Continent-wide effects of urbanization on bird and mammal genetic diversity

Urbanization and associated environmental changes are causing global
declines in vertebrate populations. In general, population declines of the
magnitudes now detected should lead to reduced effective population sizes
for animals living in proximity to humans and disturbed lands. This is
cause for concern because effective population sizes set the rate of
genetic diversity loss due to genetic drift, the rate of increase in
inbreeding, and the efficiency with which selection can act on beneficial
alleles. We predicted that the effects of urbanization should decrease
effective population size and genetic diversity, and increase
population-level genetic differentiation. To test for such patterns, we
repurposed and reanalyzed publicly archived genetic data sets for North
American birds and mammals. After filtering, we had usable raw genotype
data from 85 studies and 41,023 individuals, sampled from 1,008 locations
spanning 41 mammal and 25 bird species. We used census-based urban-rural
designations, human population density, and the Human Footprint Index as
measures of urbanization and habitat disturbance. As predicted, mammals
sampled in more disturbed environments had lower effective population
sizes and genetic diversity, and were more genetically differentiated from
those in more natural environments. There were no consistent relationships
detectable for birds. This suggests that, in general, mammal populations
living near humans may have less capacity to respond adaptively to further
environmental changes, and be more likely to suffer from effects of
inbreeding.