This thesis investigates the impact of two invasive ecosystem engineers on the river banks. Invasive species generate significant global environmental and economic costs and represent a particularly potent threat to freshwater ecosystems. Ecosystem engineers are organisms that modify their physical habitat. Therefore this thesis will explore the interaction of these two types of species and their impacts on the example of the impact of signal crayfish and Himalayan balsam The obtained results indicate that there are few avenues through which invasive ecosystem engineers can influence river bank processes. While many uncertainties remain, due to the intrinsic complexity of river ecosystems, a multitude of anthropogenic stressors that they are increasingly subjected to and a wide array of ecosystem services that rivers provide to people, it is important to consider the role of invasive ecosystem engineers in river management practices. on river banks. The work included analyses and development of conceptual models for the understanding of invasive ecosystem engineers, followed by four research chapters aimed at answering specific questions. A study of signal crayfish impact is primarily focused on the impact of burrows that crayfish dig as shelter and their influence on riverbank erosion. The interaction between habitat characteristics, the occurrence of burrows and erosion is analysed on three different levels of spatial scale: bank section in reach, reach in the catchment and bank section in the catchment. Bank section in reach survey (Chapter 4) focused on a reach heavily impacted by crayfish burrowing on the River Windrush, UK, in order to study the maximum effect of burrowing. Also, smaller spatial extent enabled detailed study of three sets of variables as well as an assessment of the impact that signal crayfish population density has on burrowing. Reach in catchment spatial scale expanded the survey to cover 103 river reaches in the Thames catchment and was based on a combination of habitat information from publicly available online data sets, primarily the River Habitat Survey database and rapid field surveys that recorded burrows and erosion. Bank section in catchment-scale was based on the same 103 sites, but the main focus of field observations were ten metres long bank sections for which habitat, burrows and erosion information were collected. Overall, burrowed banks were more likely to be characterised by cohesive bank material, steeper bank profiles with large areas of bare bank face, often on outer bend locations and were associated with bank profiles with signs of erosion. There were indications that signal crayfish burrowing is contributing to the river bank erosion, but no conclusive results have been made. Study of the impact of the Himalayan balsam was undertaken on eight sites at the River Brenta in Italy and it was focused on three main aspects. Firstly it was established that extent of Himalayan balsam domination over native vegetation varies widely depending on the habitat conditions and native plants encountered. Secondly, it was established that there are no conclusive differences in the extent of erosion and deposition on transects covered by native vegetation and Himalayan balsam. Thirdly, measurement of traits of individual plants showed significant differences in traits of individual plants that are known to have consequences for river bank erosion and deposition.

Investigations were carried out into the allelopathic potential of the invasive alien annual Himalayan Balsam (Impatiens glandulifera Royle) using a series of bioassays, including ones developed or adapted for this study. They were evaluated for their suitability to detect three of the four main modes of allelochemical release, namely leaching, exudation and decomposition. Assays which involved the measurement of lettuce radicles and hypocotyls gave reliable results and allowed a range of different Impatiens material, both living and dead, to be assessed and ranked according to the allelopathic effects demonstrated. Attempts were made to isolate resource competition from allelopathy using separately grown but connected donor and receiver plants and a density dependent design where single I. glandulifera plants were grown in pots with variable numbers of receiver plants. Results proved inconclusive. Initial experiments showed that the allelopathic potential of I. glandulifera varied according to the organ from which the material was derived. Pods, leaves and stems produced the greatest inhibition of lettuce seedlings. Effects on germination were not significant at most of the concentrations tested. Live roots of I. glandulifera plants produced pronounced orange staining of the agar into which they were placed and showed clear evidence of distance dependent inhibition of lettuce radicles. Effects were limited to growth rather than germination of the test plants. Germinating I. glandulifera seeds caused a significant inhibition of lettuce radicle elongation when the two species were grown together in an agar medium. The inhibitory effects increased significantly with increasing exposure time. Increasing I. glandulifera seedling number also produced significant reductions in lettuce radicle length. Dormant seeds, by contrast, stimulated growth. Dead seeds did not produce significant changes to the growth of the test plants. When rhizosphere soil was gathered from pot grown I. glandulifera plants, the results were mixed. Initial samples inhibited growth, whereas those collected from dying plants over a period of weeks stimulated growth. Further experimentation is required before the indications of allelopathic interactions demonstrated here can be applied to the behaviour of wild populations of I. glandulifera.

