Low‐head dam removal increases functional diversity of stream fish assemblages

Abstract

Despite the growing number of dam removals, few have been studied to understand their impacts on stream fish communities. An even smaller proportion of dam removal studies focus on the impacts of low‐head dam removals, although they are the most common type of dam. Instead, the majority of removal studies focus on the impacts of larger dams. In this study, two previously impounded Illinois Rivers were monitored to assess the impacts of low‐head dam removal on the functional assemblage of stream fishes. Study sites were sampled each fall from 2012–2015 (pre‐dam removal) and 2018–2020 (post‐dam removal) in three locations: the tailrace, impoundment, and river channel. Fishes were aggregated into habitat and reproductive guilds, relating community changes to habitat, environmental metrics, and stream quality. Prior to removal, the slackwater guild was the most prevalent habitat guild throughout both rivers, while nest builders and benthic spawners were the most abundant reproductive guilds. During the two years following removal, fish assemblage throughout both rivers shifted to a more evenly distributed representation of habitat and reproductive guilds, while restoration of lotic habitat conditions increased, as surface water temperatures decreased and QHEI, IBI, and dissolved oxygen increased. This shift in environmental metrics and increase in overall stream quality increased, particularly in the formerly impounded reaches, indicate diminished habitat homogeneity, and a shift towards natural habitat diversity. This habitat diversification likely led to the restoration of a range of potential niches, thereby increasing the array of guild types inhabiting these rivers, while simultaneously preventing single‐guild dominance.

Fish assemblage and functional trait responses to small‐dam removal

“…Although we found significant changes in some habitat variables at above the dam site, the exact mechanisms underlying this change could not be identified in our study. Other studies found similar shifts in assemblage structure at impounded sites after dam removal and suggested habitat alteration as the driver of the observed change in assemblage structure (Hogg et al, 2015;Jones et al, 2022;Maloney et al, 2008;Poulos et al, 2014). We observed no significant change in assemblage structure at the control sites on Three and Twenty Creek coincident with the timing of dam removal in Twelvemile Creek, suggesting that dam removal and subsequent change to habitat characteristics were the drivers of the change in assemblage structure observed in Twelvemile Creek.…”

“…Restoration via dam removal assumes that the ecological benefits will outweigh the ecological disturbance and economic cost caused by dam removal (Thomson et al., 2005). Despite the possible negative effects, over 1900 dams have been removed in the United State since 2000, yet relatively few studies document the effect of dam removal on fish populations or use carefully collected pre‐ and post‐removal data (American Rivers, 2019; Bellmore et al., 2017; Jones et al., 2022). Small‐dam removal studies are underrepresented in the literature and most dam‐removal studies are short‐term (1–2 years), with only 5% of studies longer than 5 years (Bellmore et al., 2017).…”

“…Accordingly, reductions in functional diversity may indicate loss of niche space exploited by species (Mouillot et al., 2005), providing additional insights on the ecosystem function and resilience of an assemblage than taxonomic diversity alone (Tilman et al., 1997). Few have explored the change in functional diversity in response to dam removal (Bellmore et al., 2017; Jones et al., 2022), and to the best of our knowledge no studies explore these responses using traditional functional diversity metrics such as functional richness or Rao's quadratic entropy. As dam‐removal frequency increases, understanding how traits and functional diversity of fish respond to dam removal is essential for creating predictive models capable of informing conservation and management actions across taxonomic and geographical boundaries.…”

Fast but transient recovery of aquatic and terrestrial communities after a large dam removal

“…Functional ecology offers a powerful framework to better understand the causes and consequences of species-environment relationships (Southwood, 1977;Townsend and Hildrew, 1994;Violle et al, 2007) with countless implications in conservation biology and restoration ecology. It is a common practice in dam removal projects to investigate changes in the functional characteristics of fish communities (Shaffer et al, 2018;Jones et al, 2023), aquatic macroinvertebrate communities (Pollard and Reed, 2004;Hansen and Hayes, 2012;Kil and Bae, 2012;Poulos et al, 2019;Mahan et al, 2021), and riparian vegetation (Foley et al, 2017). In the present study, we explored taxonomic changes of these three communities as a first step to report ecosystem recovery after dam removal.…”

Adfluvial migration and passage of Steelhead before and after dam removal at a major Great Lakes tributary

“…Those that have, primarily described changes in the river community using perhaps more basic metrics such as community composition, species richness, abundance or density (e.g., Catalano et al, 2007;Stanley et al, 2007;Maloney et al, 2008;Burroughs et al, 2010;Kornis et al, 2015;Magilligan et al, 2016;Poulos and Chernoff, 2017;Bubb et al, 2021). Furthermore, such studies often used methods that have provided information regarding only a snap-shot in time and/or space such as electrofishing, hydroacoustics, snorkeling or eDNA (e.g., Dorobek et al, 2015;Hogg et al, 2015;Duda et al, 2020;Scherelis et al, 2020;Huang et al, 2023;Jones et al, 2023;Kiffney et al, 2023;Whittum et al, 2023) and were often narrowly-scoped temporally, spatially or both (Bellmore et al, 2017;Foley et al, 2017;Whittum et al, 2023). These methods may not fully reflect fish responses, and do not reveal any direct information about fish movement and timing.…”