Evaluation of invasive and non‐invasive methods to monitor rodent abundance in the Arctic

Monitoring rodent abundance is critical to understand direct and indirect trophic interactions in most northern terrestrial ecosystems. However, logistic constraints can prevent researchers from using capture–mark–recapture methods, a robust approach to estimate abundance. Our objective was to determine the correlation between abundance estimates of Arctic lemmings obtained from live‐trapping data with spatially explicit capture–recapture models ( SECR ; N/ha) and abundance indices obtained from snap‐trapping along trap lines (N/100 trap‐nights), winter nest sampling along transects with distance sampling models (N/ha), burrow counting within quadrats (N/100 m 2 ), and incidental observations (N/100 observer‐hr). We also evaluated the impact of reduced sampling effort on the bias and precision of each abundance estimate. Data from brown ( Lemmus trimucronatus ) and collared lemmings ( Dicrostonyx groenlandicus ) were collected each year from 2007 to 2016 on Bylot Island, Nunavut, Canada. Snap‐trapping ( r  =   0.90) and incidental observations ( r  =   0.92) yielded the highest correlations with live‐trapping densities for brown lemmings, the most abundant species. When combining abundance of both lemming species, snap‐trapping ( r  =   0.77) and incidental observations ( r  =   0.90) also yielded the highest correlations. Indices from winter nests and burrows were also correlated ( r  >   0.50) with live‐trapping densities, but to a lesser degree. We found that bias generally increased when effort was reduced for methods involving modeling of capture or detection probabilities (i.e., live‐trapping, winter nests), but remained low for the other methods. In contrast, precision of estimates remained high when using SECR models, but decreased substantially for the other methods during years of low lemming abundance. Non‐convergence of SECR and distance sampling models generally increased when reducing effort and was frequent in years of low lemming abundance. Interestingly, collecting >200 h of incidental observations generated highly reliable estimates of lemming abundance compared to results from live‐trapping, indicating that such non‐invasive method can provide valuable data at low cost. We provide guidelines on other invasive or non‐invasive methods that can be used when small mammals cannot be live‐trapped and suggest the effort required to achieve a given precision.