Resource-driven release from lag phase triggers the world's fastest and largest coastal plant invasion

What determines the duration of invasion lag phases remains one of the major unresolved questions in invasion ecology. Lag phases are often attributed to genetic adaptation, propagule limitation, or demographic stochasticity, yet the mechanisms by which environmental conditions regulate establishment success remain poorly understood. Here, we investigate the explosive expansion of Spartina alterniflora along the coast of mainland China, the most rapid plant invasion recorded globally. Combining field experiments, demographic theory, and historical invasion records, we identify resource-regulated Allee effects as a mechanism governing the invasion lag phase. Experimentally, sediment accretion substantially increases clonal propagation and sexual reproduction, weakening demographic constraints associated with establishment and accelerating the transition from sparse founder patches to highly reproductive meadows. A theoretical model reveals that sediment-driven positive feedbacks weaken demographic constraints during establishment and shorten the duration of Allee effects. Historical invasion records spanning both sides of the Pacific Ocean support the predicted negative relationship between sedimentation rate and lag-phase duration. Invasion lag phases can arise solely from interactions between species traits and environmental resource availability. Contrary to the prevailing view that Allee effects are fixed properties of species, our results show that their duration is regulated by environmental resource availability. More broadly, environmental resources can govern ecological transitions by controlling transient dynamics during establishment, providing an explanation for why some introduced populations remain dormant for decades, whereas others rapidly become dominant ecosystem engineers.