A unique pathway for indirect effects of COVID-19
Theory in ecology and evolution helps us understand patterns and processes in natural systems and in human society. The coronavirus disease 2019 (COVID-19) pandemic is a profound example of a pathogen generating indirect effects by altering host behavior. Interestingly, this may be the only example of those indirect effects generated by a pathogen being primarily related to behavioral changes of individuals of the host species who were not infected. Six months since the first documented infection, the pathogen has infected only a small proportion of the global population (15 million confirmed cases as of July 30, 2020; Dong et al. 2020), yet it has resulted in widespread changes in the way billions of people live (e.g., Corlett et al. 2020, Gössling et al. 2020, Rutz et al. 2020). These behavioral changes, arising primarily to avoid infection, have had cascading indirect effects of the pathogen on the environment such as decreased concentrations of some greenhouse gasses in the atmosphere, changes in air quality and environmental pollution, and decreases in anthropogenic sound (e.g., Corlett et al. 2020, Chen et al. 2020, Isaifan 2020, Zambrano-Monserrate et al. 2020). Plus, the decrease in human activity (i.e., anthropause) has provided a strong natural experimental framework for researchers to uncover how humans affect wildlife communities (Rutz et al. 2020). Based on ecological and evolutionary theory, these indirect effects from pathogens changing the behavior of their hosts should be prevalent in nature because we know organisms–humans and many other species–commonly change their behavior in response to pathogens–either as a result of getting infected, or to avoid getting infected (Curtis 2014, Weinstein et al. 2018). However, a recent and comprehensive review (i.e., Buck and Ripple 2017) only uncovered one example where a pathogen generated indirect effects by infecting the host (i.e.., a pathogen indirectly affected prey abundance by reducing feeding rate in the predatory crawfish it infected; Haddaway et al. 2012) and found no published examples of indirect effects from a pathogen generated by behavioral avoidance of getting infected (Buck and Ripple 2017). Indeed, indirect effects stemming from host avoidance of parasites is rare as well (Buck 2019). Given that the majority of people changing their behavior has not been infected, COVID-19 has provided a compelling and rare example of indirect effects by a pathogen being caused by behavioral avoidance of infection. Likewise, COVID-19 provides us with a stark reminder that humans are a part of, and not separate from, the principle eco-evolutionary processes that shape our world. We as ecologists and evolutionary biologists can use the pandemic to advance our knowledge base, and also use our knowledge base to inform responses to the pandemic.