Estimating deer density from the number of pellet groups per individual: strip transect vs line transect

Abstract

Abstract Red and fallow deer have become overabundant species in some areas of the Iberian Peninsula, namely in enclosures located in agro-silvo-pastoral systems called “montado” in Portugal and “dehesa” in Spain. Reliable methods to estimate deer density in such conditions are required for proper population management. Inconsistent conclusions regarding the best method to estimate deer abundance from pellet group counts show the need for additional information. In this study, instead of the defecation rate and the pellet groups’ mean time of decay, we use the number of pellet groups per individual (Pgi) to improve the precision and cost-effectiveness of the faecal standing crop approach to estimate deer density. We conducted fieldwork in two fenced areas, which replicated the conditions found in large enclosures in “montado”/”dehesa”. These were small areas easy to monitor and where the number of deer was known. Two methods for estimating Pgi were compared: strip transect and line transect. Results show that the two methods in both study areas presented good and similar levels of precision. For evaluating the accuracy, line transect estimates were used as accepted values. The discrepancy between the line transect estimate and the strip transect estimate in one of the study areas revealed that systematic double counting occurred while executing the strip transect method. Although, bearing in mind the practical recommendations herein made to overcome this problem, strip transect, which is considerably less time-consuming than line transect, appears to be the method of choice to estimate Pgi and deer density in the studied conditions.