Multiple mechanisms underlie rapid expansion of an invasive alien plant.

• With growing concerns over serious ecological problems, a particular challenge is to reveal the complex mechanisms underlying rapid expansion of invasive species. Ageratina adenophora is of particular interest in addressing this question. • We used geographic information systems and logistic regression to identify the geographic and environmental factors contributing to the presence of A. adenophora. Join-count spatial statistics with reproduction mode examination were employed to elucidate the spatiotemporal dispersal mechanisms. • Multiple factors have significantly contributed to the rapid expansion of A. adenophora. Its biological traits, favoring dispersal by water and wind coupled with local spatiotemporally heterogeneous geography and ecology, promote invasion downstream and upstream along river valleys, while other factors associated with human activities facilitate its invasion over high mountains and across river valleys, providing new scope for progressive invasions. We further identified an unusual invasion event of A. adenophora subsequent to a great flood that amplified its dispersal ability from vegetative propagules and seeds. • These findings suggest that dynamic interactions of multiple factors in heterogeneous ecogeographical environments - a 'combinatorial' invasion mechanism - would generate an unexpected invasion rate of an alien species or a seemingly stochastic invasion event.