It counts who counts: an experimental evaluation of the importance of observer effects on spotlight count estimates

Abstract

Spotlight surveys conducted by volunteers is a promising method to assess the abundance of nocturnally active mammals, but estimates are subject to bias if different observer groups differ in their ability to detect animals in the dark. We quantified the variation amongst volunteer spotlight observers with respect to their ability to detect and estimate distance to realistic animal silhouettes at different distances. Detection probabilities were higher for observers experienced in spotlighting mammals than for inexperienced observers, higher for observers with a hunting background compared with non-hunters and decreased as function of age but were independent of sex or educational background. If observer-specific detection probabilities were applied to real counting routes, point count estimates from inexperienced observers without a hunting background would only be 43 % (95 % CI, 39-48) of what inexperienced hunters with a hunting background would obtain and 29 % (25-33) of what experienced spotlight observers would detect. Mean estimated distances to objects did not deviate from true distances (no bias) but were highly imprecise. Female non-hunters estimated distances less precisely than other observers and precision increased with age. The study shows that observer effects may influence abundance estimates and underlines the importance of testing and accounting for observer effects when designing citizen science-based population survey programmes.

The potential for citizen science to produce reliable and useful information in ecology

“…In detecting organisms, volunteers were comparable in some cases (mammals on territories [Newman et al. ]), but not if expertise comes from lengthy experience (cryptic marine invertebrates [Foster‐Smith & Evans ], nocturnal mammals [Sunde & Jessen ], African ungulates in herds [Steger et al. ]).…”

Bridging the Benefits of Online and Community Supported Citizen Science: A Case Study on Motivation and Retention with Conservation-Oriented Volunteers

“…Online tools and resources provide a cost-effective option for that training (Ellul et al 2001;Lye et al 2012;Silvertown et al 2013;Sunde and Jessen 2013). Augmenting cyberlearning with social media tools in citizen science projects may provide opportunities to bridge seemingly divergent motivations across diverse participants.…”

Do Weather and Moon Have Any Influence on Spotlighting Mammals? The Case of Hare in Upland Ecosystem

“…Therefore, there is currently a difficulty in designing a study that takes weather and moonlight effects into account; consequently, their influence is generally overlooked in hare spotlight counts (Strauß et al, 2008;Sunde and Jessen, 2013). In upland and shrubland ecosystems in particular we are unaware of any studies concerning the influence of abiotic factors on spotlighting brown hares.…”

“…Spotlighting (spotlight count or night lighting) is widely used to survey mammals during night (Scott et al, 2005;Sunde and Jessen, 2013). This is a favored method of estimating abundance due to its efficiency, repeatability and no necessity for animals capture (Langbein et al, 1999).…”

“…However, numerous studies have shown that several factors can affect the spotlight counts of mammals. These include the animal's activity patterns and behaviour (Brown and Peinke, 2007), visibility (Parkes, 2001; Barnes and Tapper, 1985), relief and amount of vegetation cover (Ralls and Eberhardt 1997;Langbein et al, 1999;Tizzani et al, 2013), observer experience (Sunde and Jessen, 2013), time of night and moonlight (Langbein et al, 1999;Scott et al, 2005) and weather conditions (Langbein et al, 1999).…”