Factors affecting tiger habitat and detection
Alluvial grasslands support high densities of tiger prey and contain important sources of water (Eisenberg and Seidensticker 1976, Sunquist 1981). Thus, tigers may select areas with more grassland to look for prey and obtain water. Unlike land-cover types with relatively low understory density, grasslands may also provide adequate cover for tigers' hunting activities (Sunarto et al. 2012). Correlation length can be considered a measure of connectivity since it represents the average distance a tiger can move within a land-cover patch before encountering a patch boundary. Contiguous land-cover patches may facilitate movement and dispersal across the landscape (Short Bull et al. 2011). Our results regarding tiger preference for contiguous land-cover patches at a fine spatial scale, support those of Sunarto et al. (2012), which found that tigers in Sumatra prefer large contiguous forest tracts with dense understory.
Since the understory of riverine forest is typically more dense than Sal forest (Sunquist 1981, Lehmkuhl 1994), understory vegetation may be more likely to obstruct the field of view of cameras in riverine forest than in Sal forest, which could have lowered detection probability. In addition, as animal/walking trails in riverine forests are perhaps smaller and less defined than other forest types, tigers may traverse riverine forests along multiple, unpredictable routes and consequently be detected less often by our cameras. There are two possible reasons why detection probability decreased over time. By monitoring breeding tigers from 1973 and 1989, Smith and McDougal (1991) showed that the distribution of births throughout a year was not significantly different from a uniform distribution, although a peak in births did occur between May and July. Prior to this peak, females may be less active, which could lower detection probability as compared to time periods earlier in the year. Another possible explanation is that tigers are generally less active and mobile in March and April when temperatures increase (Seidensticker 1976). By formally accounting for variation in detection probability as a function of riverine forest and time, the N-mixture model used in this study provides more reliable estimates of tiger habitat suitability across the entire study site (Royle 2004).
Habitat change across different land management regimes
Protected areas in many temperate and tropical regions have experienced declines in wildlife habitat as a result of human impacts (Liu et al. 2001, DeFries et al. 2005, Linderman et al. 2005). While the overall estimated suitability of tiger habitat in Chitwan National Park was high over the 20 year study period (Table 4), persistent and increasing human pressures may have degraded habitat suitability through time. We found that habitat inside the park became less suitable for tigers from 1989 to 1999 in the areas closest to human settlements. Likewise, Nagendra et al. (2008) indicated that forest degradation from 1989 to 2000 was more extensive along the park periphery than in the multiple-use areas outside the park. Nagendra et al. attribute this comparatively higher forest degradation inside the park to high natural resource demands by local people. A household living near Chitwan National Park, on average, collects approximately 2,200 kg of fuelwood and fodder annually from within the park (Stræde and Treue 2006). We found that habitat suitability inside the park increased somewhat in the areas closest to human settlements from 1999 to 2009, however, habitat suitability decreased in areas deeper inside the park. Although it is possible that this shift resulted from natural forest dynamics, these changes may indicate that human resource collection activities occurred deeper inside the park during that time frame. A similar pattern was shown inside China's Wolong Nature Reserve, where local people traveled farther from their homes to collect fuelwood after the fuelwood supply was depleted in the areas nearest to them (He et al. 2009).
Stræde and Treue (2006) indicated that households living near Chitwan National Park on average collect 6%, 5%, and 17.8% of their annual timber, fuelwood, and fodder needs, respectively, from inside the park. The majority of timber and fuelwood collected from inside the park was Sal (Stræde et al. 2002), which is considered the most valuable (thus most exploited) tree species in the region (Nagendra 2003). Removal of forest understory and woody biomass from the park may have reduced hiding cover for tigers (Sunarto et al. 2012) and induced land cover fragmentation (Gasparri and Grau 2009). In addition, Stræde and Helles (2000) noted that demand for park resources, especially grasses, by local people has been steadily increasing. Therefore, the decline in grassland area inside the park, and consequential loss of highly suitable tiger habitat, may be partially attributed to substantial anthropogenic pressure (Peet et al. 1999, Stræde and Helles 2000).
