Sampling adequacy dictates the detection of global patterns: A meta-analysis of butterfly diversity on mountains

Mountain environments are critical biodiversity hotspots and understanding species responses to elevation is crucial in an era of global warming. Butterflies, as sensitive bioindicators, are altering their zonation along elevational gradients. Here we synthesized 66 studies published over the last 32 years from 28 countries to: 1) systematically identify patterns and gaps in mountain butterfly studies, 2) highlight methodological and geographical biases, and 3) quantify butterfly responses to elevation via a global meta-analysis. We assessed butterfly variables grouped into three categories: species diversity (e.g., richness and abundance), functional traits (e.g., body size and melanization) and life-history traits (e.g., development time). Our meta-analysis revealed a significant decline in species diversity with increasing elevation worldwide. In contrast, both functional and life-history traits showed a significant increase. The negative diversity pattern was significantly stronger in tropical mountains, while the decline occurred across both high and low mountains. Critically, this diversity decline was only robustly detected when studies sampled over 70% of the mountain's elevational gradient, underscoring the crucial influence of sampling adequacy on the patterns of butterfly diversity along mountains. Our findings demonstrate that comprehensive sampling is essential to accurately detect macroecological patterns. We therefore urge expanded sampling of mountainous gradients to provide the data necessary for the conservation of butterflies and to fully understand the role of mountains as climate refuges.