Megabarcoding reveals a tale of two very different dark taxa along the same elevational gradient

Our planet is entering the sixth mass extinction without detailed spatio-temporal distribution and abundance information for most insect species. Particularly lacking is this information for dark taxa although they contain most animal species. This problem can be addressed with high-throughput single specimen barcoding (megabarcoding), but it remain unknown to what extent one dark taxon can be used as a proxy for another because of similar biodiversity patterns. This is here tested and we show that two Diptera taxa (Phoridae, Mycetophilidae) have very different spatio-temporal distributions across the same elevational gradient. We applied COI megabarcoding to samples collected every 14 days over three seasons at six elevations (800 to 1,800 m) in a temperate forest ecosystem in Northern China (Baihua Mountain Reserve). Elevation, precipitation, and temperature were used as environmental variables to understand the data obtained for 17,179 specimens representing 492 mOTUs of Phoridae and 2,281 specimens of 148 mOTUs of Mycetophilidae. Congruent is that the alpha diversity for both taxa increased in the summer and there is evidence for a mid-domain effect and phylogenetic clustering in high elevations. However, the extent and pattern of seasonal variation differed considerably. Phorid communities exhibited shifts between spring, summer and autumn seasons, but surprisingly little difference between elevations. In contrast, mycetophilid communities were structured along the elevational gradient and dependent on precipitation. Our study highlights the urgent need for obtaining detailed spatio-temporal information for the most diverse dark taxa, but also illustrates that these data can be readily obtained with megabarcoding.