As one of the oldest mountain ranges in China, the Qinling Mountains occupy an important position in the biogeography of China [21,22]. It is the dividing line between the subtropical zone and the warm temperate zone and also the dividing line between the Palearctic region and the Oriental region in the world zoogeographic divisions [23,24]. Taibai Mountain, the main peak of the Qinling Mountains, is 3767 m above sea level and is to the east of the Qinghai–Tibet Plateau [25,26]. The complex vegetation types and diverse climate make Qinling a “treasure house of biodiversity” [22,27]. As early as 1996, the Global Environmental Protection Fund (GEF) invested more than USD 10 million in the construction of the Qinling Nature Reserve group. The vegetation types in the middle Qinling Mountains are well-known, but there has been little research on butterflies [28,29,30]. The species composition, richness and spatial distribution pattern of butterfly biodiversity in this region are not clear, which poses obstacles to resource conservation. At present, there are many studies on butterfly community diversity and its influencing factors, especially the effects of habitat differences and climate factors on butterfly diversity [31,32,33]. For example, changes in altitude can significantly change the richness of butterflies [34,35]. In the study of the mountains in northern Oaxaca, Mexico, the increase in altitude leads to changes in temperature and humidity, which reduces the abundance of butterfly species [36]. However, in Pyrcz' s research, the abundance of butterflies in the higher elevations of the Ecuadorian Andes was higher than that in the lower elevations [37]; seasonal variation plays a decisive role in the spatial distribution of butterfly communities. Butterfly species usually synchronize their life cycles with seasonality. Different seasons bring different precipitation, which directly affects the change in butterfly species. In the study of Rio Doce State Park, butterfly diversity was more abundant in the dry season than in the wet one and their contribution to species turn over and nestedness variations overlap with seasonal variations [38].

The counties in the middle part of the Qinling Mountains are divided into two types. One includes the warm temperate coniferous, broadleaf mixed and deciduous broadleaf forests and the northern slope mountain brown soil and mountain brown land zone. Their butterflies belong to the Palearctic Realm (including WB, CC, ZZ, TB, northern HY and QS). The other group is the deciduous broad-leaved mixed forest with evergreen broad-leaved tree species in the northern subtropics and the southern slope of yellow–brown soil and yellow–brown land zone. Their butterflies belong to the Oriental Realm (including FP, NS, LB, TB and southern CG). Overall, the southern slope of the northern mountains shows abundance of butterfly species by blocking the cold of the north Qinling. The north and south form two different climate regions, where the northern region is cold and dry, while the southern one is hot and humid; butterflies in the middle part of the Qinling Mountains seem to prefer the southern slope with higher temperature and humidity [39,65,66]. In addition, the species richness of TB, ZZ and NS counties is relatively high, which is closely related to human disturbance factors and degree of protection. In the transect survey, we found that these three counties have more protected areas, less human disturbance and more virgin forests and undeveloped ecosystems. In terms of species and numbers, the closer the county is to the Qinling Mountains, the richer and higher the butterfly diversity.

Combined with our collected data and platform data, the biodiversity of butterflies in the ecosystem of the middle Qinling Mountains is affected by altitude gradient, seasonal change and habitat type. The dilution curve of Alpha species richness indicates that more species could be observed in plots with different seasons, elevation gradients and habitat types. From the perspective of spatial and temporal distribution patterns, although butterfly communities in different altitudes and seasons share some species, species composition differs greatly and community similarity is low, which provides a scientific basis for species diversity monitoring and conservation in this region.

Butterfly groups are sensitive to environmental and climate changes and are good ecological probes [11]. Elevation gradient changes are an important factor affecting biodiversity because there are many environmental variables, notably temperature and humidity [31,32,33], and different combinations of vegetation types that contribute to environmental heterogeneity at different altitudes [34]. In the mountain ecosystem, altitude is also an important factor shaping butterfly community composition. Species richness along the altitude gradient presents three modes: increasing, decreasing or a mid-altitude peak [35]. In studies of butterfly communities in the Himalayan Yanshan Mountains and the Alps, butterfly populations first increase and then decline sharply with altitude. In this study, the highest butterfly diversity was observed in the mid-altitude area of the middle Qinling Mountains, which supports the peak pattern of mid-altitude diversity. At the highest altitudes (>3000 m), on the top of Taibai Mountain, the peak of the middle part of the Qinling Mountains in Taibai county, only a small number of Nymphalidae and Papilionidae butterflies were found, which were all collected during the summer. Butterfly communities were the most abundant in coniferous and broadleaved mixed forests.

In addition, butterfly groups may respond to seasonality to some extent, which may be related to rising temperature and precipitation. The seasonal effects of precipitation on species diversity reflect the general operation of non-neutral mechanisms in natural communities. The optimum development period of butterfly larvae is correlated with the availability of plant leaves and new plant tissues and the availability of resources regulates the activity patterns of butterflies and affects butterfly diversity [67,68]. The precipitation in the middle part of the Qinling Mountains is mainly concentrated from August to October. The butterfly community richness and abundance were highest in summer, because it was in the period of the highest temperature and drought transition, which was consistent with Lourenço’s study [38]. Interestingly, the number of species and species abundance in spring were significantly higher than in autumn, but the Shannon diversity and Simpson diversity indices were lower than in summer, indicating that the effective number of common species and dominant species was higher in autumn than in spring and that species abundance could not accurately reflect the level of biodiversity. Because estimates of abundance assign equal weight to all species, they do not contain information about relative abundance of species.

According to the Red List of Species in China (Volume iii): Invertebrates, we evaluated the endangered species level of the 427 butterfly species collected, including 170 endemic species in China, 44 protected species and 2 wild animals under state key protection of Class II in China. We also found seven protected species in the IUCN Red List of Threatened Species. This could help develop better conservation measures for these endangered butterfly species. Understanding the combined impact of climate change and habitat loss on biodiversity is urgent in the field of ecology and conservation. Our research study on the middle Qinling area on butterfly diversity provides a database to help biologists provide more suitable protection schemes, reduce human activities and maintain the stability of the habitats. This is a basis to protect the butterfly species diversity, minimize the interference of secondary forest and mixed forest understory economic crops and promote the natural restoration or increase the heterogeneity of the environment. For example, more protected areas, more diversity monitoring and seasonal observation in areas where protected species are common would help protect endangered species. At the same time, in farmland planting areas, advocating the implementation of green ecological planting and the rational use of pesticides, as well as reducing environmental pollution, would effectively protect species diversity.