Comparison of adult census size and effective population size support the need for continued protection of two Solomon Island endemics

Because a population’s ability to respond to rapid change is dictated by
standing genetic variation, we can better predict a population’s long-term
viability by estimating and then comparing adult census size (N) and
effective population size (Ne). However, most studies only
measure N or Ne, which can be misleading. Using
a combination of field and genomic sequence data, we here
estimate and compare N and Ne in two
range-restricted endemics of the Solomon Islands.
Two Zosterops White-eye species inhabit the small island
of Kolombangara, with a high elevation species endemic to the island (Z.
murphyi) and a low elevation species endemic to the Solomon Islands (Z.
kulambangrae). Field observations reveal large values
of N for both species with Z.
kulambangrae numbering at 114,781 ± 32,233 adults,
and Z. murphyi numbering at 64,412 ± 15,324 adults. In
contrast, genomic analyses reveal that Ne was much lower
than N, with Z. kulambangrae estimated at 694.5
and Z. murphyi at 796.1 individuals. Further, positive
Tajima’s D values for both species suggest that they have experienced a
demographic contraction, providing a mechanism for low values
of Ne. Comparison
of N and Ne suggests that Z.
kulambangrae and Z. murphyi are not at
immediate threat of extinction but may be at genetic risk. Our results
provide important baseline data for long-term monitoring of these island
endemics, and argue for measuring both population size estimates to better
gauge long-term population viability.