Modelling the spatial dispersal of aquatic invertebrates to predict (re-)colonisation processes within river catchments
Worldwide, many lotic ecosystems are heavily impacted by anthropogenic disturbance, leading to a significant decline in freshwater biodiversity. In recent years, increasing efforts have been directed towards the restoration and revitalisation of disturbed streams and rivers to reverse this trend. Although it is widely acknowledged that species dispersal is the key to the recolonisation of restored streams and rivers and ultimately to their ecological recovery, dispersal often remains unaddressed in restoration ecology. For this reason, the present thesis had two main objectives: 1) The development and application of a dispersal modelling approach that considers taxon-specific dispersal distances and dispersal barriers. 2) The validation and comparison of this dispersal modelling approach, based on taxon-specific dispersal distances and barriers, against a purely distance-based approach. Therefore, this thesis is divided into two main chapters dealing with these objectives. Following this structure, background information and main results are summarised in the next paragraphs for each chapter. In chapter 2, we present an approach to predict larval (aquatic) and adult (terrestrial) dispersal ranges of three lotic insect species (Hydropsyche dinarica [Trichoptera], Calopteryx virgo [Odonata] and Dinocras cephalotes [Plecoptera]) within one life cycle. The actual species’ distributions (presence / absence) were obtained from a total of 1,198 sites evenly distributed within the Ruhr catchment, North Rhine-Westphalia, Germany. The predictions for aquatic and terrestrial dispersal were made for two scenarios: with and without dispersal barriers included in the predictive modelling. In-stream dispersal barriers included weirs, dams, culverts and impounded water bodies, whereas terrestrial barriers were related to the stream corridor (degraded riparian vegetation) and different forms of land use (urban land use, coniferous and deciduous or mixed forest, open land, road infrastructure). We applied a “least-cost” modelling approach and combined each species’ life-cycle-specific dispersal capabilities and the corresponding dispersal barrier’s “friction” costs in a grid-based GIS model.Among the three model species, H. dinarica was the best disperser and was predicted to be able to reach between 81% (without barriers) and 67% (with barriers) of all river sections in the model catchment within one life cycle. Aerial (terrestrial) dispersal was by far the most important dispersal mechanism. For validation purposes, we conducted a logistic regression analysis to identify sample sites with environmentally suitable habitats. Within these sites that are not considered constrained by habitat limitations, the comparison of actual and predicted absences revealed a better match, if barriers were included in the dispersal models. At the same time the mismatch of actual absences and predicted presences decreased. Our results suggest that dispersal models can contribute to a better assessment of the potential recolonisation of rivers. Yet, the dispersal of lotic insects may be considerably overestimated if dispersal barriers remain unaddressed. Chapter 3 deals with a study area within a heavily modified catchment, where formerly polluted streams are now free of untreated wastewater. Additionally, the morphology of streams has been improved by physical habitat restoration. Both water quality and structural improvements offered a unique opportunity to investigate the recolonisation of restored sections by benthic invertebrates. As dispersal is a key mechanism for recolonisation, we developed a method to predict the dispersal of 18 aquatic insect taxa to 35,338 river sections (section length: 2 m) within the catchment. Source populations of insect taxa were sampled at 33 sites. In addition, 14 morphologically restored sites were sampled and constituted the validation dataset. As in chapter 2, we applied a “least-cost” modelling approach within a raster-based GIS model, combining taxon-specific aquatic and terrestrial dispersal capabilities with the “friction” that physical migration barriers impose on dispersal of aquatic and terrestrial stages. This taxon-specific modelling approach was compared to a conservative modelling approach, assuming a Euclidean distance of 5 km, based on a former study, as the maximum dispersal distance for any source population regardless of dispersal barriers.