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 impact population dynamics. The worldwide amphibian decline constitutes an important example. Although mass amphibian mortalities have been linked to chytrid fungus infections (Lötters et al. 2009), the pathogen alone is not sufficient to cause of ongoing declines (Alford et al. 2007; Rollins-Smith et al. 2011; Scheele et al. 2019). Global warming, another culprit of population declines, degrades amphibian condition (Reading 2007), increasing susceptibility to the fungus (Garner et al. 2009; Rollins-Smith et al. 2011; Cohenet 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 the beforementioned amphibian mass mortality events (Lötters et 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). It is important to note that other strategies to monitor and manage wildlife diseases exist, like targeted surveillance on single species that dominate transmission (Streicker et al. 2013; Charlier et al. 2022).
Effects of multiple stressors (e.g., environmental stressors plus infection) can perpetuate cycles, where hosts in poor condition may not respond adequately to infection (e.g., reduced infection resistance or tolerance), further reducing their condition and increasing susceptibility to stressors and additional infections (Beldomenico & Begon 2016). As most known pathogens are multi-host (Woolhouse et al. 2001), such cycles could affect population and community-level dynamics (Beldomenico & Begon 2016). For example, 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 other species. White-nose syndrome, an emerging fungal disease in bats, constitutes another notable example. While the disease has severely decimated some bat species populations, other sympatric and closely related species have been largely unaffected while sustaining transmission (Langwig et al. 2012, 2016; Cheng et al.2021).
Although most of the taxa examined (arthropods, molluscs, amphibians, and fish) are not commonly associated with zoonotic events, insights are gained by identifying generalities across taxa and comparing them with other systems. For instance, we found that pathogen intensity increased in hosts exposed to environmental stressors, suggesting negative implications for public health. Under stressful conditions, individuals could become superspreaders, amplifying pathogen transmission potential and disease risk (Lloyd-Smith et al. 2005; Gervasi et al.2015; Faust et al. 2017). Consequently, they could increase intra- and inter-species transmission and pose a risk for spillover to humans and domesticated animals (Plowright et al. 2017; Faustet al. 2018). For example, nutritional stress has been identified a primary risk factors for Hendra virus infection in flying foxes (Pteropus sp.), leading to spillover events that affected both livestock and humans (Plowright et al. 2015; Becker et al.2022; Eby et al. 2023).