Fallow Deer and Wild Boar Pellet Group Disappearance in a Mediterranean Area

1. What they did

Counting dung pellet groups is a common way to estimate how many deer or wild boar live in an area, especially where animals are hard to see. The count only works if researchers know how fast dung disappears between visits. No one had measured this in a Mediterranean climate, or for wild boar and fallow deer at all.

In Maremma Natural Park on the coast of central Italy, the researchers collected fresh dung and set it out themselves:

• Fresh dung. Fallow deer pellets came from a 40-ha enclosure where cleared transects were checked every 2 days, so they were no more than 48 hours old. Wild boar dung came from heavily used sites visited daily and was judged to be no more than 2 days old. All dung was refrigerated until placed.
• Experimental groups. Fallow deer groups held about 64 pellets on average, so groups of 60 pellets were used. Wild boar groups held about 4.5 pellets of varying size and shape, so whole natural groups were used.
• Placement. At the start of each season (January 1993 to January 1994), they placed 30 groups of each species, 7 m apart along a random transect, in each of four habitats: maquis scrub, pinewood, sand dunes and meadow. That is 120 groups per species per season.
• Checks. They visited the groups 1 week after placement and then every 3 weeks until about half had gone from at least two habitats. A fallow deer group counted as gone when 5 or fewer pellets were left; a wild boar group when no visible sign remained. They also recorded dung beetle burial and rooting by wild boar.

2. What they found

Season mattered more than habitat

• For both species, dung disappeared slowly in winter, spring and summer, and much faster in autumn.
• Season explained most of the variation in fallow deer pellets remaining (50.6% after 1 month, 14.7% after 3 months); habitat explained only about 5%.

After 1 month

• Winter: about 99% of fallow deer groups and 100% of wild boar groups were still there.
• Autumn: 88% of fallow deer groups had gone, but only 5% of wild boar groups.
• After 1 week, no groups of either species had gone in winter or spring; fallow deer losses began only in autumn.

After 3 months

• About 78% of groups of both species were still there after a winter start.
• In autumn, all fallow deer groups had gone, and 44% of wild boar groups.
• Fallow deer dung disappeared faster than wild boar dung overall.

How long dung lasted (mean persistence)

• Fallow deer: longest in winter (about 164 days), shortest in autumn (about 13 days). Within each season, groups lasted longest in the dunes.
• Wild boar: longest in spring (about 218 days), shortest in autumn (about 62 days).

What removed the dung

• Dung beetles buried fallow deer pellets in 92 of 480 groups, 69% of them in autumn. In autumn, beetles arrived at some pellets less than 1 hour after placement.
• Beetles buried whole fallow deer pellets, but broke wild boar pellets into pieces first.
• Wild boar rooting sped up the loss of fallow deer pellets in 69 groups, mostly in summer and autumn and mostly in maquis.

3. What they concluded

• In Mediterranean areas, pellet counts for both species are best done in winter, when dung disappears most slowly, and dung should be counted over a 1-month period.
• Their correction factors are maximum values, because all their dung was set out on day zero; dung found in the wild will include younger groups, so the factors will slightly overestimate the number of groups produced.
• Rain and dung beetles probably explain the fast loss in autumn, but their effects could not be separated. In 1993, 80% of the year’s rain fell between September and December, and wild boar density was very high, so rooting may have had an unusually large effect.
• Wild boar dung may last longer because boar are not ruminants: their dung holds partly undigested food and more fibre, while fallow deer dung is finely digested. Pellet size may also play a part.
• Autumn pellet counts should be used with great caution, for example to judge habitat use. Where ungulates are scarce, empty plots in autumn cannot show whether no dung was dropped or it had already disappeared.
• Dung collected in autumn for diet studies is very likely less than 1 month old.
• The study covered only 1 year and 1 site; more sites and years are needed to know whether the figures apply generally.

4. How tracking was or was not used

The study is about one kind of sign: dung. The researchers judged dung freshness when collecting it, then followed set-out dung over time to see how long it stayed recognisable, and recorded signs of what removed it (dung beetle burial and wild boar rooting). No trailing or footprint work was involved.

5. Tracker or observer certification

Not reported. Nothing is said about who collected, aged or checked the dung. The age of the wild boar dung (2 days or less) was judged at collection.

6. Other technologies, skills or innovations

• An experimental design with known-age dung placed at the same time in four habitats in every season.
• Correction factors with 95% confidence limits for the proportion of dung groups likely to have gone after 1 and 3 months, by season and habitat.

7. Implications for conservation (from the paper)

• Wild boar and fallow deer can reach high densities and damage crops and woodland, so managers need reliable local density estimates, often from pellet counts.
• In Mediterranean regions, the timing of pellet counts strongly affects their reliability: winter counts lose the least dung, while autumn counts can miss most fallow deer dung within a month.