Linkages between riparian invasive plants, hydromorphology and salmonid fish in Scottish rivers

Abstract

Invasions by non-native species are reported as one of the greatest threats to global biodiversity, and the invasion of riparian ecosystems by invasive non-native plants (INNP) presents a common and difficult challenge for river and fishery managers. Whilst the various impacts of INNP are well-documented in a range of global studies, the type and extent of ecological changes that riparian INNP invasions induce in invertebrate and salmonid fish communities remains poorly understood. To address these gaps in the literature, this thesis assesses: (1) how riparian INNP alter the abundance, diversity and composition of freshwater macroinvertebrate communities, in relation to environmental variables; (2) how the structure of riparian terrestrial invertebrate communities differs at heavily invaded sites, and whether there is evidence of a difference in INNP species effect and (3) how juvenile salmonids utilise the altered aquatic and terrestrial prey resources at sites with greater INNP cover, and the relative importance of INNP to prey selection in relation to population dynamics and environmental stressors. Recent field survey data was used to quantify changes in the freshwater and terrestrial invertebrate communities of 24 low order streams in central Scotland. Analyses indicated that whilst greater INNP cover reduced local freshwater macroinvertebrate diversity, their effects were generally subordinate to that of physicochemical variables, though there was evidence of a legacy effect of invasion that presents a constant pressure on freshwater macroinvertebrate communities. Similarly, greater INNP cover reduced terrestrial morphospecies diversity, but also reduced abundance and increased spatial heterogeneity through loss of species at the site scale. INNP cover was found to be the strongest predictor across all assessments of terrestrial invertebrate communities. Juvenile salmonids were observed to change their predatory selection of two taxonomic orders at more heavily invaded sites, but broadly changed their feeding patterns in response to community and environmental stressors, indicating a lesser effect of riparian INNP invasions on salmonid communities. The findings presented in this thesis suggest that riparian INNP are important and significant contributors to reductions in the diversity and overall quality of both freshwater and terrestrial invertebrate communities. However, it appears that the impacts of riparian INNP are less severe for salmonid fish compared to invertebrate communities, perhaps due to their resilience and adaptability in a highly stressful environment. This thesis suggests that efforts to improve the quality of low order streams by actively managing severe riparian INNP invasions are merited, and suggests that there is a scale of community responses which may provide guidance when planning INNP management strategies. However, there is clearly a trade-off between the often significant economical investment required to treat INNP invasions and the relative uncertainty concerning any recovery that may be achieved post-treatment.