Thompson, M; (2014) The Effect of Large Woody Debris Restoration on Stream Ecosystems. Doctoral thesis , UCL (University College London).

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Abstract

The natural physical and biological states of rivers have been altered increasingly by habitat modification and pollution. Successful river restoration is therefore critical for mitigating impacts on river ecosystems. However, restorations are typically constrained to small patches within individual rivers and often lack standardised or adequate monitoring and assessment. This has led to poor diagnosis of both the “problem” and the effectiveness of the “solution” as comparable data are rare. In a literature review and meta-analysis I identified three areas limiting the development of river restoration as a science: 1) prior monitoring to determine the chief ecological constraint was not common practice; 2) the primary driver for restoration was ecological (e.g. an increase in biodiversity) but monitoring was often inadequate to detect ecological response to restoration or not undertaken; 3) target species (e.g. brown trout) or assemblages (e.g. invertebrates) are often the focus of monitoring, but it is not yet clear how habitat restoration affects more complex organisational levels (e.g. food webs, community mass-abundance scaling, ecosystems) and how this regulates populations of target taxa. These three areas were the focus of the subsequent data chapters. Conserving naturally fallen large woody debris (LWD) in rivers and felling trees to mimic LWD is one current vogue in river restoration-management thought to increase habitat diversity and positively correlate with increasing invertebrate biodiversity. Invertebrates were sampled between reaches with and without LWD and across a nutrient gradient, which included 19 base-rich streams, to test whether the chief determinant of invertebrate density and composition was either habitat quality or water quality. Five chalk streams were sampled before and after at control, reference (i.e. natural LWD) and treatment (i.e. felled) sites to test for a general restoration effect. A combination of spatially nested scales (i.e. stream-reach-habitat), invertebrate structural measures (e.g. α-diversity, abundance and biomass) and food web metrics (e.g. trivariate analysis) have been tested so that the ecological impact of restoration could be quantified. Large-scale pressures, namely fine sediment and orthophosphate (and their potential covariates) were impacting invertebrate community structure. Quantitative estimates of abundance and biomass increased within LWD habitat and reference reaches respectively. Evenness, α- and β-diversity were not significantly higher in restored LWD. Community mass-abundance scaling was not affected by either experimental or natural LWD. This implies that LWD is having an effect, but this is restricted to the invertebrate community. Many of the patterns decribed here would not have been detected using standard protocol methods (e.g. invertebrate kick net sampling). This highlights the need for the development of standardised biomonitoring practices that are able to diagnose root causes of degradation and demonstrate ecological recovery following restoration. If the aim of restoration is to predictably increase fishes biomass or biodiversity, for example, I anticipate larger-scale restorations, which address multiple stressors, and longer-term monitoring will be needed.

Type:	Thesis (Doctoral)
Title:	The Effect of Large Woody Debris Restoration on Stream Ecosystems
Language:	English
URI:	https://discovery.ucl.ac.uk/id/eprint/1455018

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