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.