Resilience and regime shifts: Do novel communities impede ecological recovery in a historically metal‐contaminated stream?

Abstract

1. Novel communities that result from exposure to contaminants and other anthropogenic stressors often persist in ecosystems that have experienced regime shifts.Because these systems may not return to pre-disturbance conditions after removal of a stressor, understanding the ecological consequences of regime shifts has important implications for how restoration success is defined. Long-term observational studies can potentially identify regime shifts in disturbed ecosystems; however, experimental approaches may be necessary to demonstrate their ecological consequences.2. We report results of a long-term (28 years) observational study and a series of stream mesocosm experiments that investigated a regime shift in a mining-contaminated watershed. We tested the hypothesis that establishment of a novel, metal-tolerant macroinvertebrate community in a previously contaminated stream impeded recovery of sensitive species, despite significant improvements in water quality over the past two decades.3. We observed significant improvements in abundance and species richness, but differences in community composition and trophic structure persisted downstream from a former source of metal contamination. These downstream communities were dominated by large, metal-tolerant caddisflies that likely impeded colonization by metal-sensitive groups (e.g. grazing mayflies). Mesocosm experiments conducted with reference and downstream communities demonstrated that novel communities retained their tolerance to metals, but were significantly more sensitive to other stressors. We suggest that the failure of downstream communities to recover represented a contaminant-induced regime shift that resulted from the sustained dominance of metal-tolerant species. 4. Synthesis and applications. Restoration ecologists generally consider increased species diversity or abundance of recreationally important species as indicators of restoration success. However, few stream restoration projects have quantified ecological resilience, and none have included a significant experimental component. Our research is the first to experimentally demonstrate that novel communities in a stream recovering from historical metal pollution have lower resilience | 2699 Journal of Applied Ecology WOLFF et aL. S U PP O RTI N G I N FO R M ATI O N Additional supporting information may be found online in the Supporting Information section at the end of the article. How to cite this article: Wolff BA, Duggan SB, Clements WH. Resilience and regime shifts: Do novel communities impede ecological recovery in a historically metal-contaminated stream? J Appl Ecol. 2019;56:2698-2709. https ://doi.

Seasonal impoundment alters patterns of tidal wetland plant diversity across spatial scales

“…Although the overall diversity of dominant plants did not differ between impounded and tidal brackish wetlands, the species identity and functional attributes of dominant plants did differ. As species presence, relative abundance, and functional traits can alter ecosystem function (Lefcheck et al 2015, Stotz et al 2019), management‐induced changes in dominant plant composition likely affect important ecosystem processes, including carbon cycling (Kroeger et al 2017), climate change resilience (Stagg et al 2016), and long‐term restoration potential (Wolff et al 2019). As the functional traits of rhizomatous perennial graminoids found in tidal brackish wetlands and non‐rhizomatous annual graminoids or perennial forbs found in impounded brackish wetlands are not equivalent, changes in dominant species could lead to important, but understudied, shifts in ecosystem function (Hooper et al 2005).…”

Metals alter membership but not diversity of a headwater stream microbiome

“…CC-BY-NC-ND 4.0 International license available under a was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprint (which this version posted May 2, 2020. ; https://doi.org/10.1101/2020.04.30.071522 doi: bioRxiv preprint macroinvertebrate membership between upstream and downstream locations previously reported for this same location (18).…”

“…CC-BY-NC-ND 4.0 International license available under a was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made The copyright holder for this preprint (which this version posted May 2, 2020. ; https://doi.org/10.1101/2020.04.30.071522 doi: bioRxiv preprint Previous investigations have shown that experimental metal exposure resulted in much greater effect on composition from macroinvertebrate communities sourced from sites upstream of California Gulch (18). In this mesocosm experiment we expected to see greater changes in microbiome membership of upstream microbiomes in response to the metal treatments, since this site has historically had lower metal exposure and we anticipated the microbes would be more sensitive to metal stress.…”

Abrupt Change in a Subtidal Rocky Reef Community Coincided with a Rapid Acceleration of Sea Water Warming

“…Only a minority of species, which we called 'resilients', showed some recovery to date. The novel community that emerged from the climate shift of the 1990s, with many once characteristic species lost, reduced complexity, biotic homogenization, greater diversity, and domination by alien species, may be able to persist [70], and there is little opportunity to go back for ecosystems under altered regime [16]. Recent analyses of Ligurian Sea temperature suggest that another phase of rapid warming is occurring, 2014 to 2018 having been the warmest years on records [28,46], thus conforming to the expected further rapid warming over the whole Mediterranean Sea [71,72].…”