First signs of macroinvertebrate recovery following enhanced restoration of boreal streams used for timber floating

Abstract

Although ecological restoration generally succeeds in increasing physical heterogeneity, many projects fail to enhance biota. Researchers have suggested several possible explanations, including insufficient restoration intensity, or time-lags in ecological responses that prevent detection of significant changes in short-term monitoring programs. This study aims to evaluate whether benthic macroinvertebrate communities responded to an expanded set of stream restoration measures within a study period of one to five years after completion of the restoration project. We studied 10 forest streams in northern Sweden that were channelized in the past for timber floating. Managers subjected six of these streams to habitat restoration, on each of these we selected two reaches, located in close proximity but differing in restoration intensity. In "basic" restored reaches, the restoration managers broke up the channelized banks and returned cobbles and small boulders to the main channel. In "enhanced" restoration reaches, they added additional large wood and boulders to reaches previously subjected to basic restoration, and rehabilitated gravel beds. The remaining four streams were not restored, and thus represent the baseline impacted (channelized) condition. We surveyed stream benthic assemblages before the enhanced restoration (year 2010) and three times afterward between 2011 and 2015. Five years after restoration, macroinvertebrate assemblages at the enhanced restored reaches were more differentiated from channelized conditions than those at basic-restored reaches. This reflected increased relative abundances of the insect orders Ephemeroptera and Trichoptera and the bivalve molluscs Sphaeriidae and decreased relative abundances of Chironomidae (Diptera). Analysis of functional traits provided further insights on the mechanistic explanations driving the recovery, e.g., indicating that the augmented channel retention capacity at enhanced restored reaches favored taxa adapted to slow flow conditions and more effectively retained passive aquatic dispersers. The increased restoration intensity in enhanced restored reaches has resulted in shifts in the composition of benthic macroinvertebrate assemblages, including increases in more sensitive taxa. These shifts became fully apparent five years after the enhanced restoration. Our results emphasize the value of longer-term monitoring to assess ecological responses following restoration, and of undertaking additional restoration as a valuable management option for previously restored sites that failed to achieve biotic recovery.

Diverging response patterns of terrestrial and aquatic species to hydromorphological restoration

“…; Pilotto et al. ), however, and their results are consistent with ours that indicate the increased breadth of habitat conditions produced by restoration in degraded rivers allowed for the establishment of richer communities with more diverse trait combinations for all studied taxonomic groups.…”

Contrasting Responses among Aquatic Organism Groups to Changes in Geomorphic Complexity Along a Gradient of Stream Habitat Restoration: Implications for Restoration Planning and Assessment

“…Channelized systems lack larger rock and wood structures which might moderate water currents during high flow events, which might disturb the construction of filtering nets in the case of H. siltalai. Pilotto et al [26] also found that Pisidium spp., in particular, were favored by the enhanced restored sites, with this effect having strengthened with time since restoration. In all the restored sites, but especially the enhanced restored sites, species of shredders and scrapers were more abundant.…”

“…The negative relationship between the longitudinal variation in depth and macroinvertebrate Shannon diversity appeared driven by the most recently restored sites, which were characterized by the most variation in stream depths in this complexity dimension. This might reflect a time-lag in recovery of macroinvertebrates following the physical disturbance associated with restoration activities, e.g., inputs of large substrate, disturbance of microhabitats including macrophytes [26,30]. Even though the enhanced restoration sites had the most wood, the negative relationship between total volume of instream wood and macroinvertebrate richness likely reflects disturbance from recent enhanced-restoration activities.…”

“…Consequently, structural features or (hydro)geomorphic complexity have become a central focus in river management and restoration [18][19][20][21][22]. However, the ecological outcomes of most restorations of geomorphic complexity in streams have been disappointing, with weak or even negative effects on benthic biodiversity [20,[23][24][25]; see [26].…”

Interpreting insect declines: seven challenges and a way forward

“…At local scales, the missing zero effect squanders the opportunity to measure future recolonisation of unoccupied sites, through (i) natural processes associated with spatially structured population dynamics (Ovaskainen & Saastamoinen, 2018;Dallas et al, 2020), (ii) rehabilitation of sites following mitigation of threatening processes (e.g. Corlett, 2016;Pilotto et al, 2018), or (iii) as a result of extra-limital processes such as shifting geographic ranges due to climate change and species invasion (e.g. Walther et al, 2009;Hill et al, 2012;Hill et al, 2017;Rabl et al, 2017).…”