Faecal sampling along trails: a questionable standard for estimating red fox Vulpes vulpes abundance

In most studies that estimate abundance of foxes from faeces counts, scanning is done along trails and roads or along linear features such as hedges, because it is supposed that foxes defecate mainly along these structures. As a consequence, only part (i.e. trails or linear features) of the total habitat is searched and results are possibly biased if usage by foxes of these searched features is subject to spatial or temporal variation. We therefore investigated three methods for counting red fox Vulpes vulpes faeces, that differ in the shape of the sampling units: trails and two alternatives; i.e. transects and squares. We searched for faeces using these three methods in two study areas (the Upper Rhine Valley and the Black Forest valleys) at 61 study plots and found a total of 257 fox faeces. Methods for estimating abundance should ideally have high accuracy and high precision. As actual fox densities in the areas were unknown, we were unable to assess the accuracy of our sampling methods and thus focused on method precision. We fit separate negative binomial regression models for each method with the number of faeces found as the dependent variable and a set of landscape variables as possible explanatory variables. The transect method detected significant differences in the number of faeces found between the study areas and was most precise. Even though we did find more faeces with the trail method, the precision of this method was lower than that of the transect method. For the methods trail and square, variance in the number of faeces found was large in comparison to their mean. Bias caused by methods that only sample part of the habitat is not limited to faecal counts and red fox studies, but can also occur with other species and methods.