Reintroduced Eurasian otters in Italy 623
FIG.4 Vortex sensitivity analysis, modelling the variation in the probability of survival of the reintroduced otter population for four uncertain parameters: (a) the number of lethal equivalents (increments of 0.629), (b) carrying capacity K (increments of 5), (c) per cent catastrophes (increments of 1%), and (d) severity for reproduction (increments of 0.1).
this difference in the treatment of samples from different years, genotyping was reasonably successful, and our success rate of 32% was in the range of that reported in previous studies (e.g. 14%, Lanszki et al., 2008; 21%, Ferrando et al., 2008; 41%, Prigioni et al., 2006b; 73%, Janssens et al., 2008). Throughout the otter’s European range, Ho is 0.35–0.69, and the mean number of alleles per locus is 4.9, ranging from 2.6 in southern Italy to 6.8 in Norway (Mucci et al., 2010). Although these numbers can- not be directly compared, as four of our microsatellites dif- fered from those used by Mucci et al. (2010), both the heterozygosity and polymorphism recorded in the Ticino population were as low as those reported for relict popula- tions of southern Italy (Verduci, 2019), and consistent with the small founder population. Although in the mid 1990s the EEP coordinator consid-
ered it unlikely that Asian otters had contributed to the pro- gramme’s captive population (Vogt, 1995), mtDNA analysis confirmed the suspected B-line. Although otter signs are conspicuous, small isolated populations can remain un- detected for several years (Gariano & Balestrieri, 2018). Our findings do not support the hypothesis that a residual, autochthonous population persisted that may have been reinforced by the reintroduction programme. Since reintroduction, the size of the otter population
probably fluctuated randomly on the edge of extinction, between 2–3 and ,10 individuals, unable to spread throughout the aquatic habitats of the River Ticino valley. As the river has been judged suitable for otters by three dif- ferent analyses (Prigioni, 1995; Montanari & Boffino, 2000; Prigioni & Balestrieri, 2011), the insufficient number of founders may be a major cause of low population growth. Allmodels in the population viability analysis suggest that the number of founders was also too few to ensure the long-
term survival of the population; the simultaneous release of three pairs of otters would have increased population size and probability of persistence, although the risk of extinction in the short term would still have been high. Similarly, the extinction probability of a larger group of 10 otters in Uppland, Sweden, has been reported to be 49% within 50 years (Ebenhard, 2000). Such small groups also tend to lose half of the heterozygosity over 50 years, decreasing popula- tion fitness (Ebenhard, 2000; Reed & Frankham, 2003). Inbreeding depression plays a major role in genetic sto-
chasticity and increases extinction risk for most threatened mammals (O’Grady et al., 2006). Consistently, according to population viability analysis, inbreeding depression is the factor that may contribute most to the variation in the ex- tinction risk of the small populations on the River Ticino. Considering that the genetic variability of the founder population was less than expected by EEP managers (Randi et al., 2001), there is reason to assume that the num- ber of lethal equivalents was higher than the default value used by Vortex. Under such conditions, stochastic events such as reproduction failure or road mortality would be ex- pected to drive this small isolated population to extinction. There has been debate concerning a possible reinforce-
ment of the reintroduced otter population (Balestrieri et al., 2016). Although B-line otters have also been reintro- duced in the UK and the Netherlands (Duplaix & Savage, 2018), the confirmed genetic composition of the Ticino population is amajor hurdle to its reinforcement with wild- caught otters, for example from Austria, where otters are widespread and perceived as a threat by fish farmers (Kranz, 2000). On the other hand, although in the 2000s the population was completely isolated, recent expansion of otter populations in the French (Barthelemy & Arthur, 2018), Austrian (Kranz, 2018) and Swiss Alps (Angst, 2018)
Oryx, 2022, 56(4), 617–626 © The Author(s), 2021. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605321000107
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