Dispersal of lotic organisms is a fundamental control of riverine biodiversity, yet the abiotic and biotic drivers of dispersal remain poorly understood, which renders predictions of invertebrate dispersal highly uncertain. This uncertainty is supposed to contribute to the asymmetry of biological pathways during degradation and recovery. In Phase I, A16 focussed on the measurement of migration distances and proportions of dispersive macroinvertebrate specimens in the urban Boye/Emscher and the adjacent rural/urban Rotbach catchment using stable isotope labelling, drift samples and traps for flying adult insects. Altogether 12 hololimnic and merolimnic species were investigated at seven sites. The results suggest that larval densities drive the emigration rates of dispersive population members (including those of adult stages of merolimnic species), while their individual and species-specific traits drive migration distances. In Phase II field measurements of both emigration and migration will be extended to the rural Biber/Kinzig catchment and compared to the findings from Phase I. Both dispersal components will be contrasted against abiotic and biotic drives (e.g., population density, habitat availability, dispersal barriers) and compared between catchments. In Phase II, the site-scale measurements of dispersal will be complemented by estimates of gene flow between populations at the catchment scale. Haplotype alpha and beta diversity at 12 sites each per catchment will allow to estimate the gene flow between sites and within catchments. The results will be contrasted against abiotic and biotic drivers (e.g., larval densities, species dispersal capabilities and migration barriers) to investigate if and how the drivers link to long-term dispersal at broader spatial scales. In concert, the results obtained from the field studies in Phase I and II will inform the development of dispersal kernels, i.e. density-distance functions that describe the proportion of dispersive population encountered as a function of the distance to the source population. The kernels will be used to develop species dispersal models (SDispMs), which will allow to analyse and disentangle the role of abiotic and biotic drivers of emigration and migration at the scale of entire catchments. The investigation of abiotic and biotic drivers will build upon the findings of related modelling projects in Phase I. In Phase III, the outcome of related model projects will be linked dynamically to SDispMs of A16, which will allow to model the dispersal of macroinvertebrates conditional on dynamic changes of descriptor variables, i.e. “on the fly”.

Julian Enß (University of Duisburg-Essen)

Biotic and abiotic drivers of macroinvertebrate dispersal

Dispersal of lotic organisms is a fundamental control of riverine biodiversity, yet the drivers and dynamics of dispersal processes still remain poorly understood, which renders predictions of invertebrate dispersal highly uncertain. Knowledge gaps exist as to the population’s proportion of members that actually disperse, the members’ individual dispersal capabilities and the role of spatial habitat heterogeneity (niche availability). Accordingly, dispersal predictions for lotic macroinvertebrates continue to rely on indirect proxies (i.e. dispersal traits), rather than on direct measurements of realised dispersal distances. Further, biotic interactions at the population (e.g. density) and community levels (competition, predation, parasitism) are likely to initiate (or inhibit) dispersal, but are rarely considered in dispersal studies.

A comprehensive field dispersal experiment will be performed for selected hololimnic and merolimnic as well as dispersive and philopatric species in both model catchments (Emscher/Boye, Kinzig). The population sizes will be estimated by benthic samples (hand nets) of all relevant microhabitats and emergence traps at eight sampling sites each per catchment. The distances crossed by individual members of a population will be quantified by an array of instream (drift nets) and terrestrial dispersal traps (Malaise and sticky traps) at four sites each per catchment. Isotope enrichment (¹⁵NH₄Cl) upstream of dispersal sites will be used to label large quantities of the model species’ populations. Dispersal traps will be located at predefined distances up- and downstream (dispersal along the stream corridor) as well as perpendicular to the stream course (overland dispersal).

Environmental and spatial variables at various spatial scales (e.g. land cover, riparian conditions, hydrology, physical habitat, physico-chemistry) will complement the biological data, to determine niche availability and ultimately, to test spatial versus environmental and biological versus abiotic descriptors of realised dispersal. Comparing the urban Emscher/Boye catchment with the rural Kinzig catchment will allow me to address catchment-related characteristics and stressor history, i.e. legacy effects on dispersal patterns.

Contact: julian.enss@uni-due.de

First Supervisor: PD Dr. Christian Feld (University of Duisburg-Essen, Aquatic Ecology)
Second Supervisor: Prof. Dr. Bernd Sures (University of Duisburg-Essen, Aquatic Ecology)
Mentor: Dr. Caroline Winking (Emschergenossenschaft und Lippeverband)