Design, evaluate, refine: camera trap studies for elusive species

The rapid expansion of camera trap surveys for elusive species has led to the widespread application of this technique, often with little standardization across studies. For example, even when targeting the same species, the amount of effort (i.e. trap nights or camera days) can vary widely from 450 trap nights (Trolle Maffei Silver et al., 2004) and still others as potential indices of abundance (O'Brien, Kinnard Thompson et al., in press). The authors examine this issue by comparing animal size with the number of photo pairs only, discounting photos when only one camera fired. The fact, however, that one of two opposing cameras did not trigger may be more related to the idiosyncrasies of camera placement rather than animal size in this instance. The authors used 50 cm height for camera placement which is substantially higher than other studies designed to photograph ocelots at 20 cm (Trolle Engeman, 2003) and surrounding the specific use of camera trap data as an index of species abundance (Carbone et al., 2001, 2002; Jennelle, Runge & MacKenzie, 2002). But given the fact that trap success appears highly correlated between years, trap effort is straightforward to calculate, and in the field, cameras are likely to be subject to fewer sources of error than other indices (e.g. variable ability of technicians in variable field conditions), it seems reasonable to explore this issue further. Perhaps new camera-trapping studies should focus on calibration of trapping rates to independent assessments of species density so that the substantial information gained from camera studies on both target and non-target species could be made more useful for long-term species conservation. Correlation between trap success (capture frequency per 1000 trap nights) for terrestrial mammals in 2005 and 2006 (n=26, rs=0.846, P<0.0001). Data from Tobler et al. (2008), Table 1. There is no question that the rise of these machines has opened up new avenues for the study of elusive species. However, there are still substantial methodological issues to explore and far too often remote camera studies give little forethought to study design and subsequent data analysis. This problem may be compounded as methodology becomes outpaced by technology (e.g. faster, more sensitive digital cameras, directly downloadable images from base stations or satellites, etc.). Studies such as Tobler et al. are still needed to further refine methodology, thereby allowing us to build more thoughtful and useful remote camera studies in the future. Thanks are due to Guy Cowlishaw for the opportunity to comment and to T.E. Mc Namara for proofreading.