Avoiding bias in estimates of population size for translocation management.

Bickerton, Katherine T; Ewen, John G; Canessa, Stefano; Cole, Nik C; Frost, Fay; Mootoocurpen, Rouben; McCrea, Rachel (2023). Avoiding bias in estimates of population size for translocation management. Ecological Applications, 33(8), e2918. Ecological Society of America 10.1002/eap.2918

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Mark-recapture surveys are commonly used to monitor translocated populations globally. Data gathered is then used to estimate demographic parameters, such as abundance and survival, using Jolly-Seber (JS) models. However, in translocated populations initial population size is known and failure to account for this may bias parameter estimates which are important for informing conservation decisions during population establishment. Here, we provide methods to account for known initial population size in JS models by incorporating a separate component likelihood for translocated individuals, using a maximum-likelihood estimation, with models that can be fitted using either R or MATLAB. We use simulated data and a case study of a threatened lizard species with low capture probability to demonstrate that unconstrained JS models may overestimate the size of translocated populations, especially in the early stages of post-release monitoring. Our approach corrects this bias; we use our simulations to demonstrate that overestimates of population size between 78-130% can occur in the unconstrained JS models when detection probability is below 0.3 compared to 1-8.9% for our constrained model. Our case study did not show an overestimate; however accounting for the initial population size greatly reduced error in all parameter estimates and prevented boundary estimates. Adopting the corrected JS model for translocations will help managers obtain more robust estimates of population size of translocated animals, better informing future management including reinforcement decisions, and ultimately improving translocation success. This article is protected by copyright. All rights reserved.

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