Current and future latitudinal gradients in stream macroinvertebrate richness across North America

Freshwater ecosystems are highly vulnerable to warming climates. However, macroecological studies of climate-change effects on aquatic biodiversity are rare because of a lack of standardized large-scale surveys, e. g., along large latitudinal gradients. Our goal was to assess the overall richness pattern and projected differences in present and future patterns of the stream insect orders Ephemeroptera (E), Plecoptera (P), Trichoptera (T), and combined EPT along an extensive latitudinal gradient across North America (30 to 70 degrees N). We applied Bioclimatic Envelope Models (BEMs) to project present-day and future climatically suitable areas for EPTs on a spatial resolution of 10 arc-min (20 km x 20 km) across North America. To overcome issues related to spatially biased sampling, we assessed climatically suitable areas (CSAs) for each genus and modeled potential generic richness/grid cell, rather than assessing observed generic richness patterns directly. Projected present-day generic richness was greatest between 40 and 48 degrees N latitude, with peaks at 44, 45, and 47 degrees N for the E, P, and T orders, respectively. Our models projected that CSAs would shift an average 4.2 to 5.2 degrees, 4.4 to 5.3 degrees and 3.4 to 4.1 degrees N latitude by 2080 for E, P, and T genera, respectively. Overall, the present-day projected generic EPT richness is highest in the warm and cool temperate zones and shows a unimodal richness pattern that is projected to shift northward under climate-change conditions. A similar northward shift of richness patterns might also apply to other aquatic insects with relatively narrow thermal sensitivity and terrestrial, winged adults, e.g., freshwater Diptera or many aquatic Coleoptera. This large-scale application of genus-by-genus models gives a first approximation of the likely consequences of climate-change effects on freshwater biota across North America.