Hello All, Many ECOLOG readers requested that I summarize the responses I received to my question about whether or not game cameras could be used to estimate population density or abundance for small mammals without the requirement to identify individual animals. So, following is a summary of the responses I received. I hope the summary will be helpful. If anyone decides to experiment with any of these methods I would love to hear about your experiences and viewpoints.
Cheers, Greta "This will likely be an unsatisfying answer, but occupancy modeling techniques could help you quantify the proportion of area occupied (PAO) by each species or multiple species. Anyway, there are several models that come to mind from the book (4.1.6 being one relevant example) Hierarchical Modeling and Inference in Ecology by Royle and Dorazio. The authors provide code for most of the models and the text is well written. Potentially you could set the cameras to take pictures at regular intervals, in this case the pictures could be similar to doing an aerial survey, point count or numerous other population survey methods. The pictures or even videos (the longer the video the longer the exposure the higher the probability of detection) could be treated as repeat visits, which would then facilitate a hierarchical modeling approach to separate the observation and process error. Using the hierarchical formulation covariate estimates related to occurrence could then be used to map the species distribution and/or density. You would have to do some reading, thinking about assumptions and the like, but at first blush I think this idea may have merit. Track plate arrays would also be a nice addition to your existing data." "Most likely the small mammals you see in your seed patches don't have any unique marking for individual identification. What you might want to do is trap and mark them (using a sharpie or fur clipping) at the side or top. If you can mark them uniquely, then you might even be able to monitor visitation rates of individuals in patches. One of my profs (Douglas Morris, Lakehead University) used to fur clip small mammals in Canada and use video recording to study their foraging behavior. I my doctoral research on desert gerbils, I marked them with PIT tags and had special readers under their foraging patches. I carried out some food augmentation experiments to look at how that affects coexistence among two species of gerbils, and their anti-predator behaviors. I hope this helps." "You need be able to identify individuals. With large mammals, usually with cats, this is generally based on strips or dots fur patterns. In general two cameras are used to get pictures from both sides of an individual. Since with small mammalls fur patterns may be homogeneus I believe that you would have to catch them at least once and come up with some visible marker which could rest for all the study period. Since you would need investments on traps anyway I am not sure if it would be an advantage to invest on cameras as well, rather than run a traditional recapture with traps. But if funds are available the use of cameras could be possible. I would borrow some and run a trial experiment first if possible." "If you are just interested in getting just an estimate of abundance of the small mammals, you could use activity as a surrogate for density. Activity, can be measured directly from the number of visits in the camera traps. A simpler and cheaper method is to use tracking plots. Clear small (e.g. .5x.5m) plots on the ground, and sprinkle some soft soil/sand on it, and then just count the # of crossing of small mammals on these plots, or estimate what percentage of the plot is covered by their tracks. This should give you a good estimate of density, because usually activity and density are highly correlated. It might actually be even better to measure activity because, since small mammals hoard seeds, even though the actual density maybe small (i.e. you might not see a change in density with your treatments), but the overall activity of the residents might increase because now there is more food in the system." Citations for relevant papers: Journal of Applied Ecology 2008,45:12281236 Estimating animal density using camera traps without the need for individual recognition J. Marcus Rowcliffe, Juliet Field, Samuel T. Turvey, and Chris Carbone Using Counts to Simultaneously Estimate Abundance and Detection Probabilities in a Salamander Community Author(s): C. Kenneth Dodd, Jr. and Robert M. Dorazio Source: Herpetologica, Vol. 60, No. 4 (Dec., 2004), pp. 468-478 Wildlife Research, 2010, 37, 456465 A comparison of the effectiveness of camera trapping and live trapping for sampling terrestrial small-mammal communities Natasha De Bondi, John G. White,C, Mike Stevens and Raylene Cooke
