Camera trapping photographic rate as an index of density in forest ungulates

Abstract

Summary1. Calibrating indices of animal abundance to true densities is critical in wildlife studies especially when direct density estimations are precluded by high costs, lack of required data or model parameters, elusiveness and rarity of target species. For studies deploying camera traps, the use of photographic rate (photographs per sampling time) as an index of abundance potentially applies to the majority of terrestrial mammals where individual recognition, and hence capture-recapture analysis, are unfeasible. The very few studies addressing this method have either been limited by lack of independence between trapping rates and density estimations, or because they combined different species, thus introducing potential bias in camera trap detection rates. This study uses a single model species from several sites to analyse calibration of trapping rates to independently derived estimations of density. The study also makes the first field test of the method by Rowcliffe et al. (2008) for density derivation from camera trapping rates based on modelling animal-camera contacts. 2. We deployed camera traps along line transects at six sites in the Udzungwa Mountains of Tanzania and correlated trapping rates of Harvey's duiker Cephalophus harveyi with densities estimated from counts made along the same transects. 3. We found a strong, linear relationship (R 2 = 0AE90) between trapping rate and density. Sampling precision analysis indicates that camera trapping rates reach satisfactory precision when trapping effort amounts to 250-300 camera days. Density estimates using Rowcliffe et al.'s (2008) gas model conversion are higher than from transect censuses; we discuss the possible reasons and stress the need for more field tests. 4. Synthesis and applications. Subject to rigorous and periodic calibration, and standardization of sampling procedures in time and over different sites, camera trapping rate is shown to be, in this study, a valid index of density in the target species. Comparative data indicate that this may also apply to forest ungulates in general. The method has great potential for standardizing monitoring programmes and reducing the costs of wildlife surveys, especially in remote areas.

Influence of camera-trap sampling design on mammal species capture rates and community structures in southeastern Brazil

“…Thus, for relative abundance indices to be useful, some authorities recommend the calibration of indices using camera trap data (Carbone et al 2001) and the evaluation of the index accuracy (Diefenbach et al 1994) to demonstrate the existence of a functional relationship between the index and the true value (Diefenbach et al 1994). However, the calibration process is not trivial, and there is no firm consensus on the best strategy to follow (e.g., O'Brien et al 2003, Rowcliffe et al 2008, Rovero & Marshall 2009.…”

“…Despite these limitations and recommendations, the index of relative abundance, without calibration, obtained from the rate or frequency of capture has been widely used in camera-trap studies (e.g., Jácomo et al 2004, O'Brien et al 2003, Silveira et al 2003, Trolle & Kéry 2005, Weckel et al 2006, Kasper et al 2007, Rowcliffe et al 2008, Rovero & Marshall 2009, without consideration of the variations in species capture probability (Gu & Swihart 2004). These issues are relevant and need to be better understood because camera-trap data have been used in management and conservation studies around the world, including ratings of the distribution and monitoring of threatened species.…”

“…the use of camera-trap records as surrogates for abundance and population density of species whose individuals cannot be identified (Carbone et al 2001, Jennelle et al 2002, Yasuda 2004, Larrucea et al 2007, Srbek-Araujo & Chiarello 2007, Rowcliffe et al 2008, Rovero & Marshall 2009, Harmsen et al 2010. Despite these limitations and recommendations, the index of relative abundance, without calibration, obtained from the rate or frequency of capture has been widely used in camera-trap studies (e.g., Jácomo et al 2004, O'Brien et al 2003, Silveira et al 2003, Trolle & Kéry 2005, Weckel et al 2006, Kasper et al 2007, Rowcliffe et al 2008, Rovero & Marshall 2009, without consideration of the variations in species capture probability (Gu & Swihart 2004).…”

Monitoring wild pig populations: a review of methods

“…The use of cameras has obvious advantages in efficiency over conventional survey techniques (like tracking plots): fewer field visits and reduced staff labor time and more data resulting from continued activation and reduced downtime from adverse weather conditions (Minta and Mangel 1989;De Bondi et al 2010;Meek et al 2012). For example, not including the cost of equipment, camera trapping was nine times cheaper than line transect counts for African forest ungulates (Rovero and Marshall 2009).…”

“…Regular maintenance and calibration of the camera traps remains essential. Theft of camera traps can result in loss of equipment and data (Rovero and Marshall 2009) and may jeopardize the monitoring effort. In some highly public areas, camera traps are not suitable because of theft, and the incidental capture of people on images can create privacy issues and legal obligations for data management and use ).…”