| dc.description.abstract | 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
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.
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. | en_US |
