Long-term species monitoring programmes have revealed catastrophic declines in insect populations and disruption of biological communities that are contributing to biodiversity loss. Such discoveries have been possible because of standardised methods, such as line transects, of counting butterflies and other insects. However, line transects are not feasible in many tropical and mountainous habitats, so alternative methods must be explored.To tackle this issue, we devised time-constrained (30-min) counts and compared butterfly diversity estimated through timed counts and line transects in three tropical habitats in India (evergreen forest, dry deciduous forest and an urban woodland). We tested the relative performance of the two methods in sampling species richness and abundance, as well as numbers of rare, endemic and specialist butterflies.We observed greater overall species richness, and more species of habitat specialists and endemics per sample in time-constrained counts in the evergreen forest, but not in the other two habitats. Thus, time-constrained counts were relatively more efficient in detecting species 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.Our study thus shows that time-constrained counts are a suitable if not a superior alternative to line transects to conduct butterfly diversity surveys and population monitoring in complex tropical landscapes. Due to methodological flexibility and simplicity, time-constrained counts may be particularly useful to study the impacts of climate change, habitat fragmentation and land use practices on butterfly, Odonata and other insect populations in populous and tech-ready tropical countries using citizen science frameworks.

Insights into the impacts of habitat and climate change, efficacy of management practices and so forth on the persistence of insect populations depend on long-term monitoring programmes that gather data using standardised methods.We devised time-constrained (30-min) counts as a standard method for sampling diversity and monitoring butterfly populations in tropical habitats where species diversity is high but linear transects are impractical.Our study showed that timed counts are as efficient as line transects in capturing species diversity and more efficient in sampling specialist and endemic species in tropical evergreen forests.image