Ecological monitoring should be a vital component of any conservation project so that the effects of management can be assessed (Kremen, Merenlender & Murphy 1994). Management plans always emphasize that monitoring should take place and, more recently, some have attempted to define limits of acceptable change beyond which management action should be taken (Alexander 1996). However, if we are to define these limits, we must be certain that our survey techniques can detect them.
The use of distance sampling techniques to estimate animal population densities has become increasingly popular since the production of the computer package transect (Laake, Burnham & Anderson 1979) and subsequently distance (Buckland et al. 1993; Laake et al. 1994). In African forests this technique has mainly been used to estimate the density of primate groups (Whitesides et al. 1988; White 1992, 1994; Plumptre & Reynolds 1994), as many species are highly visible. Surveyors of most other mammals in these forests have resorted to indirect estimation techniques rather than direct observation because visibility is often poor and some species cannot be approached in safety. In these surveys animal densities were calculated from line transects, counting signs that animals leave behind, usually nests (apes: Ghiglieri 1984; Tutin & Fernandez 1984; Wrogeman 1992; White 1994; Hashimoto 1995; Ihobe 1995; Marchesi et al. 1995; Plumptre & Reynolds 1996, 1997; Hall et al., 1998a) or dung (elephants Loxodonta africana: Wing & Buss 1970; Short 1983; Merz 1986; Barnes & Jensen 1987; Dawson 1990; Ruggiero 1990; Fay & Agnagna 1991; Barnes 1993; Plumptre & Harris 1995; ungulates: Plumptre 1991; White 1992, 1994; Plumptre & Harris 1995). Indirect counts require conversion factors to be calculated to convert the count of dung or nests to animal density. These factors include the rates of production and the decay rates of dung/nests. Some studies have avoided the need to correct for the rate of decomposition of dung or nests by counting the number that appear over a certain time period. The same transects are visited repeatedly during this period at intervals shorter than the quickest time to decay. These are referred to here as marked nest counts or clearance plot dung counts (Plumptre & Reynolds 1996; Staines & Ratcliffe 1987).
Where the animals/signs of interest occur in groups, then conversions have to be made to calculate animal/sign density from group density. This has been particularly common in primate surveys (Whitesides et al. 1988; Plumptre & Reynolds 1994), where individual density has been calculated using mean group size. Whitesides et al. (1988), in addition, recommended that a measure of mean group spread be calculated for primate studies, using a perpendicular distance measure to the nearest animal to the transect to calculate the perpendicular distance from the transect to the centre of the group. They argued that this was necessary because monkeys further from the transect tended to be missed.
Each of these conversion factors has an associated error. Few studies report their standard errors or 95% confidence limits, and where they do these errors are of the count only and rarely include errors from conversion factors. If line transects are to be used for monitoring populations of mammals then the true errors of the estimate should be calculated. This paper investigates how variation in conversion factors and measures of effort affect the error of the density estimate. Table 1 summarizes the errors associated with different methods of analysis.
|Method||Production rate||Decomposition rate||Group size||Group spread|
|Sightings of single animals||–||–||–||–|
|Sightings of groups||–||–||+||+ / –|
|Clearance plot dung counts||+||–||+||–|
|Marked nest counts||+||–||+||–|