Biases in butterfly survey effort and patterns of protocol used across biogeographic regions.

Butterflies are widely recognized as effective bioindicators due to their short life cycles, close associations with specific host plants, and rapid responses to environmental change. They are comparatively easier to identify than many other insect groups, making them valuable for biodiversity monitoring. Despite this, there is no consensus or standardized guidelines on butterfly monitoring protocols in many regions of the world. This lack of standardization can compromise data quality, limit comparability across studies, lead to inefficient use of resources, and ultimately weaken our capacity to track and understand butterfly population trends at regional and global scales. Here, we systematically reviewed 435 published studies on butterfly surveys to understand: (1) butterfly survey effort across biogeographic regions and identify areas in need of monitoring and method development, (2) examine historic trends in the use of butterfly survey protocols, and (3) assess whether studies explicitly justify their choice of protocol. Our results show that butterfly survey efforts have increased over time across all biogeographic regions but remain strongly biased towards temperate regions of the Global North, particularly the Palearctic and Nearctic. We found high diversity in protocol terminology (n = 29 protocol names), and wide use of different protocols (n = 7 protocol types). Line transects were the most frequently used protocol type, with at least 50% of studies using this protocol at any point in time since 1970. However, the use of alternative protocol types has surged in recent years. Despite this diversity, explicit justification for protocol choice was generally lacking. Together, these patterns show major gaps in monitoring coverage and inconsistency in methodological practice without justification. Overall, our findings demonstrate the need to intensify butterfly monitoring in underrepresented regions, particularly the Afrotropical, Neotropical, and Papuan regions, as well as highlighting the need for more systematic, comparative evaluations of survey protocols across different biomes to support evidence-based protocol selection, facilitating robust cross-study comparisons and informing global butterfly conservation efforts.