Giraffe Translocation Population Viability Analysis

Most populations of giraffes have declined in recent decades, leading to the recent decision to upgrade the species to vulnerable status, and some subspecies to endangered. Translocations have been used as a conservation tool to re-introduce giraffes to previously occupied areas or establish new populations, but guidelines for founding populations are lacking. To provide general guidelines for translocation projects regarding feasibility, we simulated various scenarios of translocated giraffe populations to identify viable age and sex distributions of founding populations using Population Viability Analysis (PVA) implemented in Vortex software. We explored the parameter space for demography (population growth rates: λ = 1.001, 1.010, 1.024), and the genetic load (number of lethal equivalents: LE = 2.5, 6.29, 12.6), examining how variation in founding numbers (N = 5 to 80 females) and sex ratios (M:F = 0.1 to 0.5) affected 100-year probability of extinction and genetic diversity. We found that even very small numbers of founders (N ≤10 females) can appear to be successful in the first decades due to transient positive population growth, but with moderate population growth rate and moderate genetic load, long-term population viability (probability of extinction <0.01) was only achieved with ≥30 females and ≥3 males released. To maintain >95% genetic diversity of the source population in an isolated population, 50 females and 5 males are recommended to comprise the founding population. Sensitivity analyses revealed first-year survival and reproductive rate were the simulation parameters with the greatest proportional influence on probability of extinction and genetic diversity. These simulations highlight important considerations for translocation success, and data gaps including true genetic load in wild giraffe populations.

population of simulated individuals using demographic parameters from literature (Lee and 79 Strauss 2016, Dagg 2014) and publications of the Giraffe International Studbook (Lackey 2009). 80 Males bred from ages 2 to 25, and females bred from ages 3 to 29, the maximum age of survival 81 for both sexes was 30 years, females always produced 1 calf, and sex ratio at birth was equal 82 (Dagg 2014). Within the model parameters, offspring were dependent upon their mother for 1 83 year, meaning that if the mother died during the calf's first year, then the calf also died (Dagg 84 2014). All males aged 2 and above were in the breeding pool. Demographic rates are given in 85   Table 1. 86 To parameterize our PVA, we based our demographic rates on published observations of means 87 and variances of age-specific survival and fecundity for wild giraffe populations throughout 88 Africa (Lee and Strauss 2016), and data for reproductive longevity and inbreeding depression 89 from the global zoo population (Lackey 2009). We used data from IUCN Red List assessments 90 to compute mean population growth rates for 7 growing giraffe subpopulations, including some   and reproduction due to environmental variation at 0.5.

127
We assumed zero translocation-related mortality, so no additional mortality effect above normal 128 levels due to the process of capturing, relocating, and releasing. If mortality is expected during 129 the translocation process, our simulation results should be interpreted using the actual number of 130 successful live releases. We assumed zero post-release dispersal movements because many 131 translocations will likely be into fenced or otherwise constrained areas, and because we wanted 132 to keep track of every individual in the translocated population. 133 We projected 198 PVA scenarios. We simulated populations with various numbers of 2-year-old 134 females released (5, 10, 20, 30, 40, 50, 60, and 80 females) and different numbers of 2-year-old 135 males released to vary the sex ratio (SR) at release (SR = males / females, range = 0.1 to 0.5). 136 We simulated each combination of number of females and males at the 3 levels of asymptotic size and sex ratio for successful translocations with success defined 4 ways: probability of 150 extinction PE < 0.05 and PE < 0.01; and genetic diversity GD > 80% and GD > 90%. 151 We performed a sensitivity analysis to determine which demographic or inbreeding parameter 152 was most influential to long-term viability by comparing outcomes from simulations that were 153 identical in every way except that we reduced a single demographic or inbreeding parameter by approximately 25%. We simulated release of 30 females and 3 males using moderate 155 demographic rates (Table 1, middle columns) and moderate genetic load (LE = 6.29) as the 156 reference simulation. We then simulated 6 identical populations except each had a single change:

163
Population projections from all our scenarios followed the same general post-release trajectory. year) viability depended on population growth rate and inbreeding genetic load, as well as which 170 success criteria were used, but minimum founding population varied from 10 to 60 females and 1 171 to 15 males ( Table 2).

172
The 100-year PE declined as the number of released females, and sex ratio increased (Fig. 1). 173 Assuming fast population growth demographic rates and minimum inbreeding genetic load, the 174 100-year PE was below 0.05 when release included at least 10 females and 3 males ( Table 2). 175 Under more conservative demographic rates (λ = 1.001) and realistic inbreeding genetic loads minimum founding population sizes were similar to those required for the criteria of PE < 0.05, 180 i.e. 30 females and 3-6 males (  reproductive rate were the most influential on long-term viability metrics of PE and GD (Table   202 3). Changes in first-year mortality and reproductive rate both increased PE > 10% and reduced 203   During the "founders' years" from year 2 to 23 in all our projections, initial population growth 244 will likely be positive if 2 or more females are translocated. However, this initial growth is a transient effect of the young-skewed age distribution of the founding population, and very few 246 translocations of small (<20 females) founding populations will be viable in the long term in 247 isolation due to a lack of genetic diversity (Fig 1). Increasing the number of females in the initial with assumptions made during planning. As our sensitivity analysis showed, if demographic 282 rates such as juvenile survival or reproductive rates are below critical levels, the translocated 283 population will be considerably less viable than was predicted.

284
The ideal female age class for translocations in terms of population growth rate is the youngest 285 age class that can immediately begin reproducing, but adults are logistically challenging to move 286 due to their large body size. All of our population projections used the logistically more tractable 287 age class of 2-year-olds for translocation, as this age class is often chosen for translocations.