Preserving identity in capture-mark-recapture studies: increasing the accuracy of minimum number alive (MNA) estimates by incorporating inter-census trapping efficiency variation

Quantifying abundance is often key to understanding ecological and evolutionary processes in wild populations. Despite shortcomings in producing accurate abundance estimates, minimum number alive (MNA) remains a widely used tool, due to its intuitive computation, reliable performance as an abundance indicator, and linkage to individual life-histories. Here, we propose a novel efficiency-modified MNA (eMNA) metric, which aims to preserve MNA's favourable aspects while remedying its flaws, by incorporating (a) growth correlates to back-age individuals first captured as adults, and (b) estimates of undetected persistence beyond last capture based on time-varying capture efficiency. We evaluate eMNA through samplings of a simulated baseline population parameterised using data from a long-term demographic study of European badgers (Meles meles), under three different levels of capture efficiency (low; intermediate/real based on badger field data; high). We differentiate between eMNA's performance as an abundance estimator-how well it approximates true abundance (accuracy)-and as an abundance indicator-how tightly it correlates with population abundance and changes thereof (precision). eMNA abundance estimates were negatively biased at all capture efficiencies. However, this bias was negligible at intermediate-to-high capture efficiency, particularly once low-information terminal sampling years (the first year and final three years of simulated studies) were removed. Excluding these years, eMNA under-estimated abundance by only 3.5 badgers (1.5% of population) at intermediate (real) capture efficiencies, and performed as a precise abundance indicator, with half the standard deviation of Cormack-Jolly-Seber probabilistic estimates and proving robust to inter-sampling variation in capture efficiency. Using undetected persistence probabilities to parameterise survival regression, we recreated baseline age-based survival relationships, albeit with some negative bias for under-represented ages. We offer considerations on the continued limitations of using eMNA for abundance estimates, minimum study duration for reliability, the metric's benefits when individual identity is required, and potential for further improvement.

Automated summary

This study proposes a modified version of the minimum number alive (MNA) metric, called efficiency-modified MNA (eMNA), which improves the accuracy and precision of MNA by incorporating growth correlates and estimates of undetected persistence. Experimental results using simulated badger population data show that eMNA performs well in terms of accuracy and precision at intermediate to high capture efficiencies and is robust to changes in capture efficiency.