This study represents the first nationwide survey of forest ants in Nepal. Species lists are important tools for species conservation because they provide a solid understanding of the current state of the biota, which can serve as a foundation for conservation actions [56]. The species–accumulation curve revealed that the curve approached the asymptote but was not completely leveled off (Figure 2). Despite the fact that the majority of the species were captured in the study areas, additional sampling efforts involving multiple collection methods would result in the capture of a few more species. Crematogaster (208 individuals, 42 occurrences, 6 species) and Camponotus (181 individuals, 40 occurrences, 6 species) were the most diverse genera. Of all the species, Oecophylla smaragdina was recorded most often (38 occurrences) during the study, while the genus Pheidole was recorded from Tarai and Siwalik and the hills of the western, central, and eastern regions. Seventeen genera were represented by only one species in our collection, and eight genera were collected from a single site. In Nepal, the richness of Oriental genera (such as Oecophylla, Polyrhachis, Prenolepis, Carebara, Lophomyrmex, Leptogenys, Meranoplus, and Tetraponera) outnumbered the Palaearctic genera (such as Myrmica, Lasius, and Temnothorax) as in southern China [57]. Several genera were found to be common throughout Tarai and Siwalik and the hills of western, central, and eastern regions, while others were found to be peculiar to a specific location (Figure 3 and Figure 5), with the Palearctic genera being reported at high elevations. Although it is difficult to make an exact comparison of this study with other studies because objectives, sampling methods, the area covered, and identification levels vary, our findings are consistent with ant inventories in our neighboring countries, particularly in dominant subfamilies and commonly occurring genera, such as southern China [57], Yunnan, China [58], northwestern Siwalik, India [59], southwest China [60], Jammu and Kashmir, India [61]. Furthermore, our findings agree with those of a previous study that recognized Camponotus, Pheidole, and Crematogaster as the three most species-rich genera on a global scale [62]. Weaver ants (O. smaragdina) are conspicuous arboreal ants with more than 2700 site records from 21 countries [10]. They share beneficial traits with other ant species and cause cascading effects to lower trophic levels, reducing pest number and damage in orchards and forests [63].

The variation in the distribution of the forest ants of Nepal with elevation is in line with other studies. The highest number of ant genera in our study was recorded below 1000 m asl, and there was a decreasing pattern with the increase in elevation (Figure 4) as reported by Subedi and Budha [64] when they extrapolated available elevation-related records of ants of Nepal from 200 m to 4550 m. A decreasing pattern is one of the most common patterns of ant species richness [64,65]. A number of studies from different parts of the world also reported a decreasing pattern of species richness with elevation, such as those from Mount Kinabalu [66], the Mediterranean and the oro-Mediterranean parts of Montenegro [67], rainforest in subtropical Queensland [68], wet forest on Costa Rica’s Atlantic slope [69], Imbak Canyon [70], Hengduan Mountains [71], and eastern Himalaya [15]. The most common pattern of species richness along the elevation gradient in the Nepal Himalaya seems to be a unimodal pattern [72,73,74]. However, the elevation with maximum richness is not similar for different taxa [75,76].

In Nepal, we recorded eight common tramp species, including M. pharaonis, P. longicornis, T. melanocephalum, and T. destructor. These four species are amongst twelve cosmopolitan ants that have become significant ecological, agricultural, and/or household pests [77]. Wetterer [78] considered P. longicornis to be the most widely dispersed ant species, with records from both the Old and New Worlds in both the northern and southern hemispheres. Tramp and invasive species are geographically widespread, and observations of functional groups and biogeography may help to better understand the factors that contribute to their spread [79]. Most of the widespread ants belong to the subfamily Myrmicinae and, more specifically, to the following functional groups: cryptic, opportunist, and generalized myrmicine [79].

Ants are indispensable components for the maintenance and appropriate functioning of most terrestrial ecosystems and resulting ecosystem services and disservices [80]. Ant species composition strongly influences seed dispersal by ants [81], and ant communities are very useful for the rapid assessment of terrestrial ecosystem health [82]. Ants play a crucial ecological role by helping in soil formation and by increasing its fertility [83,84]. Three ant species, including O. smaragdina, have been identified as predators of the sal heartwood borer (Hoplocerambyx spinicornis) in nature [85]. In our study, we also frequently observed O. smaragdina in Sal forests.

Of the 42 cosmopolitan ants, only 12 species are major ecological, agricultural, and/or household pests [77], and 4 of them were recorded in Tarai and Siwalik and the mid-mountains of the western, central, and eastern regions of the country. Invasive ant characteristics, such as greater abundance, aggressive behavior, and attraction to high-carbohydrate resources, may result in a high and low risk of deleterious effects on plants, resulting in a negative conservation impact [86]. Bharti and Sharma [16] revealed that the most invasive ant species occurred at lower altitudes, indicating their correlation with the disturbance of the ecosystem.

When used as bioindicators, ants can help detect early signs of habitat disturbance and can help develop management strategies [87]. Invasive ant eradication is critical for biodiversity conservation; however, more than half of the global eradication efforts were not successful [88]. The initiatives aimed to eradicate 11 species around the world, namely Anoplolepis gracilipes, Linepithema humile, Pheidole megacephala, Wasmannia auropunctata, Solenopsis invicta, S. geminata, T. melanocephalum, Lepisiota frauenfeldi, Myrmecia brevinoda, Monomorium indicum, and P. longicornis [88]. Two of the eleven species causing problems around the world, namely P. longicornis, and T. melanocephalum, were also recorded in our study. Other invasive ants, such as A. gracilipes and P. megacephala that are common in neighboring countries are not yet known in Nepal. We believe that studies focusing on the tramp and invasive species of ants in urban areas should be conducted in order to complete their inventory in the country.

MoFE [89] identified invasive species expansion as one of the direct drivers of deforestation and forest degradation, posing a very high effect in Tarai and the Siwaliks, a low effect in the mid-mountains, and a very low effect in the high mountains. Using ants as bioindicators to assess ecosystem health, Bharti and Sharma [16] discovered that the invasive species are seriously threatening the native species of the primary and secondary forests of the Himalaya. They also stated that while ant invasions are currently limited to lower mountain ranges, as global temperatures rise, they may spread to highlands.

The vast majority of insect species are herbivores, but only nine insect orders, including Hymenoptera (ants, wasps, bees, hornets, and sawflies), have species that feed on live plants [90]. Larger carpenter ants (Camponotus sp.) may excavate solid wood in living tree trunks, making them major forest pests [91]. In Nepal, pests and pathogens have invaded all types of forests, resulting in serious consequences, such as forest degradation and biodiversity loss [92].

The early detection of invasive species, pests, and pathogens is critical for preventing damage because early control activities are more feasible and effective [92,93]. Thorough monitoring of invasive ant species encroachment in forest areas is critical for protecting fragile ecosystems and maintaining sound forest health. With increased global travel and trade, several other undocumented tramp/invasive ants may occur or be introduced into the country, particularly in urban areas. A more concentrated collection of tramp/invasive ants in urban areas is necessary to identify them and prevent their possible incursion into natural forests.