<b>Table S1</b>. Comprehensive list of species collected in this study.

Tropical elevation gradients host some of the most diverse assemblages of organisms, but current empirical data provide competing hypotheses on how this elevational biodiversity is structured: species are either most numerous in tropical lowland rainforests with a decline in diversity with elevation, or middle elevation ecosystems host the highest species richness by combining low- and high-elevation elements. In this study, we investigated the distribution of scolytine beetles—including phloem, ambrosia, and seed feeding beetles of the weevil subfamily Scolytinae—along a tropical elevational gradient from 200m a.s.l. to 3500m a.s.l., to identify areas of highest and lowest species richness, abundance, and other biodiversity variables. Beetles were sampled using unbiased, passive flight intercept traps at eight elevations regularly spaced by 500 elevational m on Mt Wilhelm, Papua New Guinea, with consistent collecting effort at each site. Despite collecting over 9600 specimens of 215 species, our sampling effort did not fully exhaust the diversity within our study region, as suggested by species accumulation curves. Scolytine species richness conforms to a unimodal distribution, peaking at lower- to mid-elevations between 700m and 1200m a.s.l. Abundance is greatest at the lowest sites, driven by a few extremely abundant species. These results support the findings of previous studies that reported skewed unimodal elevational biodiversity distribution, highest at lower and particularly at middle elevations. Alternative models, such as a monotonous decrease from lowlands to higher elevations, and a mid-elevation maximum, showed lesser fit to our data. The turnover rate—beta-diversity between elevation steps—is greatest between the highest elevations. Species richness of dominant tribes—Dryocoetini, Xyleborini, and Cryphalini—was highest at mid-elevations ranging from 700m to 2200m. In terms of abundance, separate analyses of individual taxa show a different pattern in each: abundance of <i>Euwallacea</i> spp. uniformly decreases with elevation, while the abundances of other genera are driven by individual species dominant at different elevation. The <i>Coccotrypes</i> and the phloem-feeding <i>Cryphalus</i> have experienced evolutionary radiation in New Guinea, and most of their species remain undescribed. Species not yet know to science are most likely to be found at lower and middle elevations, where overall diversity is highest.