Seasonal humidity patterns in the tropics shape vegetation and insect communities, with life history traits offering insight into how seasonality influences species richness, abundance, and composition. To explore these effects, we surveyed adult butterflies across 18 one-hectare plots in Manas National Park, Assam, northeastern India (12 within the park, 6 in adjoining rural habitats), characterized by environmental variables including vegetation structure, nectar, invasive shrubs, and mammalian megafauna visitation. We recorded 121/4053 butterfly species/individuals (average species per plot 32.1±14.07 SD) in post-monsoon and 104/2727 species/individuals (24.8±11.93 SD) in pre-monsoon, indicating a seasonal turnover. As elsewhere in the tropics, forests were richer in species than grasslands, and this habitat effect was consistent across both seasons. Species richness peaked at intermediate tree and shrub cover, following gentle hump-shaped patterns. Abundance was also higher in post-monsoon and highest in rural habitats. In contrast, diversity remained unaffected by season. Megafauna visitation to the plots reduced butterfly richness, while megafauna activity signs at vines increased it, regardless of season. In multivariate analyses, season alone explained only around 2% of the variation in butterfly species composition, yet this effect was statistically highly significant and aligned with species’ life history traits. Species typical for post-monsoon tended to develop on forbs or tall grasses and inhabit large global ranges, pre-monsoon species tended to have larger wingspan and longer flight periods. The consistency of butterfly–environment relationships across seasons implies utility of short-term surveys for understanding such patterns, although long-term seasonally informed studies remain superior to them.

The butterfly fauna of the megadiverse Republic of India contains 1386 species. The species richness in its 36 federal states and union territories primarily reflects the peculiar geography of the Indian peninsula, which is isolated from the northerly situated Palearctic realm by arid deserts in the west and the Himalayas Mts. in the North, while just a narrow eastern conduit connects it with the rest of the Indo-Malayan Realm (Das et al., Insects 2023). Here, we further develop on these findings by focusing on species identities, relating the presences of individual species per federal states to physical geography, climate, land covers, and socioeconomic conditions of the states, interpreting the patterns by life history traits of individual species, and considering the phylogenetic signal in the environment x life history relationships. Physical geography was the strongest predictor of the states’ butterfly fauna compositions, followed by climate, land covers, and socioeconomics. The main faunal structures separate the humid Northeast from the rest of the country; distinguish humid Western Ghats states from the rest; and group together peninsular mountains. Analysing life histories showed that butterflies of the humid northeastern and southwestern states tend to be larger and develop on woody plants or large grasses; those of arid and high-altitude states tend to be smaller and develop on small forbs; whereas those utilising broad larval host plant scopes tend to be associated with shrubs and vines and inhabit large geographic ranges. More information on Indian butterflies’ life histories would likely yield more intricate insights.