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).