Monitoring the occupancy and abundance of wildlife populations is key to evaluate their conservation status and trends. However, estimating these parameters often involves time and resource-intensive techniques, which are logistically challenging or even unfeasible for rare and elusive species that occur patchily and in small numbers. Hence, surveys based on field identification of signs (e.g. faeces, footprints) have long been considered a cost-effective alternative in wildlife monitoring, provided they produce reliable detectability and meaningful indices of population abundance. We tested the use of sign surveys for monitoring rare and otherwise elusive small mammals, focusing on the Cabrera vole (Microtus cabrerae) in Portugal. We asked how sampling intensity affects true positive detection of the species, and whether sign abundance is related to population size. We surveyed Cabrera voles’ latrines in 20 habitat patches known to be occupied, and estimated ‘true’ population size at each patch using DNA-based capture-recapture techniques. We found that a searching rate of ca. 3 min/250m2 of habitat based on adaptive guided transects was sufficient to provide true positive detection probabilities > 0.85. Sign-based abundance indices were at best moderately correlated with estimates of ‘true’ population size, and even so only for searching rates > 12 min/250m2. Our study suggests that surveys based on field identification of signs should provide a reliable option to estimate occupancy of Cabrera voles, and possibly for other rare or elusive small mammals, but cautions should be exercised when using this approach to infer population size. In case of practical constraints to the use of more accurate methods, a considerable sampling intensity is needed to reliably index Cabrera voles’ abundance from sign surveys.
Bibliographical noteFunding Information:
Open access funding provided by FCT|FCCN (b-on). This study was funded by Fundo Europeu de Desenvolvimento Regional (FEDER) through the Programa Operacional Factores de Competitividade (COMPETE) and national funds through the Portuguese Foundation for Science and Technology (FCT) within the scope of the projects ‘MateFrag’ (PTDC/BIA-BIC/6582/2014) and ‘Agrivole’ (PTDC/BIA-ECO/31728/2017). DP was supported by the FCT grant SFRH/BD/133375/2017. TM was supported by the FCT grant SFRH/BD/145156/2019. PB was supported by EDP Biodiversity Chair. JP was supported by the European Union’s Horizon 2020 research and innovation programme under project EnvMetaGen (grant agreement no 668981). RP was supported by FCT, through a research contract under the Portuguese Decree-Law nr 57/2016.
Data Availability StatementData are available from the corresponding author upon reasonable request.
- Cabrera vole
- Genetic non-invasive sampling
- Population monitoring
- Rarefaction curves
- Sampling intensity