A response to Hesford & MacLeod (2022): Rejection of a model estimating high densities of mountain hares in the Peak District, England.

Abstract

A recent Game and Wildlife Conservation Trust (GWCT) report (Hesford & MacLeod 2022) suggested densities of mountain hares (Lepus timidus) may reach 52 - 125/km2 in parts of the Peak District, England. These are notably higher than previous and current estimates of 5 - 33 hares/km2 (Matthews et al. 2018; Bedson et al. 2022). We review Hesford & MacLeod (2022) who based their methods on those used in a review of mountain hare survey methods in Scotland (Newey et al. 2018). This review demonstrated a weak, non-significant relationship between hare encounter rates using spotlight surveys of walked transects at night and estimated densities derived from spatial capture-recapture (SCR) methods on managed heather moorland (p=0.08). Newey et al. (2018) recommended that this relationship should not be used to estimate hare densities. We reproduce the Newey et al. (2018) linear model and confirm its poor predictive ability and show that removal of one outlier reduces an already marginal relationship to a near-flat line (p=0.80). Hesford & MacLeod (2022), nonetheless, used this relationship to estimate hare densities along non-randomly placed transects. We conclude that reportedly high mountain hare densities estimated by Hesford & MacLeod (2022) are biased and based on a model with little predictive power; more recent Distance Sampling estimates are from 37 - 96% lower (Bedson et al. 2022). It is important that wildlife monitoring methods robustly account for survey bias and error, detection probability and variation between habitats, especially if results are to inform potential population management interventions.