Monitoring carnivore populations at the landscape scale: occupancy modelling of tigers from sign surveys

Abstract

Summary1. Assessing spatial distributions of threatened large carnivores at landscape scales poses formidable challenges because of their rarity and elusiveness. As a consequence of logistical constraints, investigators typically rely on sign surveys. Most survey methods, however, do not explicitly address the central problem of imperfect detections of animal signs in the field, leading to underestimates of true habitat occupancy and distribution. 2. We assessed habitat occupancy for a tiger Panthera tigris metapopulation across a c. 38 000-km 2 landscape in India, employing a spatially replicated survey to explicitly address imperfect detections. Ecological predictions about tiger presence were confronted with sign detection data generated from occupancy sampling of 205 sites, each of 188 km 2 . 3. A recent occupancy model that considers Markovian dependency among sign detections on spatial replicates performed better than the standard occupancy model (DAIC = 184AE9). A formulation of this model that fitted the data best showed that density of ungulate prey and levels of human disturbance were key determinants of local tiger presence. Model averaging resulted in a replicatelevel detection probability p t ðSʽ p t Þ = 0AE17 (0AE17) for signs and a tiger habitat occupancy estimate ofŵðSʽŵÞ = 0AE665 (0AE0857) or 14 076 (1814) km 2 of potential habitat of 21 167 km 2 . In contrast, a traditional presence-versus-absence approach underestimated occupancy by 47%. Maps of probabilities of local site occupancy clearly identified tiger source populations at higher densities and matched observed tiger density variations, suggesting their potential utility for population assessments at landscape scales. 4. Synthesis and applications. Landscape-scale sign surveys can efficiently assess large carnivore spatial distributions and elucidate the factors governing their local presence, provided ecological and observation processes are both explicitly modelled. Occupancy sampling using spatial replicates can be used to reliably and efficiently identify tiger population sources and help monitor metapopulations. Our results reinforce earlier findings that prey depletion and human disturbance are key drivers of local tiger extinctions and tigers can persist even in human-dominated landscapes through effective protection of source populations. Our approach facilitates efficient targeting of tiger conservation interventions and, more generally, provides a basis for the reliable integration of large carnivore monitoring data between local and landscape scales.

Occupancy in continuous habitat

“…For example, Wintle et al (2005) defined plots for acoustic surveys of owls and arboreal marsupials on the basis of the distance at which species may be detected, which they note was similar to the home-range radius. In other cases, investigators have chosen plots to be larger than a home range, but the plots were not large enough to escape the effects of home-range size on occupancy, as judged by our results (e.g., Karanth et al 2011). Plots in this intermediate size range are more prone to problems of interpretation than very large or very small plots.…”

“…O'Connell and Bailey (2011) worried that plots smaller than a home range would violate the assumption of closure [i.e., constant occupancy] between replicate samples. Karanth et al (2011) justified their choice of plot size in a landscape-scale study of tigers (Panthera tigris) thus ''Because our goal was to measure true occupancy (proportion of area occupied) rather than intensity of habitat use by tigers (MacKenzie and Royle 2005, MacKenzie et al 2006, the cell size chosen was, on average, larger than the maximum expected tiger home range size.'' We know of no attempt to address the quantitative relationships between plot size, home-range size and occupancy.…”

Sinks as saviors: Why flawed inference cannot assist tiger recovery

“…Tiger densities in better-protected reserves are three-to fivetimes higher than in areas with poorer protection and extractive human uses (2,3). At other ecologically comparable sites, such as Kaziranga and Corbett (2), which curtail human uses, tiger densities are five-times higher (17-19 tigers per 100 km 2 ).…”

“…Global spatial analyses show >70% of wild tigers now live and reproduce in 6% of source habitats in protected reserves, and their survival in surrounding landscape sinks depends on replenishment from sources (2)(3)(4). The sober reality is that the species has been extirpated from 93% of its former range because of conflict-ridden coexistence with humans.…”

Balancing Conservation and Development in Nandhaur Wildlife Sanctuary, Uttarakhand, India

“…Top-down management efforts which assume that the same factors are relevant across all PAs are likely to be less successful than management efforts tailored to individual places. Extending PA benefits to smaller landholders, highly resource-dependent households, and households subjected to higher income losses due to human-wildlife interactions are particularly promising to balance costs and losses from living in and around PAs 35 .…”