A comparison between time-constrained counts and line transects as methods to estimate butterfly diversity in tropical habitats
Long-term monitoring programmes have revealed catastrophic population declines that are contributing to biodiversity loss. These discoveries and the underlying programmes have been possible because of standardised methods of counting butterflies. Butterflies have been monitored using modified line transects, known in butterfly monitoring schemes as Pollard walks. However, line transects are not feasible in many tropical and mountainous habitats. To tackle this problem, we devised time-constrained (30-min) counts and compared butterfly diversity as estimated through this method with that estimated through line transects (500 m, 10 min) in three tropical habitats in India (evergreen forest, dry deciduous forest and an urban woodland). We tested the efficacy of the two methods to sample species richness and abundance, as well as numbers of rare, endemic and specialist butterflies. We observed greater species richness, and more species of habitat specialists and endemics per sample in time-constrained counts when compared with line transects in evergreen forest, but not in the other two habitats. Thus, time-constrained counts were more efficient at detecting species than line transects in the species-rich evergreen habitat. Apart from this difference, the two sampling methods captured similar levels of species richness and other measures of diversity. Time-constrained counts may thus be viewed as a feasible alternative to line transects to carry out butterfly surveys in tropical and mountainous landscapes. This method is particularly suited for biodiversity assessments and mapping involving citizen science initiatives, which has considerable potential in populous and tech-ready countries such as India.