Other disturbances such as flooding and fire also likely affected tiger habitat suitability. The heterogeneity in habitat suitability along the northern boundary of the park is likely due, in part, to the movement of the Rapti River, which constantly erodes, deposits alluvium, and changes course across the floodplain (Peet et al. 1999). When the river changes course some areas become inundated while other areas where the river once was are rapidly colonized by S. spontaneum grasses (Peet et al. 1999). In addition, fire, naturally or human caused, delays the succession from grasslands to riverine forest. However, fire cannot prevent succession entirely because of variations in fire intensity and occurrence (Lehmkuhl 1994). Thus, decreasing grasslands and Sal forest and the corresponding increase in riverine areas observed inside the park over the 20 year period may be due to a combination of human and natural disturbances. Despite the degradation of highly suitable areas inside the park, especially from 1999 to 2009, tiger numbers inside the park appear to be stable (Carter et al. 2012b). This is perhaps because the change in habitat suitability to date is not large or extensive enough to induce a substantial change in tiger numbers. Alternatively, a time-lag may exist between changes in habitat suitability and tiger numbers.
As the area outside the park was considered heavily degraded prior to its establishment as the buffer zone (Sharma 1990), resource collectors likely had to travel relatively far into the multiple-use area to find adequate supplies of high-quality resources. Extracting natural resources from deeper inside the multiple-use area may have induced the average decrease in estimated tiger habitat suitability from 1989 to 1999 within 1.5 km from human settlements. The prohibition of livestock grazing and institutionalization of community-based resource management outside the park shortly after the buffer zone was established in 1996 substantially altered resource consumption patterns. For example, the removal of domestic livestock likely enabled the area outside the park to support a greater density of wild prey animals (Gurung et al. 2008) and provide better hiding cover for tigers. The control of overharvesting of shared natural resources in community forests in the buffer zone may also have improved tiger habitat outside the park. This is supported by improvements in estimated habitat suitability occurring mostly in areas adjacent to human settlements that are designated as community forests. In addition, as resource extraction policies and practices influence the spatial distribution of human activities, these, in turn, affect the spatial configuration of the land cover. As such, before the buffer zone was established, uncontrolled open-access resource extraction likely fragmented the land cover. In contrast, the coordinated management of forest tracts by user-group committees after the buffer zone was established may have helped reverse this fragmentation (Nagendra et al. 2008), and thus improved tiger habitat suitability.
The improvement of estimated tiger habitat suitability outside the park is independently supported by an increasing frequency of tiger sightings over the last decade (DNPWC 2007). While searching for tiger tracks and kills from 1999 to 2003, Gurung et al. (2006) did not find any evidence of breeding females (i.e., adult female tracks with cub tracks) in the multiple-use area outside the park. From our 2010 camera trap data, we identified a female tiger living completely outside the park and accompanied by three of her cubs, suggesting that the habitat outside the park has improved over the last decade. Currently, tiger densities inside and outside the park in our study site seem to be comparable (Carter et al. 2012b). Attacks on people outside the park have also increased since the late 1990s (Gurung et al. 2008), thus efforts to mitigate human-tiger conflicts (Carter et al. 2013) are vital to fostering coexistence in Chitwan. The main purpose of community forests is to sustainably provide natural resources to local communities; however, their formation does not automatically lead to the protection and/or improvement of tiger habitat. Moreover, the demands, practices, and compositions of the community forest user groups are constantly changing, and consequently, tiger habitat may degrade in the future in areas where it is currently improving. Therefore, coupled with efforts to mitigate human-tiger conflicts, we suggest that tiger habitat suitability be monitored regularly inside and outside the park (using procedures similar to those reported here) to ensure that the land continues to support tigers.
