Giraffe translocation population viability analysis

De Lee,F Deacon,M Strauss,Z Muller,Cpj Scheijen,C Van Oosterhout,E Fienieg,Kd Carter

doi:10.3354/esr01022

Abstract

Most populations of giraffes have declined in recent decades, leading to the recent IUCN 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 and the genetic load, examining how variation in founding numbers and sex ratios affected 100 yr probability of population 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 compose 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.

Highlights

Giraffes Giraffa camelopardalis Linnaeus are endemic African ruminant ungulates and one of only a handful of extant terrestrial megaherbivores (OwenSmith 1988)
Long-term (100 yr) viability depended on asymptotic population growth rate and inbreeding genetic load, as well as which success criteria were used, but the minimum founding population varied from 10 to 60 females and 1 to 18 males (Table 2)
Our analyses evaluated the likely result of a giraffe translocation under reasonable demographic and genetic assumptions based on published literature and provided quantitative guidance to improve outcomes

Summary

Introduction

Giraffes Giraffa camelopardalis Linnaeus are endemic African ruminant ungulates and one of only a handful of extant terrestrial megaherbivores (OwenSmith 1988). Population modelling for translocation projects can offer guidance on age- and sex-classes for founder populations by providing estimates of population persistence, long-term viability, and genetic diversity (Seddon et al 2007). Our objective was to identify the minimum size and sex ratio of a founding population of giraffes that would ensure long-term population viability and genetic diversity. We present this as an example of how translocation planning can be improved using a quantitative approach. Single-population, individual-based models to project future population trajectories and estimate probability of extinction (PE), stochastic rate of population increase (r), and genetic diversity (GD) at a hypothetically ideal prospective release site under different founder population release scenarios. We performed a sensitivity analysis to determine which demographic and genetic load parameters had the greatest impact on translocation success, measured as proportional influence on PE and GD

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