**1. Introduction**

In the Indian megafauna, the greater one-horned rhinoceros (*Rhinoceros unicornis*) [1] is one of the five remaining species of rhinoceros of approximately thirty genera [2]. Rhinos belong to the order Perissodactyla (odd-toed ungulates), which includes horses and tapirs. They are also called the Indian rhino, greater one-horned rhinoceros, or great Indian rhinoceros (hereafter rhino), native to the Indian subcontinent (India and Nepal). Rhino has become extinct from Bhutan and Bangladesh with a decline in the population in their distributional range [3, 4]. The estimated population in India is around 3262 [5] and is 752 in Nepal [6]. It is listed as Vulnerable on the

IUCN Red List, as populations are fragmented and restricted to distributional areas of less than 20,000 km [2]. This species is listed in CITES Appendix-I and is protected under Schedule I of the Wildlife Protection Act.

Previously, the greater one-horned rhinoceros (*R. unicornis*) was widely distributed from the Hindukush Mountain Range in Pakistan to Myanmar, and it also inhabited the Ganges floodplains [7]. For over 200 years, the Indian rhinoceros has been eradicated from the majority of their former range of distribution due to overhunting, fragmentation of habitat caused by clearing of forests for cultivation, desperate agricultural use, and expansion of tea gardens [5]. It has also been eliminated from most of its former range due to uncontrolled fires and grasslands and swamps being reclaimed by expanding livestock and humans The Indian rhino once ranged throughout the northern part of the Indian subcontinent, along the Indus and Ganges river basins, and as far south as Bangladesh and the southern parts of Nepal and Bhutan. The Terai and Brahmaputra basins are home to a large number of them due to their alluvial grasslands. The Terai grasslands of southern Nepal, northern Uttar Pradesh, northern Bihar, northern West Bengal, and the Brahmaputra Valley of Assam are the only places where it survives as a result of habitat destruction and climatic changes. By the nineteenth century, it was only found in these areas.

We have limited information on the rhino population demographics and lifehistory traits in India. It is necessary to understand the changes in the population size, mortality, natality pattern, survival rate, and fecundity essential for strategizing conservation measures. We hypothesized that rhino survival, mortality, and natality varied by year, age, and gender during 1984–2021. We attempted to answer the following questions: 1. changes in the population size of rhinos in the Dudhwa National Park, 2. age-specific natality and mortality of rhinos, and 3. reproductive rate, fecundity, and inter-birth interval in the rhinos.

#### **1.1 Current habitat of the greater one-horned rhinoceros in India**

There are only three states in India that have rhino populations in different national parks: In Assam [5], there are Kaziranga National Park (2401), Manas Tiger Reserve (32), Pobitora Wildlife Sanctuary (92), and Orang National Park (100). In West Bengal, there are Jaldapara Wildlife Sanctuary (200) and Gorumara National Park (50), and in Uttar Pradesh, where rhinos are found after reintroduction in 1984, there is Dudhwa National Park (35–40). Apart from the population of rhinos in India, Nepal also consists of a good number of rhinos (752); the estimated population in the different protected areas [6] are Shuklaphanta wildlife reserve (17), Bardia wildlife reserve (38), Royal Chitwan National Park (694), and Parsa (3) (**Figure 1**).

#### **1.2 Study area**

Dudhwa National Park is located in the Terai region of Uttar Pradesh. The total area of the national park is 490.29 km [2], located along the Indo-Nepal border in Kheri District. Dudhwa was established in 1958 as a wildlife sanctuary for swamp deer and became a tiger reserve in 1979. The Terai ecosystem is one of the most threatened ecosystems in India. Located between the Himalayan foothills and the Gangetic plains, the region extends through Uttarakhand, northern Uttar Pradesh, Bihar, northwestern Bengal, Assam, and southern Nepal. The park falls within the upper Gangetic plain, a vast alluvial plain with an altitude ranging from 150 m in the southeast to 182 m in the north. Sal forests are very dense, and they can be divided into four types: the tropical

*An Assessment of the Population Density of Greater One-Horned Rhinoceros in Uttar Pradesh… DOI: http://dx.doi.org/10.5772/intechopen.109824*

**Figure 1.**

*Existing population of the great one-horned rhinoceros only in India and Nepal (courtesy WWF-India).*

semi-evergreen forest, the moist deciduous forest of northern India, the tropical swamp forest, and the tropical dry deciduous forest of northern Guatemala [8]. In total, the park comprises about 19% grassland. In addition to rivers, streams, lakes, and marshes, wetlands are a third major habitat type.

Dudhwa has a subtropical climate that is extremely humid, with a dry winter and three distinct seasons. The summer season begins in mid-March and lasts until mid-June. The weather gets hot with temperatures reaching 40°C. The winter season extends from mid-October to mid-March. The temperature ranges between 20 and 30°C during the day and is cold and foggy at night. Monsoons typically start in mid-July and last until September, resulting in 90% of the 150 cm of rainfall. The temperature during the rainy season ranges from 24 to 30°C.

#### **1.3 Methods**

Based on the annual reports and other public sources, we obtained secondary data about the rhino reintroduction program of 1984 and rhino birth records and mortality cases between 1984 and 2021. The annual reports provide detailed information on each case, including the estimated age, gender, cause of death, location, and mortality date. Life-history traits such as natality, age-specific survival, fecundity, mortality, and inter-birth interval were calculated [9, 10]. The rhino population growth model is predicted using a generalized linear model equation (1), specifically logistic non-linear least squares regression [10].

$$\mathcal{Y} = \beta\_1 / (\mathbf{1} + \mathbf{exp}(-\beta\_0 + \beta\_2 \ast \mathbf{x}) \tag{1}$$

Where *β<sup>1</sup>* is the asymptote value (maximum population size). *β<sup>0</sup>* is the constant; *β<sup>2</sup>* is the growth parameter; and *x* is the year.
