Strong human association with plant invasion success for Trifolium introductions to New Zealand

It has proven difficult, when focused only on biological determinants, to explain why some plant species become naturalized in or invade new locations, whereas others fail. We analyzed the invasion of Trifolium (true clover) species into New Zealand, assessing a range of human, biogeographic, and biological influences at three key invasion stages: introduction, naturalization, and spread. We used sparse principal component analysis (SPCA) to define suites of related attributes and aggregated boosted trees to model relationships with invasion outcomes. Human and biogeographic attributes were strongly associated with success at all stages. Whereas biogeographic attributes, notably large native range, were consistently associated with success, different human factors appeared to favor success at different stages, such as presence in early trade/immigration hotspots (introduction), intentional large-scale planting (naturalization), and frequent presence as a seed contaminant (relative spread rate). Biological traits were less strongly associated with success for introduction and spread and little if at all for naturalization; we found that tall perennials with long flowering periods were more frequently selected for introduction, whereas species with extended flowering in New Zealand spread more rapidly. In addition to causal relationships, the importance of human factors may reflect indirect associations, including ecological traits associated with both human use and invasion. Nevertheless, our results highlight key roles that humans can play in facilitating plant invasion via two pathways: (i) commercial introduction leading to widespread planting and concomitant naturalization and spread and (ii) unintentional introduction and spread of species associated with human activities, such as seed contaminants.