5) The effect of urbanisation on gut microbial rhythms
Urbanization is rapidly altering wildlife environments and activity
patterns. Medium to large mammals are becoming more nocturnal to escape
human disturbance 99 whilst small mammals that are
normally nocturnal are active around the clock in urban areas100. Artificial light is causing birds and bats to
extend and reduce their activity periods, respectively101,102, and is also associated with altered
physiology and immune responses 103–105. Urban
habitats also offer different diets, with many urban animals becoming
scavengers or being provisioned by humans 106, and are
associated with pollution 107 and higher pathogen
diversity 108 than natural habitats. How these shifts
in behaviour and exposure to pathogens and pollution are affecting
health for both humans and wildlife via circadian mismatching is an
outstanding question of urgent need of attention109–111, given ongoing and rapid human encroachment
into natural habitats.
How might urbanization affect the gut microbiota, and what are the
consequences for wildlife health? Accumulating evidence from across
phylogenetically-diverse species suggests that urbanization generates a
more ‘humanized’ gut microbiota, with a higher proportion of
opportunistic pathogens 112–117. Yet, whether
urbanisation is altering microbial rhythms is still unclear. In humans,
urbanisation is associated with a loss of seasonal rhythms in the gut
microbiota 78,118, indicating that biological rhythms
might be disrupted by urban lifestyles. Wildlife health may be
negatively affected by urbanisation and artificial light if changes to
activity patters (e.g., timing of feeding) or altered diet disrupts gut
microbial oscillations (Fig. 3). Constant light or dark leads to a loss
of microbial rhythms in both chickens 32 and mice35, and this alteration is at least in part due to
sensory signalling from the brain rather than changes to feeding times119. Diets high in fat also dampen microbial rhythms
and thereby lead to dysbiosis – an imbalance in the microbiome that has
negative health outcomes 39,120,121. Together, these
indicate that urbanisation may alter microbial rhythms via multiple
mechanisms.