How many are there? Multiple-covariate distance sampling for monitoring pampas deer in Corrientes, Argentina

Abstract

Context The pampas deer (Ozotoceros bezoarticus) is an endangered species in Argentina. Scarce information exists about one of the four last populations that survive in Corrientes province, where direct counts estimated a population of <500 individuals. Aims To evaluate the status of the pampas deer population in Corrientes by applying a standardised methodology and to develop methodological recommendations for future deer monitoring. Methods We conducted six population censuses between 2007 and 2010, using line transects placed on roads throughout 1200 km2 of grasslands in the Aguapey region, Corrientes, Argentina. From a moving vehicle, we counted every pampas deer group observed along transects. We used Distance 6.0 and its multiple-covariate distance sampling engine to estimate deer density, while exploring the potential effect of roads, habitat type, hour, season, observer experience and survey effort on deer occurrence and density estimation. Key results The occurrence of pampas deer was irrespective of transect location (minor or major road) but a greater number of animals was detected over transects in minor roads and in areas covered by grasslands with young pine plantations. We estimated a density of 1.17 individuals km–2 (s.e. = 0.52), and habitat type was the most important covariate for density estimation. We estimated a total population of 1495 deer (95% CI = 951–2351, CV = 23.27%) for the Aguapey region in Argentina. Conclusions Corrientes hosts one of the largest populations of pampas deer in Argentina, with ~1000 individuals. The fact that we estimated a larger population than did previous studies could be explained either by actual population growth during the past 10 years, or by the use of more exhaustive and sophisticated sampling design and data analysis. Implications Population surveys using covariate distance sampling on ground line transects can provide more realistic population estimates than do other simpler methods. Our population estimates and methods can be used as a baseline for future monitoring of this population, as long as factors such as sampling effort, type of roads for locating transects, and habitat type are considered in future analysis.

Temperate Grassland Afforestation Dynamics in the Aguapey Valuable Grassland Area between 1999 and 2020: Identifying the Need for Protection

“…In this regard, [13] stated that six globally threatened bird species, distributed within the Aguapey basin, were highly impacted by the presence of tree plantations, which not only affected their breeding habitats but also impeded their ability to disperse between habitat patches. The impacts of temperate grassland fragmentation have also been reported in regard to the endangered Pampas deer [31]; however, a recent study suggested that Pampas deer could be positively selecting grassland patches within young tree plantations as refuge against predators [61]. The ecological effects are therefore complex and warrant further study.…”

Using spotlight observations to predict resource selection and abundance for white‐tailed deer

“…Whereas habitat‐based population models used presence‐only locations in a spatially explicit manner to develop deer population estimates, distance sampling models incorporated the distance and direction from observers to each deer to model the detection function and estimate density. We used the multiple covariate distance sampling (MCDS) engine (Marques and Buckland ) to estimate the detection function for deer in response to the proportion of forest cover and terrain ruggedness measured at 2 scales in a GIS (90 m and 120 m) and deer density (Anderson et al , Zamboni et al ). We further extrapolated deer abundance statewide based on the area surveyed during spotlight surveys.…”