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).