Influence of Connectivity, Wild Prey and Disturbance on Occupancy of Tigers in the Human-Dominated Western Terai Arc Landscape

1. What they did

Tigers now live in only about 7% of their original range, mostly in forests surrounded by farms, towns and people. The western end of the Terai Arc Landscape (TAL), along the foot of the Himalayas in northern India, is split in two at the Chilla-Motichur corridor by the Ganga. West of the river (Tiger Habitat Block I, THB I) only a small, isolated group of tigers remains; east of it (THB II) tiger numbers are high, including in Corbett Tiger Reserve.

The authors asked where tigers occur across 6,979 km² of potential habitat, and why. They tested four predictions: tigers would occupy less habitat in the cut-off western block; occupancy would follow wild prey, especially sambar and chital; human disturbance would lower occupancy, although crowded tigers in THB II would be pushed into poorer habitat; and field data on prey and disturbance would explain the patterns better than habitat measured from maps.

They overlaid a grid of 57 cells of 166 km² each, larger than a tiger’s home range of about 60 km². From November 2009 to March 2010, a team of four surveyors walked 1166 km along dry sandy streambeds, forest roads and trails, looking for tiger tracks, scats and scrapes. Each cell was surveyed within about 12 to 48 hours, with up to 40 km walked in fully forested cells. Along each 250 m segment they also recorded the tracks of ten wild prey species and of livestock, and signs of people lopping branches or cutting wood. These records became indices of wild prey, principal prey (sambar and chital) and disturbance. They then used occupancy models, which allow for tigers being present but their signs missed, to estimate the share of habitat used by tigers.

2. What they found

Signs found

• Tiger signs were found on 611 one-kilometre segments in 32 of the 57 cells (a raw estimate of 0.56). The team recorded 204 track sets and collected 89 scats.
• Tracks (pug marks) were the most common sign, because the bhabar habitat has good tracking surfaces, especially sandy streambeds.

Detecting signs

• Signs were far easier to detect in the eastern block, where tigers are more abundant: the chance of detecting tiger signs on a segment was 0.386 in THB I and 0.947 in THB II.
• Surface mattered. Sandy streambeds (82% of segments) gave the best detection, then forest roads (15%), then forest trails (3%). For example, detection in THB II was 0.951 on streambeds, 0.854 on roads and 0.696 on trails.
• The distance walked had little effect on detection.

Where tigers live

• Overall, tigers occupied 0.588 of the habitat, about 4,109 km² of 6,979 km².
• In the western block, only about 17.85% of the habitat was occupied (522 km²); in the eastern block, 88.5% (3,587 km²).
• The best models included the block together with wild prey or disturbance. Wild prey raised occupancy, disturbance lowered it, and the habitat share in a cell also had a positive effect.
• Field-based measures (wild prey, disturbance) explained occupancy better than habitat from maps, except for principal prey, which explained little.
• Several multiple-use forest divisions next to Corbett (parts of Lansdowne, Bijnor, Ramnagar and Terai west) had tiger occupancy as high as the protected source sites, and support breeding tigers.

3. What they concluded

• The break in the Chilla-Motichur corridor has cut the western block off from source populations in the east, and tiger occurrence in THB I has fallen in the seven years since the 2002 to 2003 surveys, even though prey is plentiful in western Rajaji National Park.
• In THB I the few remaining tigers select the best habitat, with plenty of prey and little disturbance. In THB II, with high tiger numbers, some tigers are pushed into poorer habitat, which brings them into conflict with people.
• Sambar and chital explained little, probably because tigers in THB I are far below the numbers the habitat could carry, and because in THB II many tigers live outside protected areas, where prey may be reduced by livestock and other human pressures.
• Field data on prey gave a different, more realistic picture than habitat models built from maps: some densely forested areas hold little wild prey.
• The Hines et al. occupancy model, with a good field design that accounts for differences in detection between blocks and substrates, is a practical and robust way to assess tigers across a large landscape.

4. How tracking was or was not used

The whole study rests on tracks and signs surveys. Surveyors walked sandy streambeds, forest roads and trails at 10 to 15 km a day, searching for tiger tracks, scats and scrapes, because tigers often travel and mark along such routes. To avoid confusing tiger signs with leopard signs, tracks and scrapes were identified by size, shape, depth and gait, and scats by their size and other signs nearby. The team took time to verify each sign and recorded only signs that were identified without doubt. They also recorded the tracks of prey and livestock and signs of human use. The authors assume that signs lasted about equally long across the study area, and they surveyed only in the dry winter months, when there is little rain to wash signs away. No trailing was reported.

5. Tracker or observer certification

Reported but not validated. All surveys were done by a team of four experienced surveyors, each with more than 5 years of experience of sign surveys in this region. No certification or test of their skill is reported. The field team thanked in the acknowledgements is Imam Hussein, Ameer Hussein, Sher Singh, Rajendar and Suniti B. Dutta.

6. Other technologies, skills or innovations

• A grid-cell survey design with sign records along 250 m segments, grouped into 1 km replicates, so that occupancy and detection could be estimated together.
• A simulation in program GENPRES to check that 57 cells were enough (bias stayed small even with as few as 18 cells).
• Proportional habitat per cell measured in a Geographic Information System (GIS), ArcGIS 9.3.

7. Implications for conservation (from the paper)

• The western block could support at least 30 tigers, but without a working corridor tigers cannot recolonize it. Corridor restoration has only recently begun, following directions from the Supreme Court of India.
• Because the western population is small and not viable, the authors say governments and conservation agencies should translocate tigers from a nearby source, such as Corbett, to prevent local extinction.
• Protected areas would benefit from standardized patrol-based law enforcement monitoring.
• Multiple-use forests next to Corbett hold breeding tigers but get little protection; 645 cases of livestock killed by tigers were reported from these areas in 2009 to 2010, and unchecked conflict could lead to tigers being killed in retaliation.
• The authors call for managing the landscape as a single conservation unit focused on tigers and other wildlife, and for linking tiger occupancy with local people’s dependence on the forest, to decide which areas should be inviolate core habitat and which areas of coexistence.