Effects of low-density development on stream biota: Evidence for biotic homogenization from an assemblage perspective

Land use/land cover change from forested to developed land is a major threat to freshwater biodiversity globally. However, existing research has focused on high-density development in urban centers. We know less about how low-density development affects stream biodiversity, even though increases in low-density development have been documented across the globe and may be tied to the homogenization of stream biotic assemblages. Here, we investigated the diversity (alpha and beta) and assemblage composition of algal diatoms and fish over a ten-year period at three points in time (2000, 2005, 2010) in forested watersheds (n = 4) and watersheds experiencing increasing low-density development (“developing watersheds”; n = 4) in the southern Appalachian Mountains of western North Carolina, USA. We employed a repeated snapshot sampling approach for the data collection. In developing watersheds, we observed higher alpha diversity and different species assemblages for both diatoms and fishes compared to forested watersheds. These differences were attributed to the establishment and/or higher abundances of cosmopolitan diatoms (e.g., Achnanthidium rivulare Potapova and Ponader, Navicula spp., Nitzschia spp.) as well as fishes (e.g., Creek Chub [Semotilus atromaculatus] and Blacknose Dace [Rhinichthys atratulus]). Most endemic species persisted in developing reaches, but in lower abundances (e.g., diatoms: Meridion alansmithii Brant; fishes: Mottled Sculpin [Cottus bairdi]). Diatom beta diversity within developing reaches was lower compared to forested reaches, and lower in 2005 and 2010 compared to 2000. Diatom assemblage composition also changed over time in developing reaches but remained stable in forested reaches. In contrast, fish beta diversity showed no significant differences between watershed types or among years, suggesting that diatom assemblages may respond more quickly or to lower levels of watershed development than fish assemblages. We conclude that biotic assemblages in streams draining developing watersheds in southern Appalachia show evidence of homogenization, but not yet extirpation of endemic taxa. Our study demonstrates the importance of understanding the effects of low-density development on biodiversity in stream ecosystems around the world before native species are lost.