Land management policies and practices inside and outside the park and their impacts on tiger habitat are not mutually exclusive. These two land management regimes adjoin each other in space, with the effects of one likely having direct and/or indirect effects on the other. For instance, while local people legally extract natural resources from the buffer zone outside the park, it appears that the products obtained do not fully substitute those collected inside the park (Stræde and Treue 2006). Therefore, given projected human population growth and current per-capita resource demands in Chitwan (Stræde and Treue 2006, CBS 2012), protecting tiger habitat will become increasingly difficult over time. As such, policies that reduce human reliance on local natural resources (e.g., encouraging the use of non-wood fuel sources and improved livestock breeds) and actively managing tiger habitat are urgently needed (Hjortsø et al. 2006, Gurung et al. 2009, Thornton 2010). Furthermore, participatory-based management of ecosystems in and around the park, wherein local people are partners in designing, implementing, and enforcing resource management actions, may also foster sustainable conservation of tiger habitat over the long-term (Agrawal and Ostrom 2001). Such institutional arrangements, for instance, may enable the creation and maintenance of grasslands and early successional vegetation types in Chitwan through coordinated management efforts, including tree thinning and grassland cutting and burning (Brown 2003, Smit 2004).
In this study we assumed that tigers within the Chitwan ecosystem remained closed to demographic change for 89 days, which may be appropriate in this context as tigers are long-lived and Chitwan has relatively long seasons (O'Brien and Kinnaird 2011). We also assumed that the determinants of tiger habitat suitability remained unchanged from 1989 to 2009, which may be appropriate as tiger-habitat relationships characterize fundamental interactions between tigers and their environment (Morrison et al. 2006). However, tigers may have adjusted their space use over the 20 year period as a result of changes in the composition and distribution of prey, the density of tigers, and/or human disturbances. For example, although tigers may be habituated to the road network that currently occurs in the study site, the construction of roads in the past may have strongly disturbed tiger space use and consequently decreased habitat suitability of areas surrounding the roads for extended periods of time.
Our results on habitat suitability only refer to the dry season (i.e., January to April). Yet, tigers may select areas differently at different times of the year. For example, the use of grasslands by tigers may decrease in the wet season because the grasslands are periodically inundated, which forces prey animals to move to drier ground in upland forests (Sunquist 1981). Nevertheless, our findings are likely valid across seasons because the factors influencing habitat suitability identified in this study are similar to those from several other studies that spanned different seasons (Seidensticker 1976, Sunquist 1981, Smith et al. 1998). Collecting longitudinal (i.e., annual) tiger occurrence data across different seasons in conjunction with data on environmental and human factors in future studies will eliminate confounding issues of temporal variation in tiger habitat selection. Integrating such information with detailed data on individual-level tiger behaviors (e.g., from GPS collars) would explicitly link individual and population-level processes, and potentially allow for the design of adaptive models of habitat suitability that better capture temporal dynamics in habitat selection (Persson and De Roos 2003).
Although wildlife abundance is generally correlated with habitat suitability (Boyce and McDonald 1999), in some cases it is possible to find low or no relation between wildlife abundance and habitat selection (Liu et al. 2011). For instance, occurrences in suitable habitat may be low due to hunting or disease or to lack of colonization due to dispersal barriers. Conversely, animal occurrences in low quality habitat may be high, because less competitive juveniles are relegated to those areas (Van Horne 1983), or animals use those areas as travel corridors. However, circumstances such as these seem to be uncommon in our study site as tiger poaching in Chitwan has been relatively low since the end of the civil war in Nepal in 2006. Furthermore, tigers seem to have colonized the entire study site before the study was performed, and with the exception of the Rapti River, no obvious barriers to tiger movement (e.g., expanses of cultivated or urban developed land) are conspicuous.
Explicitly linking habitat conditions to tiger survival and reproduction would potentially enable evaluating changes in tiger population size in areas under different management regimes. However, with the data collected in this study we were unable to quantitatively relate changes in tiger habitat suitability to tiger population dynamics and persistence. As a result, it is still unclear how the changes observed affect tiger population viability in the area.