density does not seem to play a significant role in multi-year Bd infection dynamics in this system (83). Thus, ignoring Bd infection dynamics and instead assuming all host vital rates are in the presence of high Bd prevalence significantly simplifies the model without much loss of realism.…”

Divergent population responses following salamander mass mortalities and declines driven by the emerging pathogen Batrachochytrium salamandrivorans

“…Interestingly, contrary to theoretical models and small-scale experiments, a recent large-scale study across hundreds of frog populations affected by Bd revealed that population densities only have a very limited influence on real-life disease dynamics and outcomes [47]. Instead, environmental variables such as climatic factors and disease reservoirs were found to be better predictors of amphibian population responses.…”