Country‐wide genetic monitoring over 21 years reveals lag in genetic recovery despite spatial connectivity in an expanding carnivore (Eurasian otter, Lutra lutra) population

Numerous terrestrial mammal species have experienced extensive population
declines during past centuries, due largely to anthropogenic pressures.
For some species, including the Eurasian otter (Lutra lutra),
environmental and legal protection has more recently led to population
growth and recolonisation of parts of their historic ranges. While
heralded as conservation successes, only a few such recoveries have been
examined from a genetic perspective, i.e. whether genetic variability and
connectivity have been restored. We here use large-scale and long-term
genetic monitoring data from UK otters, whose population underwent a
well-documented population decline between the 1950s to 1970s, to explore
the dynamics of a population re-expansion over a 21-year period. We
genotyped otters from across Wales and England at five time points between
1994 and 2014 using 15 microsatellite loci. We used this combination of
long-term temporal and large-scale spatial sampling to evaluate 3
hypotheses relating to genetic recovery; that (i) gene flow between
sub-populations would increase over time, (ii) genetic diversity of
previously isolated populations would increase, and that (iii) genetic
structuring would weaken over time. Although we found an increase in
inter-regional gene flow and admixture levels among subpopulations, there
was no significant temporal change in either heterozygosity or allelic
richness. Genetic structuring among the main sub-populations hence
remained strong and showed a clear historical continuity. These findings
highlight an underappreciated aspect of population recovery of endangered
species, that genetic recovery may often lag behind the processes of
spatial and demographic recovery. In other words, the restoration of
physical connectivity of populations does not necessarily lead to genetic
connectivity. Our findings emphasise the need for genetic data as an
integral part of conservation monitoring, to enable the potential
vulnerability of populations to be evaluated.