Humans have transported thousands of species across the globe, often with long-lasting consequences for ecosystems. While the number of alien species in regional species pools such as countries or islands is steadily increasing, their actual presence in local communities remains surprisingly low. My PhD project, ASAAS (Alien Species Accumulation Across Scales), explores this mismatch between regional accumulation and local representation.
The central question is whether this discrepancy reflects time lags in the spread of alien species or whether certain ecological and environmental constraints limit their ability to establish in local habitats. I study this process across multiple taxonomic groups, including plants, vertebrates, and selected invertebrates, and use large-scale datasets such as GloNAF, Alien Species First Record Database, sPlot, EVA, PREDICTS, BioTime, and AgriWeedClim.
To study alien species accumulation across scales, I combine large global databases with statistical modelling. Datasets such as GloNAF, Alien Species First Record Database, sPlot, EVA, PREDICTS, BioTime, and AgriWeedClim provide complementary perspectives, from regional species pools to local community composition. Together they allow me to trace how alien species enter new regions, how long they take to appear in local plots, and whether their spread differs across taxonomic groups.
I analyze these patterns by comparing regional and local trends over time, modelling the influence of residence time, habitat characteristics, and human disturbance on the likelihood of establishment. Particular attention is given to whether alien species saturate in local communities or whether continued propagule pressure, land-use change, and climate change open new opportunities for spread. Computer models are used to project future dynamics under different scenarios, linking historical accumulation with potential trajectories in the coming decades.
This combined approach lets me move beyond single case studies toward a broader understanding of how alien species integrate into ecosystems. By treating regional and local scales together, I can test whether the apparent mismatch between them is a temporary lag or a more fundamental feature of invasion dynamics.
The results of ASAAS will contribute to a more nuanced understanding of how alien species spread and establish across scales. By clarifying whether the low local representation of alien species is simply a matter of time or the result of ecological constraints, the project aims to inform both theory and applied biodiversity research. For example, if time lags dominate, we may expect a future surge in alien species at the community level as regional pools continue to grow. If local constraints prove stronger, then regional accumulation might overestimate the impact on ecosystems.
Another perspective opened by this project is how different drivers interact. Propagule pressure, land-use change, and climate change may accelerate local establishment and overcome barriers that have so far kept alien species rare in communities. Linking these drivers to species traits and residence time could reveal why some groups expand rapidly while others remain confined to the regional pool.
Beyond academic contributions, ASAAS has practical implications for how we monitor and anticipate biological invasions. Improved projections of spread can guide early detection, help prioritize management efforts, and refine expectations about which species and habitats are most vulnerable. The project also emphasizes the importance of integrating global datasets, showing how collaboration and data sharing can uncover large-scale patterns that individual studies cannot resolve.
In a broader sense, ASAAS is an opportunity to bridge scales in invasion ecology. It connects the big picture of global alien species accumulation with the fine-grained processes shaping local communities. Although rooted in pressing ecological questions, this project also reflects my own path as a physicist moving into quantitative ecology. It is both a scientific investigation of alien species dynamics and a chance to develop modelling approaches that help clarify one of the most important drivers of biodiversity change.