Patterns in and predictors of stream and river macroinvertebrate genera and fish species richness across the conterminous USA
Taxa richness patterns are useful for evaluating areas of greatest conservation concern. To 30 determine those patterns, we analyzed fish and macroinvertebrate taxa richness data obtained 31 at 3475 unique stream and river sites collected by the USEPA’s National Rivers and Streams 32 Assessment. We also determined which natural and anthropogenic variables most strongly 33 explained the observed patterns in regional richness. As expected, macroinvertebrate and fish 34 richness increased with the number of sites sampled per region. Therefore, we determined 35 residual taxa richness from the deviation of observed richness from predicted richness given 36 the number of sample sites in a region. Regional richness markedly exceeded average local site 37 richness for both macroinvertebrates and fish, often by an order of magnitude. Predictors of 38 macroinvertebrate-genus and fish-species residual-regional richness were quite different. Air 39 temperature was an important predictor in both cases but was positive for fish and negative for 40 macroinvertebrates. Cold, high elevation and forested ecoregions support high numbers of 41 macroinvertebrate genera but few fish species. On the other hand, warm low-elevation 42 ecoregions with high levels of agriculture support higher numbers of fish species but fewer 43 macroinvertebrate genera. Both natural and land use variables were significant predictors of regional richness. This study is the first to determine mean site and regional richness of both 45 fish and aquatic macroinvertebrates across the conterminous USA, and the key anthropogenic 46 drivers of regional richness. Thus, it offers important insights into regional USA biodiversity 47 hotspots. If the USA chooses to protect 30% of its land areas by 2030 and 50% by 2050, it would 48 be wise to focus such protections on those freshwater biodiversity hotspots.