C S I R O
P U B L I S H I N G
Wildlife Research Volume 25, 1998 © CSIRO 1998
A journal for the publication of original scientific research in the biology and management of wild native or feral introduced vertebrates
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Wildlife Research, 1998, 25, 255–260
A study of dung decay and plot size for surveying feral pigs using dung counts
Jim Hone and Warren Martin Applied Ecology Research Group, University of Canberra, Canberra, ACT 2602, Australia.
Abstract The effects of dung decay and plot size on counts of dung pellets of feral pigs (Sus scrofa) were investigated in south-eastern Australia. Greater understanding of both could potentially improve survey accuracy and field interpretation of dung counts by managers. Dung pellets persisted for up to 16 months, with over 50% disappearance within 2–5 months, depending on season. The long persistence suggests the need for caution in infering recent presence of feral pigs from the presence of intact pig dung. Partial correlation analysis showed the survival rate of pellets during the first month to be significantly negatively correlated with measures of temperature and rainfall in the month. There was a significant (P < 0.005) interaction of plot size, over the range 5–20 m2, and month on the average number of pellets per square metre per month, and a significant (P < 0.01) effect of year on average counts. The largest plot size always detected at least one dung pellet but smaller plots did not.
Introduction Many methods are used to survey feral pigs (Sus scrofa), including aerial survey, spotlighting counts, ground transects, area counts, mark–recapture and dung counts (Choquenot et al. 1996). Surveys can be made more efficient by greater understanding of what influences accuracy and precision. Field management of pigs and their effects may also be improved by increasing the ability of managers to interpret dung they find on the ground; presence of dung may show that pigs were present recently, and hence could be a focus for control. Saunders et al. (1993) reported that bait acceptance by feral pigs in Kosciuszko National Park was higher when signs of recent pig activity, including dung, was present. The use of dung counts for indices or estimates of total abundance can be influenced by patterns of decay of pellets and plot size. The pattern of dung decay may vary over time and could influence the interval between surveys to prevent complete disappearance of all pellets since the previous survey. Field managers often want to know how long dung pellets will persist in the field, to aid their interpretation of dung they find. The decay of dung of other mammal species has been described as exponential in several studies (Taylor and Williams 1956; Batcheler 1975; Wigley and Johnson 1981; Aulak and Babinska-Werka 1990), whereas a linear pattern of decay was reported for dung of red-necked wallabies (Macropus rufogriseus) and redbellied pademelons (Thyogale billardierii) in Tasmania (Johnson 1977, in Southwell 1989). Plot size has been shown to influence dung counts of deer (Smith 1968; Batcheler 1975), with the conclusion that counts of pellets per square metre can be lower on larger plots than on smaller plots, because more pellets are missed on larger than on smaller plots. This paper examines two aspects of the use of dung counts to obtain indices of abundance for feral pigs: the patterns of decay, and hence persistence of dung pellets, and the effects of plot size on counts of dung pellets. 10.1071/WR97024
1035-3712/98/030255
256
J. Hone and W. Martin
Methods Study area The study area was located in the eastern half of Namadgi National Park (35°358S, 149°08E) in southeastern Australia. The site has been described by Hone (1988, 1995) and Hone and Stone (1989). Briefly, the study area comprised mountain ranges covered with forest and woodland of Eucalyptus species and associated treeless grassy valleys. Elevation at the study sites was 800–1300 m and average annual rainfall was about 700 mm but increasing with elevation; rainfall is spread evenly throughout the year. Snow is common in winter (June–August), but the snow stays on the ground for only one or two days. Dung surveys The methods of feral pig dung surveys have been described by Hone (1988, 1995). Plots were arranged in each of seven sites, each of 4 km2 , with 100 plots randomly located within each site. Plots were permanently marked with numbered colour-coded pegs. The standard plot size was 20 m2, as used by Ralph and Maxwell (1984) in Hawaii. Sites were a minimum of 5 km apart, and plots within sites were 30–50 m apart. In the analysis of pellet counts reported here, data from all seven sites are combined. Individual dung pellets were counted monthly on plots, and all dung cleared off plots after counting. Dung of feral pigs was separated from that of other species by size, shape, colour, composition and smell. Dung decay Fresh dung pellets were collected each month for 12 successive months across all sites and placed in known locations nearby (never moved more than 10 m). The number of intact pellets remaining at the end of each month was recorded. A pellet was classed as intact if it was still whole and could be picked up without breaking. Note that intact pellets are not necessarily fresh pellets. The average number of fresh pellets established each month was 50.2 (s.e. = 7.4). The study was carried out from June 1985 to May 1986 inclusive, and counts of dung remaining continued in all months, except August 1986, to December 1986. Hence the maximum duration of monitoring dung pellets was 18 months. Each month’s fresh pellets can be considered a cohort, so estimates of survival (lx) and survival rate (px) were estimated following Caughley (1980). Survival (persistence) rate was estimated, as lx+1/lx, for the first month and similarly for the second month. Estimates for later months were not always possible because of small sample sizes. The relationships between the survival rate and monthly mean minimum, maximum and overall mean temperatures (°C) and monthly rainfall (mm) were investigated by correlation analysis after arcsine-tranformation of the survival rate proportions. Partial correlation analysis (Snedecor and Cochran 1967) was used to separate the effects of mean monthly temperatures and monthly rainfall. Plot size To examine the effects of plot size, each 10 × 2 m plot was divided prior to the study into four quarters, each 5 x 1 m. Records were kept separately of the number of dung pellets in each quarter for later analysis. Each of the 700 plots was randomly divided into one of four plot sizes: 5 × 1 m, 10 x 1 m, 5 × 2 m, and 10 × 2 m. Hence, each plot size and shape combination has data from 700/4 = 175 plots in each month, and as there was no overlap of plot sizes, there was independence between plots. The random selection of plots occurred independently in each of 5 months (July, September, October, November and December) in both 1985 and 1986. Counts of dung pellets from each plot size were converted to an average number of pellets per square metre per month, to construct a table of means for analysis. Split-plot fixed-factor analysis of variance (Snedecor and Cochran 1967) was used to test for the effects of plot size and year (main plot factors) and month (sub-plot factor) on the average number of pellets per square metre. Data were transformed to common logarithms, after adding 1, to obtain variance homogeneity (Snedecor and Cochran 1967). An analysis of the effect of sites was not possible as there were too few data for analysis at each site in some months. Hence, data were pooled across sites. This is not spatial pseudoreplication because sites and plots within sites are not explicit and not tested in the analysis.
Results Dung decay The maximum duration a pellet remained intact was 16 months. However most had decayed many months prior to 16 months. The number of months till 0.90, 0.75, 0.50, 0.25 and 0.10 of
Dung counts of feral pigs
Table 1.
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The time, in months, of specified proportions of dung pellets of feral pigs remaining intact
Month when fresh
0.90
0.75
June July August September October November December January February March April May
1–2 1 1–2
