1. INTRODUCTION
Differential resource use allows for diverse species to specialize in ecological niches and thus coexist in a particular area and this theory has been stimulated one of the most fertile fields in ecology (Chase and Leibold, 2003). The requirement-based concept of the ecological niche (Grinnell, 1917; Hutchinson, 1957) defines it as a function that links the fitness of individuals to their environment. Whenever several species co-exist, biotic interactions (like competition, predation, parasitism, mutualism) affect their fitness and behavior and may drastically affect their niches (Pearson and Dawson, 2003).
Morphologically and phylogenetically, similar sympatric species are expected to have niche overlap and competitive interactions under conditions of limited resources (Schoener, 1974; Putman, 1996). Hence, potentially competing sympatric species are expected to develop niche differentiation to avoid or decrease competition (Pianka, 1974; Schoener, 1974; Putman, 1996). The spatial aspect of niche (habitat) is the most commonly partitioned resource followed by food (Schoener, 1974, 1983; Toft, 1985). Habitat selection and the niche theory explains that similar species with similar niches should be allopatric or possess some of their behavioral aspects that separate them spatially or temporally within the same range. The coexistence of competitive species co-occurs in the same habitat as a result of the resource partitioning (Hardin, 1960). Niche breadth can be measured by observing the distribution of individual organism within a set of resource states. Some plants and animals are more specialized than others and measure of niche breadth attempt to measure this quantitatively.
Swamp deer (Cervus duvaucelii ) has been enlisted as the “vulnerable” in the red list of International Union for Conservation of Nature (IUCN) (Duckworth et al., 2015b) and is listed on Appendix I of Convention on international trade in endangered species of wild fauna and flora (CITES) (CITES, 2019). This deer was extirpated from Chitwan National Park of Nepal during the 1960s, possibly due to a combination of factors (e.g. poaching, disease and habitat loss) (Duckworth et al., 2015b). Swamp deer is one of 27 mammal species strictly protected by the National Parks and Wildlife Conservation Act 2029 (1973) of Nepal. Swamp deer mortality is largely by predation, flooding (for R. d. duvaucelii and R. d. ranjitsinhii ) and poaching. Tiger (Panthera tigris ) is a major predator of this deer and there are few reports of kills by leopard (P. pardus ) and dhole (Cuon alpinus ) (Qureshi et al., 2004). Swamp deer is predominantly a grazer (Qureshi et al., 2004), but at least R. d. duvaucelii is known to feed occasionally on aquatic plants, and aquatic plants contribute significantly to the diet of R. d. ranjitsinhi during the monsoon and winter (Qureshi et al., 1994).
Spotted deer (Axis axis ) has been enlisted as the “least concern” in the red list of International Union for Conservation of Nature (IUCN) (Duckworth et al., 2015a). Spotted deer is a plastic species capable of considerable adaptive response. This characteristic is well reflected through its wide-ranging distribution in India and even where it is an exotic (Dave, 2008). Also, most of the natural ranges of spotted deer are shared with livestock (Dave, 2008).
In Nepal, out of four localities reported by Schaller (1967), only two of them hold the species at present. Thus, there are ongoing plans in Nepal to translocate some of the deer from Suklaphanta National Park (SNP) to boost the smaller population of Bardia National Park (BNP) (Duckworth et al., 2015b). The Department of National Parks and Wildlife Conservation is also looking at the feasibility of re-introducing the species to Chitwan National Park (CNP) (Duckworth et al., 2015b). At the same time it is essential to know about the competitive behavior of this species at its natural habitat. Intensive grazing by large number of cattle have created severe condition in SNP (Bhattarai, 2012). Domestic cattle grazing and other disruptions may affect the nutritional stability of wild animals because they expend additional energy moving away from the disruptions and may be forced to forage in poor habitats instead of high-quality patches, and consequently may be competitively excluded from better habitats (Schaller, 1977). Quantitative studies on these aspects are required in order to understand the relationships between domestic cattle and herbivores, which are crucial to understanding the major factors limiting wild ungulates densities and formulation of a meaningful conservation plan for the region and species.
Thus, this study tried to quantify the potential competition based on overlap in two dimensions, habitat and space, between the indigenous and domestic species. We focused on swamp deer and spotted deer, living sympatrically in SNP along with the domestic cattle. This study assessed the vulnerability of species by determining the niche breadth regarding habitat use by the species, assessed the habitat overlap among wild and domestic herbivores and explored whether the potential exists for direct interspecific competition.