Background/Question/Methods We examined the biogeography of aquatic and semi-aquatic Heteroptera (ASH) in the Grand Canyon (GC) ecoregion (GCE) on and adjacent to the southern Colorado Plateau.

Results/Conclusions We found 14 families with 37 genera and 89 ASH taxa in 86 species in the GCE. We detected 54 ASH taxa detected within or on the rims of GC and its major tributaries, a fauna 3.8-fold higher than previously reported. We tested two groups of biogeographic hypotheses to account for these relatively high levels of diversity, demonstrating an underlying pattern of mixed biogeographic affinity and stronger landform-climate effects. Equal numbers of ASH taxa were derived from allocthonous (neotropical and nearctic) sources as compared to autochthonous (range-centered) taxa. A negative linear relationship existed between area-adjusted ASH taxon density and elevation, with more Mexican/neotropical taxa at low elevations and more nearctic taxa at higher elevations. The uplifted southern margin of the Colorado Plateau along the Mogollon Rim supported elevated ASH diversity as a function of ecotone effects and inter-provincial basin connectivity. Biogeographic range constraints were spatially hierarchical, with more taxa blocked by the Mogollon Rim than by GC; however, the roles of landform on taxon ranges were consistent from the scale of the Mogollon Rim to that of  GC, with stronger barrier/filter effects than null, corridor, or refuge effects, and little endemism. Colonization history varied across elevation and in relation to landscape evolution. Nepomorpha distribution follows an early Cenozoic river in western Grand Canyon, suggesting long-term effects of a pre-GC fluvial system. We compared ASH biogeographic patterns with those of other aquatic and terrestrial taxa in the GCE., finding evidence that taxon vagility may be related to diversity. From a conservation standpoing, no reported GCE ASH taxa are known to have been extirpated, but 52.8 percent of the extant fauna occurred at 3 or fewer localities (primarily springs), sites that may be threatened by habitat alteration or climate change.