River restoration, habitat heterogeneity and biodiversity: a failure of theory or practice?
TLDR
In this paper, the authors evaluated the reach-scale response of invertebrate species richness to restoration actions that increased channel complexity and found that there is no evidence that physical heterogeneity should not be the primary factor controlling stream biodiversity, particularly in a restoration context.Abstract:
Summary
1. Stream ecosystems are increasingly impacted by multiple stressors that lead to a loss of sensitive species and an overall reduction in diversity. A dominant paradigm in ecological restoration is that increasing habitat heterogeneity (HH) promotes restoration of biodiversity. This paradigm is reflected in stream restoration projects through the common practice of re-configuring channels to add meanders and adding physical structures such as boulders and artificial riffles to restore biodiversity by enhancing structural heterogeneity.
2. To evaluate the validity of this paradigm, we completed an extensive evaluation of published studies that have quantitatively examined the reach-scale response of invertebrate species richness to restoration actions that increased channel complexity/HH. We also evaluated studies that used manipulative or correlative approaches to test for a relationship between physical heterogeneity and invertebrate diversity in streams that were not in need of restoration.
3. We found habitat and macroinvertebrate data for 78 independent stream or river restoration projects described by 18 different author groups in which invertebrate taxa richness data in response to the restoration treatment were available. Most projects were successful in enhancing physical HH; however, only two showed statistically significant increases in biodiversity rendering them more similar to reference reaches or sites.
4. Studies manipulating structural complexity in otherwise healthy streams were generally small in scale and less than half showed a significant positive relationship with invertebrate diversity. Only one-third of the studies that attempted to correlate biodiversity to existing levels of in-stream heterogeneity found a positive relationship.
5. Across all the studies we evaluated, there is no evidence that HH was the primary factor controlling stream invertebrate diversity, particularly in a restoration context. The findings indicate that physical heterogeneity should not be the driving force in selecting restoration approaches for most degraded waterways. Evidence suggests that much more must be done to restore streams impacted by multiple stressors than simply re-configuring channels and enhancing structural complexity with meanders, boulders, wood, or other structures.
6. Thematic implications: as integrators of all activities on the land, streams are sensitive to a host of stressors including impacts from urbanisation, agriculture, deforestation, invasive species, flow regulation, water extractions and mining. The impacts of these individually or in combination typically lead to a decrease in biodiversity because of reduced water quality, biologically unsuitable flow regimes, dispersal barriers, altered inputs of organic matter or sunlight, degraded habitat, etc. Despite the complexity of these stressors, a large number of stream restoration projects focus primarily on physical channel characteristics. We show that this is not a wise investment if ecological recovery is the goal. Managers should critically diagnose the stressors impacting an impaired stream and invest resources first in repairing those problems most likely to limit restoration.read more
Citations
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Emerging threats and persistent conservation challenges for freshwater biodiversity
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TL;DR: Efforts to reverse global trends in freshwater degradation now depend on bridging an immense gap between the aspirations of conservation biologists and the accelerating rate of species endangerment.
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Toward an Era of Restoration in Ecology: Successes, Failures, and Opportunities Ahead
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The European Water Framework Directive at the age of 10: A critical review of the achievements with recommendations for the future
Daniel Hering,Ángel Borja,Jacob Carstensen,Laurence Carvalho,Michael Elliott,Christian K. Feld,Anna-Stiina Heiskanen,Richard K. Johnson,Jannicke Moe,Didier Pont,Anne Lyche Solheim,Wouter van de Bund +11 more
TL;DR: The successes and problems encountered with implementation of the WFD over the past 10 years are reviewed and recommendations to further improve the implementation process are provided.
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Multiple stressors in freshwater ecosystems
TL;DR: A collection of case studies from the first Freshwater Biological Association Conference on Multiple Stressors in Freshwater Ecosystems (FBA-2007) as mentioned in this paper, which was devoted to the management of multiple stressors in freshwater ecosystems, is a good starting point for this work.
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Habitat complexity: approaches and future directions
TL;DR: It is argued that complexity may be implicated in community persistence and ecosystem stability by acting as a decoupling mechanism in predator–prey interactions and potential application of these approaches for cross-ecosystem comparisons is discussed.
References
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TL;DR: It is shown that ecological effects of habitat heterogeneity may vary considerably between species groups depending on whether structural attributes are perceived as heterogeneity or fragmentation, and possible effects may also vary relative to the structural variable measured.
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