From Purpose to Persistence: How Intentional Introductions by Humans Influence Ecological Niches of Non-Native Plants

Aim: The intentional or unintentional transport of non-native plants is key to overcoming geographic barriers. However, it remains unclear whether such introduction pathways associate with overcoming environmental barriers, which is key for successful invasion. Here, we test how intentionality of introduction associates with niche breadth and niche harshness. Location: Europe. Time Period: 1914–2020. Major Taxa Studied: 220 plant species. Methods: Across > 60,000 invaded vegetation plots, we tested whether intentionality of introduction (intentional, unintentional, or both) and characteristics of non-native plants (native climatic niche breadth, growth form, dispersal syndrome, height, residence time) were associated with their niche breadth, quantified through habitats, climate, and co-occurring flora. We tested how the intentionality of introduction was associated with environmental harshness (drought, salinity, oligotrophy, and elevation), while accounting for land-cover and habitat types. Results: Non-native plants introduced both intentionally and unintentionally had a broader habitat range, compared to non-native plants introduced only unintentionally. A broad climatic niche in the native range was associated with a broader invaded climatic niche, while a long residence time was associated with broader habitat and biotic niches. Intentional introduction was associated with the invasion of dry habitats and forests, whereas unintentional introduction was linked to the invasion of saline, high-elevation, and disturbed environments. Main Conclusions: In addition to triggering invasions, the type of process responsible for introduction can partly explain how non-native plants overcome environmental barriers in the invaded range. The intentionality of introduction was associated with niche breadth only in terms of habitat range, while the association with niche harshness depended on the type of stress, which highlights the importance of integrative niche assessments. The relationship between intentionality of introduction and the invaded niche could relate to intentionality-specific differences in biological attributes (environmental tolerance, dispersal capacity, and preference for disturbance) and the introduction process (propagule pressure and residence time).