Drive counts as a method of estimating ungulate density in forests: mission impossible?

Abstract

Although drive counts are frequently used to estimate the size of deer populations in forests, little is known about how counting methods or the density and social organization of the deer species concerned influence the accuracy of the estimates obtained, and hence their suitability for informing management decisions. As these issues cannot readily be examined for real populations, we conducted a series of ‘virtual experiments’ in a computer simulation model to evaluate the effects of block size, proportion of forest counted, deer density, social aggregation and spatial auto-correlation on the accuracy of drive counts. Simulated populations of red and roe deer were generated on the basis of drive count data obtained from Polish commercial forests. For both deer species, count accuracy increased with increasing density, and decreased as the degree of aggregation, either demographic or spatial, within the population increased. However, the effect of density on accuracy was substantially greater than the effect of aggregation. Although improvements in accuracy could be made by reducing the size of counting blocks for low-density, aggregated populations, these were limited. Increasing the proportion of the forest counted led to greater improvements in accuracy, but the gains were limited compared with the increase in effort required. If it is necessary to estimate the deer population with a high degree of accuracy (e.g. within 10% of the true value), drive counts are likely to be inadequate whatever the deer density. However, if a lower level of accuracy (within 20% or more) is acceptable, our study suggests that at higher deer densities (more than ca. five to seven deer/100 ha) drive counts can provide reliable information on population size.

Young conifer stands form a deer browsing refuge for an oak admixture: silvicultural implications for forest regeneration under herbivore pressure

“…comm.). If well performed, this method provides reliable estimates on population size (Borkowski et al 2011). The estimated red deer density are in line with other estimates in north-eastern Poland (2.6-7.6 ind/100 ha, Borowik and Jędrzejewska 2017).…”

Predation of wildlife by free-ranging domestic dogs in Polish hunting grounds and potential competition with the grey wolf

“…We also collected reported data on annual estimates of populations of game species on hunting grounds, estimated directly via visual observations of animals during line-transect distance sampling, drive counts, or plot sampling, or indirectly via track counts (Borkowski et al, 2011;Chećko, 2011;Fonseca et al, 2007). Although survey methods differ among clubs and thus have limitations (Wawrzyniak et al, 2010), we assumed that they were reliable enough to provide comparable information on the relative abundance of game species, as have prior studies (e.g., Borkowski et al, 2011;Panek, 2006).…”

“…Although survey methods differ among clubs and thus have limitations (Wawrzyniak et al, 2010), we assumed that they were reliable enough to provide comparable information on the relative abundance of game species, as have prior studies (e.g., Borkowski et al, 2011;Panek, 2006). In addition, we summarized data on harvest levels within each hunting ground.…”

The Role of Topography in the Distribution and Intensity of Damage Caused by Deer in Polish Mountain Forests

“…The results of many studies that were carried out in various locations throughout the world indicated that the high density of ungulates has strongly impacted many ecosystems, including forests [3,4,5,6,7]. Intensive pressure from deer modified not only forest regeneration, but also limited the diversity of the herbaceous vegetation, its layout and structure, soil characteristics and the deposition of mycorrhizal fungal spores.…”