Author: Shweta Shivakumar
Key Highlights:
- The dynamics of change in a population of tigers can be determined using cameras, rigorous scientific methods, and long-term studies.
- Ecological parameters of tigers, such as abundance, survival, transience, and emigration were modeled in an analytical framework called capture-recapture.
- Although the tiger population of Nagarahole experienced high levels of mortality from 1991-2000, the population was found to be healthy.
Wide-ranging large wildlife such as tigers have complicated societies. If one has to estimate numbers and their demographics, it takes a long-term dedicated study like the one conducted by Dr. Ullas Karanth and Dr. Samba Kumar of Centre for Wildlife Studies, and Dr. Jamed D Nichols and Dr. James Hines of Patuxent Research Centre. A total of 74 tigers were photographed in Nagarahole Tiger Reserve of south India over a nine-year period, from 1991 to 2000, using motion-sensing cameras stationed over a 231 km sq area. This expansive research effort was conducted to estimate the tiger population’s yearly fluctuations, effects of tiger movement from adjacent multi-use landscapes, and tiger reactions to photo-capture through the study period.
This study takes into account the social organization of adult tigers, where residents (breeding adults who defend fixed home ranges) and temporary emigrants (animals unavailable for photographing over a subset of the study period) are mixed with tiger losses (deaths and permanent emigrants) at any given period of study. Population parameters, such as abundance, survival, transience, and emigration, among others, were estimated to understand the effects of tiger movement from landscapes adjacent to Protected Areas (PAs) on the future of the tiger population in this area. Since Nagarahole Tiger Reserve is surrounded by multi-use landscapes, such as agriculture and other forests, transient tiger movement from those landscapes into the PA was expected in the study, coupled with possible high mortality.
This study found that between March 1996 and May 2000, when the study area was constant (231 sq km), the abundance of tigers ranged from 17 to 30 (±2.1), with an estimated annual increase of 3%. Tigers in the “transience” category (tigers photographed once but with no chance of being photographed again) comprised 18% of the total tigers. About 10 % of the tigers were temporary emigrants, and the annual death/loss rate was 23% of the tigers. The recruitment estimates (births+immigration) were relatively imprecise with high error rates since younger animals were excluded from the analysis. The important takeaway from this study is that despite high levels of tiger ‘losses’ in the study region, protected wild tigers prevailed at relatively high numbers in Nagarahole Tiger Reserve. The authors speculate that this may be due to their inherent ability to reproduce rapidly during the periods when conditions are optimal such as abundant prey base. This non-invasive methodology, where the animal is not affected by the research methodology of being photographed, is a robust population dynamics estimation method of wildlife with individual body markings (such as stripes of a tiger).
Original Article: Karanth, K.U., Nichols, J.D., Kumar, N.S. & Hines, J.E. (2006) Assessing tiger population dynamics using photographic capture-recapture sampling. Ecology, 87, 2925–2937.
You can access the original article here.
