Quantifying Colonisation Pressure as a Driver of Secondary Spread During the <scp> <i>Dreissena polymorpha</i> </scp> Invasion of German River Networks

ABSTRACT Biological invasions have become a major component of global change, yet the processes governing secondary spread after initial introductions remain insufficiently resolved. A specific and central gap is the lack of a spatially explicit and temporally dynamic quantification of colonisation pressure, despite its presumed importance for establishment and spread in connected river systems. Identifying whether and where previously established non‐native populations generate the momentum to further facilitate dispersal is essential for separating spread processes from local environmental suitability Using long‐term freshwater macroinvertebrate monitoring data from Germany, we describe colonisation pressure for the zebra mussel Dreissena polymorpha as a distance‐weighted function of previously colonised sites across a river‐network framework and subsequently link it to first detections, establishment probability and several site‐level predictors. We found that colonisation pressure increased through time as the invaded range expanded and that it was consistently highest along the Rhine River, with newly colonised sites generally experiencing higher colonisation pressure in their year of first detection than uninvaded sites. At the same time, colonisation pressure did not explain observed abundances at first detection and was negatively associated with establishment probability, while variation in colonisation pressure itself was better explained by anthropogenic and invasion‐related factors than by local climatic, hydrological or topographic conditions. While several questions remain, our results position colonisation pressure as a useful mechanistic descriptor of invasion momentum in riverine systems, with direct relevance for surveillance and risk prioritisation and with potential applicability to climate‐driven range shifts, species reintroductions and the expansion of commercially or conservation‐relevant taxa in spatially structured landscapes.