Rma. Gill et al., THE USE OF PORTABLE THERMAL IMAGING FOR ESTIMATING DEER POPULATION-DENSITY IN FOREST HABITATS, Journal of Applied Ecology, 34(5), 1997, pp. 1273-1286
1. The reliability of deer population management could be improved wit
h good density estimates, but current methods are either labour-intens
ive or suffer from uncertainties regarding accuracy. 2. Visibility var
ies substantially in forests depending on stand type, age and understo
rey vegetation. In such conditions distance sampling would be an effic
ient estimation method, but observer disturbance often results in bias
when the method is applied to deer. 3. The performance of thermal ima
ging for estimating deer density by distance sampling was assessed in
seven forest deer populations. Thermal imaging equipment can detect th
e long-wave energy. radiated by natural objects, clearly revealing war
m-bodied animals even if partly obscured by vegetation. 4. Many more d
eer were detected at night using a thermal imager than along the same
transect routes in daytime. Detection distances were correlated with v
isibility but were substantially longer than the average distances at
which most animals were disturbed. Most deer were detected without cau
sing prior disturbance. 5. Densities were estimated with a coefficient
of variation ranging from 10.2-28.4%. Precision depended on sampling
effort and sample sizes obtained. 6. A Monte Carlo simulation revealed
a quadratic relationship between accuracy and visibility, with accura
cy increasing with average visibility and a tendency for deer to selec
t more open habitats within a forest. Under conditions that are likely
to be typical of temperate forests (<40% thicket and neutral selectio
n, or <70% thicket if thicket is avoided), accuracy was generally good
and changed relatively little in relation to visibility and habitat s
election. 7. Likely sources of bias as well as alternatives to thermal
imaging are discussed. It is concluded that the method would be suita
ble for estimating ungulate densities in forests with an adequate netw
ork of tracks.