Improving reliability of scat counts for abundance and distribution estimations of Lumholtz’s Tree-kangaroo (Dendrolagus lumholtzi) in its rainforest habitats

1. What they did

Lumholtz’s tree-kangaroos live high in the canopy of dense rainforest on the Atherton Tablelands of Far North Queensland, and are mostly active at night, dawn and dusk. They are hard to see, so counting their scats is one of the most promising ways to estimate how many there are and where they live. But scat counts can mislead: scats may vanish before anyone finds them (a false negative, wrongly concluding the animals are absent), old scats may linger (a false positive, wrongly concluding the animals are using the area now), and small tree-kangaroo scats can be mistaken for those of red-legged pademelons, which eat the same food.

The authors set out to find traits that tell fresh tree-kangaroo scats from old ones:

• Scat production. They collected and weighed every scat from six captive tree-kangaroos for three or four days. Vegetation in the enclosures was covered with cloth so no scat was missed.
• Laboratory decay. They kept fresh scats on leaf litter under four conditions (warm-wet, warm-dry, cool-wet, cool-dry) for ten days, and tracked mass, circumference (from photographs), pH and the first appearance of mould.
• Forest decay. In November 2010, they placed fresh scats in wire mesh cages on the forest floor at three sites (sunny regrowth, shaded old growth and beside a creek) for four days, and set pitfall traps baited with tree-kangaroo scats to catch dung beetles.

2. What they found

Scat production

• Each animal produced on average 60 to 128 scats a day, weighing 67.5 to 168.7 g in total.
• A single scat weighed on average 1.06 to 1.9 g.
• Production varied a great deal between animals, and animals given the same food did not produce consistently similar numbers or masses of scats.

Decay in the laboratory

• Scats kept wet gained mass until they levelled off at about 130% of their original mass.
• Scats kept dry lost mass: by Day 10, warm-dry scats had lost about 90% and cool-dry scats lost mass more slowly.
• Mass and circumference changed together (positive correlations in all four conditions), so scats grew when wet and shrank when dry.
• Fresh scats had a pH of about 6.1, and about 5.4 on Day 2. From Day 3 onwards every scat tested at pH 7, the highest reading the test kit could give.
• All scats grew mould. Most had some by Day 3, and all had visible mould by Day 4, with no difference between conditions.

Decay in the forest

• At the shaded old growth and creek sites, every scat disappeared within 24 hours on each of the four days.
• At the sunny site, 43% disappeared within 24 hours on Day 1, 78% on Day 2, 100% on Day 3 and 89% on Day 4.
• Because scats vanished so quickly, changes in mass and size in the forest could not be analysed.
• Pitfall traps caught on average 50.5 dung beetles at the shaded site, 38 at the creek site and 11.5 at the sunny site, which had the most variable humidity.
• No scats in the forest grew mould, though mouldy scats were often seen in two of the enclosures.

3. What they concluded

• Dung beetles likely ate most of the scats in the forest. This may be a rainforest problem: scat counts have worked well for eastern grey kangaroos in drier habitats, where few scats are lost.
• In the wet season, scats are likely to vanish quickly, raising the chance of false negatives. Wet scats also swell, which makes them less likely to be confused with pademelon scats.
• In the dry season, scats last longer, raising the chance of counting old scats (false positives), and dry scats shrink, which makes confusion with the smaller pademelon scats more likely.
• Scat surveys for this species should be done in dry periods, and only fresh scats of average size and acidic pH (and, under wet conditions, without mould) should be counted. Mouldy scats can be assumed to be at least three or four days old, and scats with pH under 7 are likely less than three days old, though weather and diet can affect both.
• Reliable estimates could come from measuring scat production, decay and dung beetle activity for each site and season at the time of the survey, or from occupancy models that account for false negative and false positive errors. More studies on traits specific to this species’ scats are needed.

4. How tracking was or was not used

The study is about scat, one of the main signs used to survey animals. No tracking was done in the field: scats came from captive animals, and the authors measured how scats change with age and weather (size, pH and mould) to help surveyors age scats and avoid confusing them with pademelon scats.

6. Other technologies, skills or innovations

• Scat circumference measured from photographs with the freehand tool in ImageJ software.
• Scat pH measured with a soil pH test kit and colour chart.
• Temperature and humidity data loggers at laboratory and forest sites.
• Pitfall traps made from PVC tubes, baited with fresh tree-kangaroo scats, to sample dung beetles.

7. Implications for conservation (from the paper)

• Lumholtz’s tree-kangaroos are listed as near threatened in Queensland, and a lack of reliable data on their numbers and habitat use makes conservation planning difficult.
• Better non-invasive methods, such as more accurate scat counts, could provide the knowledge needed to plan conservation for this species.