4. Discussion
The intense coloration and patterns seen in leafhopper species collected from mid hills zone II and high hills wet temperate zone III compared to low hills zone I and high hills dry temperate zone IV might be due to more atmospheric humidity, rainfall conditions (Table 1; Fig. Ia & Ib) as well as higher abundance of leafhopper population in those areas. Many workers have found a significant correlation of darker and brighter pigments of insects with high elevational environment, specifically to snow, extreme cold, overcrowding developmental stages and hibernation period, higher light intensity, humidity, and harmful ultraviolet radiations (Sala et al ., 2010; Fischer et al ., 2011; Guoet al ., 2013; Chatelain et al ., 2020). In the high species diversity and richness zone, there was diverse vegetation choice constituting forests, grasses, vegetable crops, horticultural crops as well as fruit trees that served for regular supply of food at different times and seasons of the year for their generations and hence, might have affected their physical appearance.
Leafhopper species richness followed a declining trend along the extreme higher and lower elevations in the region. With higher leafhopper abundance as well as species richness, peak rise occurred in mid hills zone II (650-1800m) followed by high hills wet temperate zone III (1801-2200m amsl) which shrinked towards increasing altitude. In the studies conducted by Acharya and Vijayan (2011) while observing the distribution pattern of butterflies’ species, similar was the pattern with highest number of species observed below 1800 m highest at 900-1800 m band of elevation. In the western part of Sikkim Himalayas, Chettriet al . (2010) recorded lower range of butterflies’ species richness with rising altitude. Similar were the results recorded by Fleishman et al . (1998) from the Great Basin of the USA; Vu and Yuan (2003) from Vietnam and Leingartner et al . (2014). However if the phylogenetic relatedness of the butterfly collections is to be considered, it does not increment with the elevation gradient. The elevational pattern reported for leafhopper in the present study has similarity with the pattern for Himalayan fishes (Bhatt et al ., 2012) but differ slightly from plants and other vertebrates groups from the same region (Chettri et al ., 2010; Acharya et al ., 2011). The distribution pattern reported for leafhopper species at different range of agro climatic zones in the present study might be due to number of reasons governed by various causes that include; specifically & most significantly resources available to the species, distribution pattern & congenial environmental conditions, which are the major factors determining the structure of species in a niche (Table 1; Fig. Ia &b). In general, this study is in line with the findings of most of the previous work on insects but few also attained contrasting results as in the Western Himalayan elevation gradient, Bhardwajet al . (2012) found a negative trend of butterfly diversity. The type and quantity of resources, as well as their distribution patterns, climatic conditions, and disturbance levels, are the major factors that determine the community structure of insects along spatial gradients (Fleishman et al ., 1998; Foristera et al ., 2010; Fischeret al ., 2011). The declining trend at extreme higher elevation in high hills dry temperate zone IV (above 2200m amsl) in the present study might be attributed to low mercury level and lesser humidity in those areas (Table 1; Fig. Ia &b). Temperature is the most essential character that decides and regulates the condition for diversity of plant, animal and any other kind of species of organism. However in the present study, declining trend of leafhopper species was also observed towards the extreme lower elevation in low hills zone I (350-650 m amsl) which might be due to the monocropping pattern followed in terms of cash crops that attracts only certain kind of leafhopper pest species; higher mercury level sometimes going up to 50° C in summers, unbearable for survival (Table 1; Fig. Ia &b) and higher anthropogenic activity in those areas due to development, etc . Abundance of leafhopper in the warm season was higher than in the hot summers, which might be due to the extreme weather condition which affected the mobility of leafhoppers as reported by Habekuss et al . (2009); El-Wakeil et al . (2014). Leafhoppers like any other organism require a certain threshold level of temperature and humidity congenial for their survival and continuous lifecycle; hence the extreme hot and extreme cold weather parameters prevailing at these lower and higher elevations, respectively, were the reasons for encountering lesser species diversity in such agro climatic zones. Similar were the causes mentioned by Xuet al . (2017) in terms of spatial distributions of plant diversity in his research in the Lvliang Mountains of China.
In the present research work it was observed that the mid hills zone II, as well as high hills wet temperate zone III had maximum plant diversity and a longer period of vegetation as compared to other two zones (Table 1; Fig Ia & Ib) providing complimentary habitat and food for longer duration which might have resulted in the survival of maximum number of generations. The results of the studies carried out by various workers indicated that global warming, increasing temperature and decreasing snow line will cause upward migration of species in Himalayas and mountain system due to climate change (Walther et al ., 2005; Pauli et al . 2007; Kelly & Goulden, 2008; Lenoir et al ., 2008; Fischer et al ., 2011; Chatelain et al ., 2020). Definitely, the abundance of various species at low altitudes is decreasing and not only insects will become more abundant, but also insects like leafhoppers might migrate from lower to higher elevation as stated by Chmielewski (2007) in his study with Russian wheat aphid (Diuraphis noxia ). Similar were the results of a study conducted by Sharma (2015) who delivered that changing scenario of climate and alarming effect of global warming initiated the shift in allocation of geographical areas and population dynamics of insect pest species. These transformation and modifications will affect plant diversity and crop production. Niche area of insect’s acclimatized to tropical and subtropical areas shall get transferred to colder regions due to increase in mercury level along with alterations in zones of their host plants. Melting glaciers and snow in temperate cold dry zone shall cause the area to become greener with lustrous flora and increase in fauna. Relative abundance of some major insect species and their natural enemies confined to the temperate regions might go extinct in future and in case of leafhoppers the most serious and frightening concern is the rise in plant viruses and phytoplasma transmission, thus, with the rising temperature insect transmitted diseases may also prevail. At present the plant diseases specifically transmitted by leafhoppers have no substantial control, therefore with such hypothesis the coming future of crops and food security lies in the paradigm of uncertainty.
5. Data accessibility statement :
Upon acceptance of manuscript authors are ready to provide their data and all other information (whichever required) in a publicly accessible repository Drayad.