Estimates of population deer population density are useful in guiding management decisions relating to the need and effectiveness of deer management. However, past experience with methods based on direct counts often yielded serious under-estimates in forested environments, so alternative approaches have been developed.
Initial investigations focussed mainly on faecal pellet counts as a method that would be suitable for use in dense forest vegetation. This approach relied on estimation of the rates of deposition and disappearance of pellet groups, and have been described, along with other methods and sampling considerations in:
Mayle, B. Peace, A. Gill, R. (1999) How many deer? – a guide to estimating deer population size. Field Book 18, Forestry Commission, Edinburgh.
Later studies, developed in collaboration with Strath Caulaidh Ltd. indicated that methods based on counts of pellets obtained in two successive visits in marked plots could be more efficient, particularly in upland environments.
Campbell, D., Swanson, G.M. and Sales, J. (2004). Comparing the precision and cost-effectiveness of faecal pellet group count methods. Journal of Applied Ecology, 41, 1185-1196).
A full description, and justification, of the method appears in:
Swanson, G, Campbell, D & Armstrong, H. (2008). Estimating deer abundance in woodlands: the combination plot technique. Forestry Commission Bulletin no. 128. Forestry Commission, Edinburgh.
The use of thermal imaging was also developed for estimating deer population density. Thermal imagers detect the radiant heat energy emitted from warm-bodied animals, and can make nocturnal and camouflaged animals highly conspicuous. Large mammals can be detected at distances of up to 2 km. Heat energy cannot however pass through solid objects, so animals can be concealed by vegetation. Since 1995, methods have been developed to obtain estimates of population density and trend using distance sampling, which removes bias caused by varying vegetation density. Observations are made at night, when deer are more likely to make use of fields or open areas.
This approach has proved very effective in the lowlands, where dense coniferous thicket vegetation is less extensive than in the uplands. Thermal imaging can also provide information on deer species composition, group sizes and habitat use.
Gill R.M.A., Thomas, M.L. and Stocker, D. (1997). The use of portable thermal imaging for estimating deer population density in forest habitats. Journal of Applied Ecology, 34 1273-1286.
More recently, this approach has been adapted for use on wild boar.