Monitoring roe deer populations in the Alps: the pellet group count with distance sampling approach

Monitoring roe deer populations in the Alps: the pellet group count with distance

sampling approach

Francesca Cagnacci

_{, Luca Pedrotti}

_{, Alessandra Tagliabò}

_{, Lucio Luchesa}

_,

_Barbara

Franzetti

_{, Stefano Focardi}

1 Centre of Alpine Ecology, Viote del Monte Bondone, 38040 Trento-Italy; tel. 0461 939568; Fax 0461 948190; cagnacci@cealp.it

2 Consorzio del Parco Nazionale dello Stelvio – Settore trentino, via Roma 65, 38024 Cogolo di Peio (TN-Italy); tel. 0463 746121; Fax 0463 746090; luca.pedrotti@libero.it

3 Associazione Cacciatori Trentini, via Guardini 41, 38100 Trento-Italy

4 _{Istituto Nazionale per la Fauna Selvatica, via Ca’ Fornacetta 9, 40064 Ozzano dell’Emilia (BO); tel. 051}

6512111; Fax 051 796628; franzetti@iperbole.bologna.it; stefano.focardi@infs.it

* _{After a grant by Municipality of Trento}

Roe deer population size has traditionally represented a difficult task for population ecologists, due to the preference of wood and elusive behaviour of the species. The Alpine habitat, characterised by patchiness and a poorly accessible terrain, may even enhance the difficulties. In this paper, we assessed the feasibility and efficacy of pellet group count with distance sampling as opposed to censusing methods commonly applied in game management (e.g. drives, spot counts, block censuses). The study was conducted in a 9500ha area (Monte Bondone, Trento Province, Italy) where a sample of the roe deer population was tracked by means of GPS-telemetry. Systematic grids of line transects were randomly over imposed to the study area and sampled in 2006, both in spring (winter density) and autumn (summer density), for a total survey length of 43200m and 216 transects. From October 2005 to October 2006, we carried out and experimental assessment of pellet group decay rate, placing fresh samples in different habitats and analysing monthly disappearance rates by logistic regression. In autumn, we carried out thermal imaging distance sampling and mark-resight observations in a portion of the study area. Mean monthly time to decay was mainly influenced by habitat, slope and weather condition at deposition, particularly snow cover in winter months. In spring, pellet group encounter rate was higher than in autumn (0.020 pellet groups/m and 0.009 pellet groups/m, respectively), which led to a lower %CV of pellet group density estimate (13% and 26% respectively). This corresponded to a longer time to decay in spring than in autumn (242.9 days and 57.2 days respectively) and to halved roe deer density values (2.5 deer/100ha and 5.5deer/100ha respectively). However, the disproportion between the decrease of encounter rate and time to decay between the two seasons (2 fold vs 4 fold), is likely due to a high variability of the spring mean times to decay, which may have led to biased spring density values. In autumn, pellet group count density was comparable to thermal imaging distance sampling and mark-resight estimates and consistent with hunting data. Therefore, density estimates proved more robust when pellet groups decayed in a short time, comparable to the length of the sampling period.

Pellet group count with distance sampling seemed to encompass randomness, replicability and possibility to control a priori the threshold precision/effort. However, if routinely used, a continuous monitoring of decay rate is strongly suggested.

This research has been carried out in the context of the project “BECOCERWI” (Grant N. 3479, funded by the Autonomous Province of Trento to F.C.)