Balancing act: modelling sustainable release numbers for translocations

Translocated populations often share demographic, environmental and genetic risks associated with relict populations. Models that predict translocation impacts on source and founder populations are therefore necessary to ensure that harvesting for release does not jeopardize either population. However, current models generally focus on maximizing the long-term demographic viability and retention of allelic diversity in the founder population only. We therefore risk estimating a release number that is considered 'appropriate' for maintaining viability and allelic diversity in the founder population but is in fact detrimental to the source. Our objectives were to determine a sustainable number of individuals for release that maximized the viability and allelic diversity of source and founder populations. We developed a spatially explicit individual-based model that simulated hypothetical translocation scenarios for the threatened New Zealand frog, Leiopelma pakeka, which was restricted to Te Hoiere/Maud Island and has been heavily targeted for translocations. Source and translocated populations were simulated over 200 generations, representing 200 years across various initial population sizes from 20 to 300 frogs. To account for the potential loss of allelic diversity from the source population via harvesting, along with genetic drift and founder events typically associated with translocations, an additional set of simulations determined the probability of retaining a single diploid locus within populations with rare and common alleles. Our results showed that at least 120 frogs were required for populations to remain viable and maximize allelic retention in the long-term, regardless of allelic diversity. However, harvesting more than 150 frogs led to ongoing or the onset of declines in the viability and allelic retention of source populations. We recommend that future translocations of L. pakeka harvest no more than 140 frogs from each of the sub-populations on Te Hoiere/Maud Island. We also recommend the described approach to simulate hypothetical translocation scenarios for other managed species.