No evidence of genetic structure in a sky island endemic: implications
for population persistence under a shrinking thermal niche
Abstract
Mountain habitats physically isolated from one another (“sky islands”)
represent a unique system for studying dispersal in seemingly isolated
populations. The Cape Fold Belt of southwest South Africa forms a sky
island archipelago of high-altitude mountain fynbos of which the Cape
Rockjumper Chaetops frenatus is an avian-endemic. Continued contraction
of habitat due to increasing temperatures may be causing further
isolation of C. frenatus populations beyond their dispersal capacities,
resulting in currently declining populations in warmer areas of their
habitat. In this study, we sequenced two mitochondrial loci and one
nuclear locus of 73 C. frenatus samples from 13 localities representing
8 mountain ranges. We found (1) low overall genetic diversity, (2) no
evidence for geographically-based genetic structuring, and (3) no
evidence for inbreeding within localities. While this may indicate birds
are effectively dispersing, it may also indicate strong selective
pressure is being placed on their specific genotype. Haplotype networks
suggested that C. frenatus may have experienced a bottleneck or founder
effect in their recent genetic past —- a result supported by a
significantly negative Tajima’s D value. As the first avian genetic
study to arise from a range-restricted species of the Cape Fold Belt sky
islands, our results show no evidence that C. frenatus are unable to
disperse across inhospitable lowland habitat, and thus may not
experience isolation due to climate change. We thus potentially found
further support that selective pressure in species with highly
specialized habitat niches may have a stronger effect than dispersal
limitations.