The Vulnerable marsh deer Blastocerus dichotomus, the largest native cervid in South America, is declining throughout its range as a result of the conversion of wetlands and overhunting. Estimated densities in open wetlands of several types are 0.1–6.8 individuals per km2. We undertook the first unmanned aerial vehicle (UAV) survey of the marsh deer to estimate the density of this species in a 113.6 km2 area under forestry management in the lower delta of the Paraná River, Argentina. During 6–8 August 2019, at a time of year when canopy cover is minimal, we surveyed marsh deer using Phantom 4 Pro UAVs along 94 transects totalling 127.8 km and 8.6 km2 (8.1% of the study area). The 5,506 photographs obtained were manually checked by us and by a group of 39 trained volunteers, following a standardized protocol. We detected a total of 58 marsh deer, giving an estimated density of 6.90 individuals per km2 (95% CI 5.26–8.54), which extrapolates to 559–908 individuals in our 113.6 km2 study area. As it has generally been assumed that marsh deer prefer open habitats, this relatively high estimate of density within a forestry plantation matrix is unexpected. We discuss the advantages of using UAVs to survey marsh deer and other related ungulates.

Wild ungulates, and particularly deer, can cause severe damage to commercial plantations, resulting in reduced tolerance of their presence by forestry producers. The marsh deer Blastocerus dichotomus, categorized as Vulnerable on the IUCN Red List, is declining throughout South America. A population of c. 500 individuals survive within a matrix of commercial plantations in the lower delta of the Paraná River, the southernmost stronghold for the species. Local forestry producers usually report that damage to plantations is attributable to marsh deer, thus justifying persecution of the species. Seventy-six forestry producers (representing c. 33% of the total plantation area of the lower delta) were interviewed using a semi-structured questionnaire to assess perceived levels of tree damage, associated economic losses, and attitudes towards the deer. Simultaneously, plantation stands were surveyed to quantify the actual tree damage caused by this ungulate. Seventy-six percent of producers reported damage to trees by deer (i.e. browsing, fraying caused by antler rubbing) but most of them perceived low levels of damage per property (median < 0.2%), with negligible economic effects. However, 5% of producers (all of them with ≤ 2 km2 in production, usually family enterprises) perceived high levels of damage and economic losses, and supported deer hunting as a management option. Field surveys indicated that damage caused by deer could be more severe than perceived by producers, although spatially confined within the landscape. Monitoring of damage perception by forestry producers, and compensation schemes to assist small producers are necessary for adequate management of this threatened marsh deer population.

Aerial surveys have been used successfully to estimate vertebrate populations in open habitats. The marsh deer Blastocerus dichotomus, categorized as Vulnerable on the IUCN Red List, lives in such habitats and is suitable for aerial counting because it is conspicuous. This species, the largest South American deer, is native to Argentina, Boliva, Brazil, Paraguay and Peru but no reliable information has previously been available on its populations in Bolivia. From May to August 2007 we conducted aerial transects to survey marsh deer in three large areas of savannah. We used a modified mark–recapture method to improve the accuracy of the counts and estimated density and abundance. The corrected, estimated density of the marsh deer was 0.24 km−2 in the northern La Paz Department, 0.12 km−2 in Mamoré and 0.15 km−2 in Iténez. These densities are similar to the mean density of the species on other South American savannahs. This is the first large-scale survey of the marsh deer in Bolivia and the first to provide information about the density of the species in the Amazon. We recommend the creation of protected areas in these savannahs, and wildlife and domestic health programmes to conserve the marsh deer of this region.