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

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

All change at the water's edge: invasion by non-native riparian plants negatively impacts terrestrial invertebrates

Riparian zones are complex, dynamic habitats that play a critical role in river ecosystem functioning. Terrestrial invertebrates comprise much of the diversity found in riparian habitats and facilitate the transfer of energy between aquatic and terrestrial systems. However, the consequences for terrestrial invertebrates of invasion of riparian zones by invasive non-native plants (INNP) remain poorly understood. Responses of terrestrial macroinvertebrate morphospecies to invasion by two common INNP, Fallopia japonica (Japanese knotweed) and Impatiens glandulifera (Himalayan balsam) were assessed, relative to local environmental factors. Terrestrial invertebrates were collected from 20 sites on low order streams in June and August alongside data on physical attributes and land use. Greater cover of F. japonica and I. glandulifera cover reduced total invertebrate abundance and morphospecies diversity at the individual sample scale, whilst increasing spatial heterogeneity of invertebrates at the site scale. Impatiens glandulifera reduced morphospecies diversity at the site scale with increasing cover, but this was not observed for F. japonica. INNP affected terrestrial invertebrate morphospecies abundance and diversity, to a greater extent than prevailing environmental conditions. Our findings therefore offer support for managing riparian plant invasions to improve habitat heterogeneity, restore terrestrial invertebrate diversity and repair aquatic-terrestrial linkages

Stream invertebrate diversity reduces with invasion of river banks by non-native plants

1. Invasion of riparian zones by non-native plants is a global issue and commonly perceived as a challenge for river and fishery managers, but the type and extent of ecological changes induced by such invasions remain poorly understood. Established effects on sediment delivery, allochthonous inputs and channel shading could potentially alter aquatic macroinvertebrate assemblages, with implications for in-stream ecological quality. 2. We assessed responses in the diversity, quality and heterogeneity of stream macroinvertebrate communities to riparian invasion by non-native plants. Macroinvertebrates were collected from 24 sites on low order streams in central and southern Scotland during spring and autumn. The effect of invasive non-native plants (INNP) on macroinvertebrates was assessed relative to that of local physical and chemical factors. 3. INNP cover was associated with stronger effects than other factors on local diversity of macroinvertebrates (33% reduction at the highest INNP cover) but also increased macroinvertebrate abundance across sites. Invaded sites were also associated with lower macroinvertebrate biomonitoring scores. Community composition differed between invaded and uninvaded sites in autumn, but not in spring. However, INNP influence on macroinvertebrate composition was generally secondary to that of physicochemical variables (e.g. channel shade, substrate diversity). 4. We demonstrate that the influence of INNP extends beyond well-known impacts on plant communities to reductions mainly in stream macroinvertebrate diversity. Combined with the negative impact on pollution-sensitive macroinvertebrate taxa this raises concerns over the ecological health of streams with heavily invaded riparian zones. Our findings suggest that efforts to improve low order streams by actively managing severe riparian invasions are merited, but the size and uncertainty of the likely ecological gains must also be evaluated against the effort involved

Unexpected changes in community size structure in a natural warming experiment

Natural ecosystems typically consist of many small and few large organisms. The scaling of this negative relationship between body mass and abundance has important implications for resource partitioning and energy usage. Global warming over the next century is predicted to favour smaller organisms, producing steeper mass-abundance scaling and a less efficient transfer of biomass through the food web. Here, we show that the opposite effect occurs in a natural warming experiment involving 13 whole-stream ecosystems within the same catchment, which span a temperature gradient of 5-25 °C. We introduce a mechanistic model that shows how the temperature dependence of basal resource carrying capacity can account for these previously unexpected results. If nutrient supply increases with temperature to offset the rising metabolic demand of primary producers, there will be sufficient resources to sustain larger consumers at higher trophic levels. These new data and the model that explains them highlight important exceptions to some commonly assumed 'rules' about responses to warming in natural ecosystems