Impatiens glandulifera is an invasive species throughout Europe. It was introduced to the wild in 1918 by escape from private gardens as well as the transport of garden soil and waste. Impatiens glandulifera prefers moist soils with moderate shade and commonly grows in riparian environments. In the Umeå municipality in northern Sweden, where this study is focused, the number of Himalayan balsam observations has dramatically increased over the last few years. This study aimed to determine habitat and anthropogenic land-use factors that may have contributed to the spread of Himalayan balsam in the Umeå municipality. This was done by using data on recorded observations of the species in a database called Artportalen. In order to determine how abundant and in which habitats Himalayan balsam occurs in the Umeå municipality, data on recorded observations in Artportalen from the Umeå municipality region for the period 01-01-2021 to 31-12-2021 was used along with maps from Google Earth Pro and Google maps. It was found that Himalayan balsam is now common in the region and does not show a definitive preference for riparian habitats or for forest or open areas in the Umeå municipality. Human activities seemed to be the main contributor to the spread of Himalayan balsam in the Umeå municipality.

Invasive alien species are recognized as a major problem worldwide. They affect native flora and fauna negatively, resulting in loss of biodiversity and increased economic costs of the species’ themselves and management efforts. One of the most widely distributed invasive plant species – the Himalayan balsam (Impatiens glandulifera) – is inflicting damage to native plants in Sweden by overshadowing due to its tall stature and capacity to increase population numbers quickly over long distances. The purpose of this study was to examine the distribution of Himalayan balsam in urban versus rural areas in south, central, and northern Sweden. Five land cover classes were also compared to investigate potential differences in where Himalayan balsam is frequently observed. Species observations from Species Observation System (Artportalen by SLU Swedish Species Information Centre) were combined with land cover data from the Swedish Environmental Protection Agency to assess the distribution of Himalayan balsam over different cover classes. The results showed no significant differences for urban versus rural areas nor latitude, but some of the land cover classes were significantly different from one another. Himalayan balsam observations in forests on wetlands are significantly more reported compared to all other classes, and exploited lands and open lands had more reported observations than forests outside wetlands. Observations by the public are often characterised by opportunistic reporting behaviour, ultimately leading to bias in which species are reported. In conclusion, this study tested means to assess the distribution of Himalayan balsam among land cover classes and its results are consistent with other studies showing Himalayan balsam favours in moist habitats over other land cover classes. / Invasiva främmande arter ses som ett stort hot runt om i världen. Inhemsk flora och fauna blir kontinuerligt negativt påverkad av invasiva arter vilket resulterar i förlust av biodiversitet och ökade ekonomiska kostnader. En av de mest utspridda invasiva växtarterna – jättebalsamin (Impatiens glandulifera) – skadar inhemska växtarter i Sverige genom att överskugga dem samt skapa stora bestånd genom sin kapacitet att spridas långa distanser. Denna studies syfte var att undersöka jättebalsaminens utbredning i stads- och lantliga miljöer, samt geografiska områden i södra, centrala och nordliga delar av Sverige. Fem marktäckeskategorier undersöktes också för att se potentiella skillnader på habitat där jättebalsamin har rapporterats befinnas sig. Data från Artportalen (SLU Artdatabanken) användes i kombination med marktäckesdata från Naturvårdsverket. Studiens resultat fann inga signifikanta skillnader mellan stadsmiljöer och lantliga miljöer. Inga signifikanta skillnader kunde hittas för geografiska områden i södra, centrala och nordliga Sverige. Studien fann däremot signifikanta skillnader i rapporterade observationer bland marktäckesklasserna där skogar på våtmarker hade fler observationer per km2 jämfört med skogar utanför våtmarker, exploaterade marker och öppna marker. Observationer från allmänheten är ofta präglad av opportunistiska beteenden gällande rapporteringar, vilket kan leda till partiska data där vissa arter rapporteras medan andra ignoreras. Sammanfattningsvis testade denna studie utbredningen av jättebalsamin på marktäckesklasser och var förenlig med tidigare studier som visar att jättebalsamin är vanligare förekommande i fuktiga habitat jämfört med andra marktäckesklasser.

The main aim of this dissertation is to determine the global occurence of geografically non-indigenous plant species - Giant Hogweed (Heracleum mantegazzianum), Himalayan balsam (Impatiens glandulifera), Japanese knotweed (Reynoutria japonika), Sakhalin knotweed (Reynoutria sachalinensis) and Reynoutria x bohemika in selected locations of the South Bohemian region, for example in district of Český Krumlov and České Budějovice. The theoretical part is focused on the characteristics of selected localities where there is an occurrence of these plants and description of the above-mentioned invasive plant species and thier subsequent liquidation. The result of the dissertation (of the work) are the map´s outputs processed in the Geographic Information System, which record the global occurance of geografically non-indigenous plant species.