Implications for biodiversity conservation and disease
transmission
While there are many examples of human activities conspicuously causing
wildlife population declines (Dirzo et al. 2014), more subtle
disruptions of host-pathogen interactions can also significantly affect
population dynamics. Take the example of the worldwide amphibian
decline: although mass mortalities have been linked to chytrid fungus
infections (Lötters et al. 2009), research shows that the
pathogen is not a sufficient cause of ongoing declines (Alford et
al. 2007; Rollins-Smith et al. 2011; Scheele et al.2019). Global warming, another culprit, also degrades amphibian’s
condition (Reading 2007), making them more susceptible to the fungus and
potentially to other stressors (Garner et al. 2009; Rollins-Smithet al. 2011; Cohen et al. 2019a, b, 2020). In the wild,
when pathogens are highly virulent, sick individuals are seldom found,
probably due to reduced survivorship and diminished activity when ill.
However, sick or dead individuals are conspicuous at infrequent times,
such as in the beforementioned amphibian mass mortality events (Lötterset al. 2009). As sick animals become abundant, they could be more
commonly detected, indicating an ongoing population decline (green lines
in Fig. 5B and C) (Beldomenico & Begon 2016).
The effects of multiple stressors (e.g., environmental stressors plus
infection) could lead to vicious cycles, where a host in poor condition
might not respond adequately to infection, further reducing the
condition and susceptibility to stressors and additional infections
(Beldomenico & Begon 2016). If we consider multi-host pathogens, which
are the majority of known pathogens (Woolhouse et al. 2001),
those vicious cycles could have not only a negative effect at the
population level but also at the community level (Beldomenico & Begon
2016). Considering multi-host pathogens, Lafferty & Holt (2003) showed
a positive association between stress and disease because transmission
did not decrease as a specific host population became rare (as in our
models with a single species), posing a threat to the most vulnerable
species and conservation.
Our findings also suggest negative implications for public health.
Animals under stress could become more competent hosts (i.e., host
ability to transmit pathogens to other hosts or vectors) as conditions
deteriorate (Gervasi et al. 2015). Changes in immunity can
influence parasite spread by increasing per-contact transmission
probabilities or lowering host recovery rates, extending the duration of
the infectious period (Altizer et al. 2006). Consequently,
increasing the probability of intra- and inter-species transmission and
posing a risk for spillover to human and domesticated animal populations
(Plowright et al. 2017). For example, nutritional stress has been
identified as one of the main risk factors for Hendra virus infection in
flying foxes (Pteropus sp.), leading to the spillover events that
affected both livestock and humans (Plowright et al. 2015; Beckeret al. 2022; Eby et al. 2023).