Detecting inbreeding depression in a severely bottlenecked, recovering species: The little spotted kiwi (Apteryx owenii)
Authors:   , Taylor Helen R. 
Published: November 2021

<p>Population bottlenecks reduce genetic variation and population size. Small populations are at greater risk of inbreeding, which further erodes genetic diversity and can lead to inbreeding depression. Inbreeding depression is known to increase extinction risk. Thus, detecting inbreeding depression is important for population viability assessment and conservation management. However, identifying inbreeding depression in wild populations is challenging due to the difficulty of obtaining long-term measures of fitness and error-free measures of individual inbreeding coefficients. I investigated inbreeding depression and our power to detect it in species that have very low genetic variation, using little spotted kiwi (Apteryx owenii) (LSK) as a case study. This endemic New Zealand ratite experienced a bottleneck of, at most, five individuals ~100 years ago and has since been subjected to secondary bottlenecks as a result of introductions to new predator-free locations. There is no behavioural pedigree data available for any LSK population and the status of the species is monitored almost exclusively via population growth. I conducted two seasons of field work to determine hatching success in the two LSK populations with the highest and lowest numbers of founders; Zealandia Sanctuary (40 founders) and Long Island (two founders). I also used simulation-based modelling to assess the feasibility of reconstructing pedigrees based on individual genotypes from LSK populations to calculate pedigree inbreeding coefficients. Finally, I used microsatellite genotypes to measure the genetic erosion in successive filial groupings of Long Island and Zealandia LSK as a result of their respective bottlenecks, and tested for inbreeding depression on Long Island. Hatching success was significantly lower on Long Island than in Zealandia in both years of the study despite significantly higher reproductive effort on Long Island. Although this was suggestive of inbreeding depression on Long Island, simulation results showed that constructing a pedigree for any LSK population based on the genetic markers and samples currently available would lead to inaccurate pedigrees and invalid estimates of individual inbreeding coefficients. Thus, an alternative method of detecting inbreeding and inbreeding depression was required. Microsatellite data showed continued loss of heterozygosity in both populations, but loss of allelic diversity on Long Island only. Individual genotypes indicated that the majority (74%) of the adult Long Island population is comprised of the founding pair (F) and their direct offspring (F1) rather than birds from subsequent generations (F2+). This is not what would be expected if survival was equal between these two filial classes. I suggest that the high levels of inbreeding (≥0.25) in F2+ birds is impacting on their survival, creating a demographic skew in the population and resulting in lower hatching success on average on Long Island when compared to the relatively outbred Zealandia birds. This inbreeding depression appears to have been masked, thus far, by positive population growth on Long Island resulting from the long life span of LSK (27-83 years) and continued reproductive success of the founding pair. Thus, it is likely that the Long Island population will go into decline when the founding pair cease to reproduce. This study highlights the challenges of measuring inbreeding depression in species with very low genetic variation and the importance of assessing the statistical power and reliability of the genetic tools available for those species. It also demonstrates that basic genetic techniques can offer valuable insight when more advanced tools prove error-prone. Monitoring vital rates such as hatching success in conjunction with genetic data is important for assessing the success of conservation translocations and detecting potentially cryptic genetic threats such as inbreeding depression. My results suggest that LSK are being affected by inbreeding depression and that careful genetic management will be required to ensure the long-term viability of this species.</p>