Current theory (e.g., consumer‐controlled theory) predicts that nutrient enrichment typically amplifies herbivory and thereby suppresses the growth and expansion of invasive plants. Herbivores can facilitate plant regrowth in the native community by stimulating complementary growth or ameliorating habitat conditions (e.g., by increasing soil oxygen and nutrient availability), but whether they have similar positive effects on invasive plants, especially under nutrient enrichment, remains unknown. Using a field nitrogen (N)‐enrichment × crab exclusion experiment, we evaluated and compared the effects of both N enrichment and crab herbivory on the growth performance of a global invasive cordgrass, Spartina alterniflora, and a co‐occurring native plant, Phragmites australis. We found that crabs consistently suppressed P. australis by density and aboveground biomass regardless of N enrichment. In contrast, for S. alterniflora, the negative effects of crabs under ambient N were replaced by positive effects under N enrichment, with crabs stimulating complementary increases in density and aboveground biomass. The differing effects between the N treatments were driven by crab burrowing activity, which increased soil N availability, and the nutrient‐use efficiency of S. alterniflora. Our findings revealed that native herbivores can have opposing effects on native and invasive plants, which broadens our understanding of how exotic plants can achieve dominance in a changing world.

Plant invasions driven by global environmental change increasingly threaten natural ecosystems. Whether reducing nitrogen (N) input can help mitigate plant invasions remainsunclear. We used ongoing N reductions in the Yangtze River to explore how N reductions affect native community recovery in estuarine marshes degraded by plant invasions. Using Google Earth images, we mapped native Phragmites australis patches and assessed changes in theirabundance in Spartina alterniflora-invaded marshes, showing that P. australis gradually recovered following reduced N input. To identify the underlying mechanisms, we transplanted N-fertilized and unfertilized S. alterniflora populations into plots with ambient and enriched N conditions and co-planted them with P. australis, respectively; the competitive advantage of S. alterniflora over P. australisdecreased with N reductions, regardless of fertilized population history, shifting the marsh from P. australisexclusion to species coexistence. Thus, nutrient reductions can shift ecosystems from being susceptible to invasion to successional recovery, offering an effective strategyfor mitigating plant invasions and facilitating landscape-scale native community recovery.