Are abundance indices derived from spotlight counts reliable to monitor red deer Cervus elaphus populations?

Management of large herbivores could be improved by investing less effort in estimating absolute abundance and more effort tracking variation over time of indicators of ecological change (IEC) describing animal performance, herbivore impact on habitat, and relative animal abundance. To describe relative changes in animal abundance, monitoring trends in numbers through indices may constitute a useful and low cost method, especially at large spatial scales. Reliability of indices to detect trends should be evaluated before they are used in wildlife management. We compared population trends estimated from spotlight counts, a standard census method for deer populations, with population size estimates of a red deer Cervus elaphus population monitored using Capture-Mark-Recapture (CMR) methodology. We found a strong negative effect of conditions of observation (e.g. rainfall) on both the number of animals (-24.4%) and the number of groups (-31.6%) seen per kilometre. After controlling for observation conditions, we found that these two abundance indices were linearly correlated with CMR estimates, with the group-based index being better correlated (r = 0.75) than the individual-based index (r = 0.68). These consistent trends between indices and CM R estimates provide support in using standardised spotlight counts as an IEC describing relative changes in abundance for the monitoring and management of red deer populations.